9.10 An aromatic compound ‘A’ on treatment with aqueous ammonia and heating forms compound ‘B’ which on heating with Br2 and KOH forms a compound ‘C’ of molecular formula C6H7N. Write the structures and IUPAC names of compounds A, B and C. 9.11 Complete the following reactions: (i) C6 H 5 NH 2  CHCl 3  alc.KOH  (ii) C 6 H 5 N 2 Cl  H 3 PO 2  H 2 O  (iii) C6 H 5 NH 2  H 2 SO 4  conc.  (iv) C6 H 5 N 2 Cl  C 2 H 5 OH  (v) C6 H5 NH 2  Br2  aq   O  (vi) C6 H 5 NH 2   CH 3 CO  2 4  (vii) C 6 H5 N 2 Cl ii NaNOi HBF2 /Cu, 9.12 Why cannot aromatic primary amines be prepared by Gabriel phthalimide synthesis? 9.13 Write the reactions of (i) aromatic and (ii) aliphatic primary amines with nitrous acid. 9.14 Give plausible explanation for each of the following: (i) Why are amines less acidic than alcohols of comparable molecular masses? (ii) Why do primary amines have higher boiling point than tertiary amines? (iii) Why are aliphatic amines stronger bases than aromatic amines? Answers to Some Intext Questions 9.4 (i) C6H5NH2 < NH3 < C6H5CH2NH2 < C2H5NH2 < (C2H5)2NH (ii) C6H5NH2 < C2H5NH2. < (C2H5)3N < (C2H5)2NH (iii) C6H5NH2 < C6H5CH2NH2 < (CH3)3N < CH3NH2 < (CH3)2NH Chemistry 280 Reprint 2025-26 UnitUnitUnitUnit Unit1010 BiomoleculesBiomoleculesBiomoleculesBiomoleculesBiomoleculesBiomoleculesBiomoleculesBiomoleculesBiomoleculesBiomoleculesObjectives After studying this Unit, you will be able to “It is the harmonious and synchronous progress of chemical • explain the characteristics of reactions in body which leads to life”. biomolecules like carbohydrates, proteins and nucleic acids and hormones; A living system grows, sustains and reproduces itself. • classify carbohydrates, proteins, The most amazing thing about a living system is that it nucleic acids and vitamins on the basis of their structures; is composed of non-living atoms and molecules. The • explain the difference between pursuit of knowledge of what goes on chemically within DNA and RNA; a living system falls in the domain of biochemistry. Living • describe the role of biomolecules systems are made up of various complex biomolecules in biosystem. like carbohydrates, proteins, nucleic acids, lipids, etc. Proteins and carbohydrates are essential constituents of our food. These biomolecules interact with each other and constitute the molecular logic of life processes. In addition, some simple molecules like vitamins and mineral salts also play an important role in the functions of organisms. Structures and functions of some of these biomolecules are discussed in this Unit. 10.110.110.110.110.1 CarbohydratesCarbohydratesCarbohydratesCarbohydratesCarbohydrates Carbohydrates are primarily produced by plants and form a very large group of naturally occurring organic compounds. Some common examples of carbohydrates are cane sugar, glucose, starch, etc. Most of them have a general formula, Cx(H2O)y, and were considered as hydrates of carbon from where the name carbohydrate was derived. For example, the molecular formula of glucose (C6H12O6) fits into this general formula, C6(H2O)6. But all the compounds which fit into this formula may not be classified as carbohydrates. For example acetic acid (CH3COOH) fits into this general formula, C2(H2O)2 but is not a carbohydrate. Similarly, rhamnose, C6H12O5 is a carbohydrate but does not fit in this definition. A large number of their reactions have shown that they contain specific functional groups. Chemically, the carbohydrates may be defined as optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis. Some of the carbohydrates, which are sweet in taste, are also called sugars. The most common sugar, used in our homes is named as sucrose whereas the sugar present Reprint 2025-26 in milk is known as lactose. Carbohydrates are also called saccharides (Greek: sakcharon means sugar). Carbohydrates are classified on the basis of their behaviour on hydrolysis. They have been broadly divided into following three groups. 10.1.1 (i) Monosaccharides: A carbohydrate that cannot be hydrolysed further Classification of to give simpler unit of polyhydroxy aldehyde or ketone is called a Carbohydrates monosaccharide. About 20 monosaccharides are known to occur in nature. Some common examples are glucose, fructose, ribose, etc. (ii) Oligosaccharides: Carbohydrates that yield two to ten monosaccharide units, on hydrolysis, are called oligosaccharides. They are further classified as disaccharides, trisaccharides, tetrasaccharides, etc., depending upon the number of monosaccharides, they provide on hydrolysis. Amongst these the most common are disaccharides. The two monosaccharide units obtained on hydrolysis of a disaccharide may be same or different. For example, one molecule of sucrose on hydrolysis gives one molecule of glucose and one molecule of fructose whereas maltose gives two molecules of only glucose. (iii) Polysaccharides: Carbohydrates which yield a large number of monosaccharide units on hydrolysis are called polysaccharides. Some common examples are starch, cellulose, glycogen, gums, etc. Polysaccharides are not sweet in taste, hence they are also called non-sugars. The carbohydrates may also be classified as either reducing or non- reducing sugars. All those carbohydrates which reduce Fehling’s solution and Tollens’ reagent are referred to as reducing sugars. All monosaccharides whether aldose or ketose are reducing sugars. 10.1.2 Monosaccharides are further classified on the basis of number of carbon Monosaccharides atoms and the functional group present in them. If a monosaccharide contains an aldehyde group, it is known as an aldose and if it contains a keto group, it is known as a ketose. Number of carbon atoms constituting the monosaccharide is also introduced in the name as is evident from the examples given in Table 10.1 Table 10.1: Different Types of Monosaccharides Carbon atoms General term Aldehyde Ketone 3 Triose Aldotriose Ketotriose 4 Tetrose Aldotetrose Ketotetrose 5 Pentose Aldopentose Ketopentose 6 Hexose Aldohexose Ketohexose 7 Heptose Aldoheptose Ketoheptose 10.1.2.1 Glucose Glucose occurs freely in nature as well as in the combined form. It is present in sweet fruits and honey. Ripe grapes also contain glucose in large amounts. It is prepared as follows: Preparation of 1. From sucrose (Cane sugar): If sucrose is boiled with dilute HCl or Glucose H2SO4 in alcoholic solution, glucose and fructose are obtained in equal amounts. Chemistry 282 Reprint 2025-26 H+ C12 H 22 O11 + H 2 O → C 6 H12 O 6 + C 6 H12 O 6 Sucrose Glucose Fructose 2. From starch: Commercially glucose is obtained by hydrolysis of starch by boiling it with dilute H2SO4 at 393 K under pressure. H + (C 6 H10 O 5 ) n + nH 2 O →393K; 2-3 atm nC 6 H12 O 6 Starch or cellulose Glucose Glucose is an aldohexose and is also known as dextrose. It is theStructure of monomer of many of the larger carbohydrates, namely starch, cellulose.Glucose It is probably the most abundant organic compound on earth. It was assigned the structure given below on the basis of the following evidences: CHO 1. Its molecular formula was found to be C6H12O6. 2. On prolonged heating with HI, it forms n-hexane, suggesting that all(CH OH )4 the six carbon atoms are linked in a straight chain. CH2OH Glucose 3. Glucose reacts with hydroxylamine to form an oxime and adds a molecule of hydrogen cyanide to give cyanohydrin. These reactions confirm the presence of a carbonyl group (>C = O) in glucose. 4. Glucose gets oxidised to six carbon carboxylic acid (gluconic acid) on reaction with a mild oxidising agent like bromine water. This indicates that the carbonyl group is present as an aldehydic group. CHO COOH Br2 water (CHOH )4 (CH OH )4 CH2OH CH2OH Gluconic acid 5. Acetylation of glucose with acetic anhydride gives glucose pentaacetate which confirms the presence of five –OH groups. Since it exists as a stable compound, five –OH groups should be attached to different carbon atoms. 283 Biomolecules Reprint 2025-26 6. On oxidation with nitric acid, glucose as well as gluconic acid both yield a dicarboxylic acid, saccharic acid. This indicates the presence of a primary alcoholic (–OH) group in glucose. CHO COOH COOH Oxidation Oxidation (CHOH)4 (CHOH )4 (CHOH )4 CH 2OH COOH CH2 OH Saccharic Gluconic acid acid The exact spatial arrangement of different —OH groups was given by Fischer after studying many other properties. Its configuration is correctly represented as I. So gluconic acid is represented as II and saccharic acid as III. CHO COOH COOH H OH H OH H OH OH H OH H OH H H OH H OH H OH H OH H OH H OH CH2OH CH2OH COOH I II III Glucose is correctly named as D(+)-glucose. ‘D’ before the name of glucose represents the configuration whereas ‘(+)’ represents dextrorotatory nature of the molecule. It should be remembered that ‘D’ and ‘L’ have no relation with the optical activity of the compound. They are also not related to letter ‘d’ and ‘l’ (see Unit 6). The meaning of D– and L– notations is as follows. The letters ‘D’ or ‘L’ before the name of any compound indicate the relative configuration of a particular stereoisomer of a compound with respect to configuration of some other compound, configuration of which is known. In the case of carbohydrates, this refers to their relation with a particular isomer of glyceraldehyde. Glyceraldehyde contains one asymmetric carbon atom and exists in two enantiomeric forms as shown below. (+) Isomer of glyceraldehyde has ‘D’ configuration. It means that when its structural formula is written on paper following specific conventions which you will study in higher classes, the –OH group lies on right hand side in the structure. All those compounds which can be chemically correlated to D (+) isomer of glyceraldehyde are said to have D- configuration whereas those which can be correlated to ‘L’ (–) isomer of glyceraldehyde are said to have L—configuration. In L (–) isomer –OH group is on left hand side as you can see in the structure. For assigning Chemistry 284 Reprint 2025-26 the configuration of monosaccharides, it is the lowest asymmetric carbon atom (as shown below) which is compared. As in (+) glucose, —OH on the lowest asymmetric carbon is on the right side which is comparable to (+) glyceraldehyde, so (+) glucose is assigned D-configuration. Other asymmetric carbon atoms of glucose are not considered for this comparison. Also, the structure of glucose and glyceraldehyde is written in a way that most oxidised carbon (in this case –CHO)is at the top. CHO H OH OH H CHO H OH H OH H OH CH2OH CH2OH D– (+) – Glyceraldehyde D–(+) – Glucose Cyclic The structure (I) of glucose explained most of its properties but the Structure following reactions and facts could not be explained by this structure. of Glucose 1. Despite having the aldehyde group, glucose does not give Schiff’s test and it does not form the hydrogensulphite addition product with NaHSO3. 2. The pentaacetate of glucose does not react with hydroxylamine indicating the absence of free —CHO group. 3. Glucose is found to exist in two different crystalline forms which are named as a and b. The a-form of glucose (m.p. 419 K) is obtained by crystallisation from concentrated solution of glucose at 303 K while the b-form (m.p. 423 K) is obtained by crystallisation from hot and saturated aqueous solution at 371 K. This behaviour could not be explained by the open chain structure (I) for glucose. It was proposed that one of the —OH groups may add to the —CHO group and form a cyclic hemiacetal structure. It was found that glucose forms a six-membered ring in which —OH at C-5 is involved in ring formation. This explains the absence of —CHO group and also existence of glucose in two forms as shown below. These two cyclic forms exist in equilibrium with open chain structure. The two cyclic hemiacetal forms of glucose differ only in the configuration of the hydroxyl group at C1, called anomeric carbon 285 Biomolecules Reprint 2025-26 (the aldehyde carbon before cyclisation). Such isomers, i.e., a-form and b-form, are called anomers. The six membered cyclic structure of glucose is called pyranose structure (a– or b–), in analogy with pyran. Pyran is a cyclic organic compound with one oxygen atom and five carbon atoms in the ring. The cyclic structure of glucose is more correctly represented by Haworth structure as given below. 10.1.2.2 Fructose Fructose is an important ketohexose. It is obtained along with glucose by the hydrolysis of disaccharide, sucrose. It is a natural monosaccharide found in fruits, honey and vegetables. In its pure form it is used as a sweetner. It is also an important ketohexose. Fructose also has the molecular formula C6H12O6 and Structure on the basis of its reactions it was found to contain a of Fructose ketonic functional group at carbon number 2 and six carbons in straight chain as in the case of glucose. It belongs to D-series and is a laevorotatory compound. It is appropriately written as D-(–)-fructose. Its open chain structure is as shown. It also exists in two cyclic forms which are obtained by the addition of —OH at C5 to the ( ) group. The ring, thus formed is a five membered ring and is named as furanose with analogy to the compound furan. Furan is a five membered cyclic compound with one oxygen and four carbon atoms. The cyclic structures of two anomers of fructose are represented by Haworth structures as given. Chemistry 286 Reprint 2025-26 10.1.3 You have already read that disaccharides on hydrolysis with dilute Disaccharides acids or enzymes yield two molecules of either the same or different monosaccharides. The two monosaccharides are joined together by an oxide linkage formed by the loss of a water molecule. Such a linkage between two monosaccharide units through oxygen atom is called glycosidic linkage. In disaccharides, if the reducing groups of monosaccharides i.e., aldehydic or ketonic groups are bonded, these are non-reducing sugars, e.g., sucrose. On the other hand, sugars in which these functional groups are free, are called reducing sugars, for example, maltose and lactose. (i) Sucrose: One of the common disaccharides is sucrose which on hydrolysis gives equimolar mixture of D-(+)-glucose and D-(-) fructose. These two monosaccharides are held together by a glycosidic linkage between C1 of a-D-glucose and C2 of b-D-fructose. Since the reducing groups of glucose and fructose are involved in glycosidic bond formation, sucrose is a non reducing sugar. Sucrose is dextrorotatory but after hydrolysis gives dextrorotatory glucose and laevorotatory fructose. Since the laevorotation of fructose (–92.4°) is more than dextrorotation of glucose (+ 52.5°), the mixture is laevorotatory. Thus, hydrolysis of sucrose brings about a change in the sign of rotation, from dextro (+) to laevo (–) and the product is named as invert sugar. (ii) Maltose: Another disaccharide, maltose is composed of two a-D-glucose units in which C1 of one glucose (I) is linked to C4 of another glucose unit (II). The free aldehyde group can be produced at C1 of second glucose in solution and it shows reducing properties so it is a reducing sugar. 287 Biomolecules Reprint 2025-26 (iii) Lactose: It is more commonly known as milk sugar since this disaccharide is found in milk. It is composed of b-D-galactose and b-D-glucose. The linkage is between C1 of galactose and C4 of glucose. Free aldehyde group may be produced at C-1 of glucose unit, hence it is also a reducing sugar. 10.1.4 Polysaccharides Polysaccharides contain a large number of monosaccharide units joined together by glycosidic linkages. These are the most commonly encountered carbohydrates in nature. They mainly act as the food storage or structural materials. (i) Starch: Starch is the main storage polysaccharide of plants. It is the most important dietary source for human beings. High content of starch is found in cereals, roots, tubers and some vegetables. It is a polymer of a-glucose and consists of two components— Amylose and Amylopectin. Amylose is water soluble component which constitutes about 15-20% of starch. Chemically amylose is a long unbranched chain with 200-1000 a-D-(+)-glucose units held together by C1– C4 glycosidic linkage. Amylopectin is insoluble in water and constitutes about 80- 85% of starch. It is a branched chain polymer of a-D-glucose units in which chain is formed by C1–C4 glycosidic linkage whereas branching occurs by C1–C6 glycosidic linkage. Chemistry 288 Reprint 2025-26 (ii) Cellulose: Cellulose occurs exclusively in plants and it is the most abundant organic substance in plant kingdom. It is a predominant constituent of cell wall of plant cells. Cellulose is a straight chain polysaccharide composed only of b-D-glucose units which are joined by glycosidic linkage between C1 of one glucose unit and C4 of the next glucose unit. (iii) Glycogen: The carbohydrates are stored in animal body as glycogen. It is also known as animal starch because its structure is similar to amylopectin and is rather more highly branched. It is present in liver, muscles and brain. When the body needs glucose, enzymes break the glycogen down to glucose. Glycogen is also found in yeast and fungi. 10.1.5 Carbohydrates are essential for life in both plants and animals. They Importance of form a major portion of our food. Honey has been used for a long time Carbohydrates as an instant source of energy by ‘Vaids’ in ayurvedic system of medicine. Carbohydrates are used as storage molecules as starch in plants and glycogen in animals. Cell wall of bacteria and plants is made up of cellulose. We build furniture, etc. from cellulose in the form 289 Biomolecules Reprint 2025-26 of wood and clothe ourselves with cellulose in the form of cotton fibre. They provide raw materials for many important industries like textiles, paper, lacquers and breweries. Two aldopentoses viz. D-ribose and 2-deoxy-D-ribose (Section 10.5.1, Class XII) are present in nucleic acids. Carbohydrates are found in biosystem in combination with many proteins and lipids. IntextIntextIntextIntextIntext QuestionsQuestionsQuestionsQuestionsQuestions 10.1 Glucose or sucrose are soluble in water but cyclohexane or benzene (simple six membered ring compounds) are insoluble in water. Explain. 10.2 What are the expected products of hydrolysis of lactose? 10.3 How do you explain the absence of aldehyde group in the pentaacetate of D-glucose? 10.210.210.210.210.2 ProteinsProteinsProteinsProteinsProteins Proteins are the most abundant biomolecules of the living system. Chief sources of proteins are milk, cheese, pulses, peanuts, fish, meat, etc. They occur in every part of the body and form the fundamental basis of structure and functions of life. They are also required for growth and maintenance of body. The word protein is derived from Greek word, “proteios” which means primary or of prime importance. All proteins are polymers of a-amino acids. 10.2.1 Amino Amino acids contain amino (–NH2) and carboxyl (–COOH) functional groups. Depending upon the relative position of amino group with Acids respect to carboxyl group, the amino acids can be R CH COOH classified as a, b, g, d and so on. Only a-amino acids are obtained on hydrolysis of proteins. They NH2 may contain other functional groups also. a-amino acid All a-amino acids have trivial names, which (R = side chain) usually reflect the property of that compound or its source. Glycine is so named since it has sweet taste (in Greek glykos means sweet) and tyrosine was first obtained from cheese (in Greek, tyros means cheese.) Amino acids are generally represented by a three letter symbol, sometimes one letter symbol is also used. Structures of some commonly occurring amino acids along with their 3-letter and 1-letter symbols are given in Table 10.2. COOH Table 10.2: Natural Amino Acids H2N H R Name of the Characteristic feature Three letter One letter amino acids of side chain, R symbol code 1. Glycine H Gly G 2. Alanine – CH3 Ala A 3. Valine* (H3C)2CH- Val V 4. Leucine* (H3C)2CH-CH2- Leu L Chemistry 290 Reprint 2025-26 5. Isoleucine* H3C-CH2-CH- Ile I | CH3 6. Arginine* HN=C-NH-(CH2)3- Arg R | NH2 7. Lysine* H2N-(CH2)4- Lys K 8. Glutamic acid HOOC-CH2-CH2- Glu E 9. Aspartic acid HOOC-CH2- Asp D O || 10. Glutamine H2N-C-CH2-CH2- Gln Q O || 11. Asparagine H2N-C-CH2- Asn N 12. Threonine* H3C-CHOH- Thr T 13. Serine HO-CH2- Ser S 14. Cysteine HS-CH2- Cys C 15. Methionine* H3C-S-CH2-CH2- Met M 16. Phenylalanine* C6H5-CH2- Phe F 17. Tyrosine (p)HO-C6H4-CH2- Tyr Y –CH2 18. Tryptophan* Trp W N H 19. Histidine* His H 20. Proline Pro P * essential amino acid, a = entire structure 10.2.2 Amino acids are classified as acidic, basic or neutral depending upon Classification of the relative number of amino and carboxyl groups in their molecule. Amino Acids Equal number of amino and carboxyl groups makes it neutral; more number of amino than carboxyl groups makes it basic and more carboxyl groups as compared to amino groups makes it acidic. The amino acids, which can be synthesised in the body, are known as non- essential amino acids. On the other hand, those which cannot be synthesised in the body and must be obtained through diet, are known as essential amino acids (marked with asterisk in Table 10.2). 291 Biomolecules Reprint 2025-26 Amino acids are usually colourless, crystalline solids. These are water-soluble, high melting solids and behave like salts rather than simple amines or carboxylic acids. This behaviour is due to the presence of both acidic (carboxyl group) and basic (amino group) groups in the same molecule. In aqueous solution, the carboxyl group can lose a proton and amino group can accept a proton, giving rise to a dipolar ion known as zwitter ion. This is neutral but contains both positive and negative charges. In zwitter ionic form, amino acids show amphoteric behaviour as they react both with acids and bases. Except glycine, all other naturally occurring a-amino acids are optically active, since the a-carbon atom is asymmetric. These exist both in ‘D’ and ‘L’ forms. Most naturally occurring amino acids have L-configuration. L-Aminoacids are represented by writing the –NH2 group on left hand side. 10.2.3 Structure You have already read that proteins are the polymers of a-amino acids of Proteins and they are connected to each other by peptide bond or peptide linkage. Chemically, peptide linkage is an amide formed between –COOH group and –NH2 group. The reaction between two molecules of similar or different amino acids, proceeds through the combination of the amino group of one molecule with the carboxyl group of the other. This results in the elimination of a water molecule and formation of a peptide bond –CO–NH–. The product of the reaction is called a dipeptide because it is made up of two amino acids. For example, when carboxyl group of glycine combines with the amino group of alanine we get a dipeptide, glycylalanine. If a third amino acid combines to a dipeptide, the product is called a tripeptide. A tripeptide contains three amino acids linked by two peptide linkages. Similarly when four, five or six amino acids are linked, the respective products are known as tetrapeptide, pentapeptide or hexapeptide, respectively. When the number of such amino acids is more than ten, then the products are called polypeptides. A polypeptide with more than hundred amino acid residues, having molecular mass higher than 10,000u is called a protein. However, the distinction between a polypeptide and a protein is not very sharp. Polypeptides with fewer amino acids are likely to be called proteins if they ordinarily have a well defined conformation of a protein such as insulin which contains 51 amino acids. Proteins can be classified into two types on the basis of their molecular shape. (a) Fibrous proteins When the polypeptide chains run parallel and are held together by hydrogen and disulphide bonds, then fibre– like structure is formed. Such proteins are generally insoluble in water. Some common examples are keratin (present in hair, wool, silk) and myosin (present in muscles), etc. Chemistry 292 Reprint 2025-26 (b) Globular proteins This structure results when the chains of polypeptides coil around to give a spherical shape. These are usually soluble in water. Insulin and albumins are the common examples of globular proteins. Structure and shape of proteins can be studied at four different levels, i.e., primary, secondary, tertiary and quaternary, each level being more complex than the previous one. (i) Primary structure of proteins: Proteins may have one or more polypeptide chains. Each polypeptide in a protein has amino acids linked with each other in a specific sequence and it is this sequence of amino acids that is said to be the primary structure of that protein. Any change in this primary structure i.e., the sequence of amino acids creates a different protein. (ii) Secondary structure of proteins: The secondary structure of protein refers to the shape in which a long polypeptide chain can exist. They are found to exist in two different types of structures viz. a-helix and b-pleated sheet structure. These structures arise due to the regular folding of the backbone of the polypeptide chain due to hydrogen bonding between and –NH– groups of the peptide bond. a-Helix is one of the most common ways in which a polypeptide chain forms all possible hydrogen bonds by twisting into a right handed screw (helix) with the Fig. 10.1: a-Helix –NH group of each amino acid residue hydrogen bonded to the structure of proteins C O of an adjacent turn of the helix as shown in Fig.10.1. In b-pleated sheet structure all peptide chains are stretched out to nearly maximum extension and then laid side by side which are held together by intermolecular hydrogen bonds. The structure resembles the pleated folds of drapery and therefore is known as b-pleated sheet. (iii) Tertiary structure of proteins: The tertiary structure of proteins represents overall folding of the polypeptide chains i.e., further folding of the secondary structure. It gives rise to two major molecular shapes viz. fibrous and globular. The main forces which stabilise the 2° and 3° structures of proteins are hydrogen bonds, disulphide linkages, van der Waals and electrostatic forces of attraction. Fig. 10.2: b-Pleated sheet structure of (iv) Quaternary structure of proteins: Some of the proteins proteins are composed of two or more polypeptide chains referred to as sub-units. The spatial arrangement of these subunits with respect to each other is known as quaternary structure. 293 Biomolecules Reprint 2025-26 A diagrammatic representation of all these four structures is given in Figure 10.3 where each coloured ball represents an amino acid. Fig. 10.3: Diagrammatic representation of protein structure (two sub-units of two types in quaternary structure) Fig. 10.4: Primary, secondary, tertiary and quaternary structures of haemoglobin 10.2.4 Protein found in a biological system with a unique three-dimensional Denaturation of structure and biological activity is called a native protein. When a Proteins protein in its native form, is subjected to physical change like change in temperature or chemical change like change in pH, the hydrogen bonds are disturbed. Due to this, globules unfold and helix get uncoiled and protein loses its biological activity. This is called denaturation of Chemistry 294 Reprint 2025-26 protein. During denaturation secondary and tertiary structures are destroyed but primary structure remains intact. The coagulation of egg white on boiling is a common example of denaturation. Another example is curdling of milk which is caused due to the formation of lactic acid by the bacteria present in milk. IntextIntextIntextIntextIntext QuestionsQuestionsQuestionsQuestionsQuestions 10.4 The melting points and solubility in water of amino acids are generally higher than that of the corresponding halo acids. Explain. 10.5 Where does the water present in the egg go after boiling the egg? 10.310.310.310.310.3 EnzymesEnzymesEnzymesEnzymesEnzymes Life is possible due to the coordination of various chemical reactions in living organisms. An example is the digestion of food, absorption of appropriate molecules and ultimately production of energy. This process involves a sequence of reactions and all these reactions occur in the body under very mild conditions. This occurs with the help of certain biocatalysts called enzymes. Almost all the enzymes are globular proteins. Enzymes are very specific for a particular reaction and for a particular substrate. They are generally named after the compound or class of compounds upon which they work. For example, the enzyme that catalyses hydrolysis of maltose into glucose is named as maltase. Maltase C12 H 22 O11  2 C 6 H12 O 6 Maltose G lucose Sometimes enzymes are also named after the reaction, where they are used. For example, the enzymes which catalyse the oxidation of one substrate with simultaneous reduction of another substrate are named as oxidoreductase enzymes. The ending of the name of an enzyme is -ase. 10.3.1 Mechanism Enzymes are needed only in small quantities for the progress of a reaction. of Enzyme Similar to the action of chemical catalysts, enzymes are said to reduce Action the magnitude of activation energy. For example, activation energy for acid hydrolysis of sucrose is 6.22 kJ mol–1, while the activation energy is only 2.15 kJ mol–1 when hydrolysed by the enzyme, sucrase. Mechanism for the enzyme action has been discussed. 10.410.410.410.410.4 VitaminsVitaminsVitaminsVitaminsVitamins It has been observed that certain organic compounds are required in small amounts in our diet but their deficiency causes specific diseases. These compounds are called vitamins. Most of the vitamins cannot be synthesised in our body but plants can synthesise almost all of them, so they are considered as essential food factors. However, the bacteria of the gut can produce some of the vitamins required by us. All the vitamins are generally available in our diet. Different vitamins belong to various chemical classes and it is difficult to define them on the basis of structure. They are generally regarded as organic compounds required in the diet in small amounts to perform specific biological functions for normal maintenance of optimum growth 295 Biomolecules Reprint 2025-26 and health of the organism. Vitamins are designated by alphabets A, B, C, D, etc. Some of them are further named as sub-groups e.g. B1, B2, B6, B12, etc. Excess of vitamins is also harmful and vitamin pills should not be taken without the advice of doctor. The term “Vitamine” was coined from the word vital + amine since the earlier identified compounds had amino groups. Later work showed that most of them did not contain amino groups, so the letter ‘e’ was dropped and the term vitamin is used these days. 10.4.1 Vitamins are classified into two groups depending upon their solubility Classification of in water or fat. Vitamins (i) Fat soluble vitamins: Vitamins which are soluble in fat and oils but insoluble in water are kept in this group. These are vitamins A, D, E and K. They are stored in liver and adipose (fat storing) tissues. (ii) Water soluble vitamins: B group vitamins and vitamin C are soluble in water so they are grouped together. Water soluble vitamins must be supplied regularly in diet because they are readily excreted in urine and cannot be stored (except vitamin B12) in our body. Some important vitamins, their sources and diseases caused by their deficiency are listed in Table 10.3. Table 10.3: Some important Vitamins, their Sources and their Deficiency Diseases Sl. Name of Sources Deficiency diseases No. Vitamins 1. Vitamin A Fish liver oil, carrots, X e r o p h t h a l m i a butter and milk (hardening of cornea of eye) Night blindness 2. Vitamin B1 Yeast, milk, green Beri beri (loss of appe- (Thiamine) vegetables and cereals tite, retarded growth) 3. Vitamin B2 Milk, eggwhite, liver, Cheilosis (fissuring at (Riboflavin) kidney corners of mouth and lips), digestive disorders and burning sensation of the skin. 4. Vitamin B6 Yeast, milk, egg yolk, Convulsions (Pyridoxine) cereals and grams 5. Vitamin B12 Meat, fish, egg and Pernicious anaemia curd (RBC deficient in haemoglobin) 6. Vitamin C Citrus fruits, amla and Scurvy (bleeding gums) (Ascorbic acid) green leafy vegetables 7. Vitamin D Exposure to sunlight, Rickets (bone deformities fish and egg yolk in children) and osteo- malacia (soft bones and joint pain in adults) Chemistry 296 Reprint 2025-26 8. Vitamin E Vegetable oils like wheat Increased fragility of germ oil, sunflower oil, RBCs and muscular etc. weakness 9. Vitamin K Green leafy vegetables Increased blood clotting time 10.5.5.5.5.5 NucleicNucleicNucleicNucleicNucleic AcidsAcidsAcidsAcidsAcids Every generation of each and every species resembles its ancestors in many ways. How are these characteristics transmitted from one generation to the next? It has been observed that nucleus of a living cell is responsible for this transmission of inherent characters, also called heredity. The particles in nucleus of the cell, responsible for heredity, are called chromosomes which are made up of proteins and another type of biomolecules called nucleic acids. These are mainly of two types, the deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Since nucleic acids are long chain polymers of nucleotides, so they are also called polynucleotides. James Dewey Watson Born in Chicago, Illinois, in 1928, Dr Watson received his Ph.D. (1950) from Indiana University in Zoology. He is best known for his discovery of the structure of DNA for which he shared with Francis Crick and Maurice Wilkins the 1962 Nobel prize in Physiology and Medicine. They proposed that DNA molecule takes the shape of a double helix, an elegantly simple structure that resembles a gently twisted ladder. The rails of the ladder are made of alternating units of phosphate and the sugar deoxyribose; the rungs are each composed of a pair of purine/ pyrimidine bases. This research laid the foundation for the emerging field of molecular biology. The complementary pairing of nucleotide bases explains how identical copies of parental DNA pass on to two daughter cells. This research launched a revolution in biology that led to modern recombinant DNA techniques. 10.5.1 Chemical Complete hydrolysis of DNA (or RNA) yields a pentose sugar, phosphoric Composition acid and nitrogen containing heterocyclic compounds (called bases). In of Nucleic DNA molecules, the sugar moiety is b-D-2-deoxyribose whereas in Acids RNA molecule, it is b-D-ribose. 297 Biomolecules Reprint 2025-26 DNA contains four bases viz. adenine (A), guanine (G), cytosine (C) and thymine (T). RNA also contains four bases, the first three bases are same as in DNA but the fourth one is uracil (U). Cytosine (C) Thymine (T) Uracil (U) 10.5.2 Structure A unit formed by the attachment of a base to 1¢ position of sugar is of Nucleic known as nucleoside. In nucleosides, the sugar carbons are numbered Acids as 1¢, 2¢, 3¢, etc. in order to distinguish these from the bases (Fig. 10.5a). When nucleoside is linked to phosphoric acid at 5¢-position of sugar moiety, we get a nucleotide (Fig. 10.5). Fig. 10.5: Structure of (a) a nucleoside and (b) a nucleotide Nucleotides are joined together by phosphodiester linkage between 5¢ and 3¢ carbon atoms of the pentose sugar. The formation of a typical dinucleotide is shown in Fig. 10.6. Chemistry 298 Reprint 2025-26 Fig. 10.6: Formation of a dinucleotide A simplified version of nucleic acid chain is as shown below. Base Base Base Sugar Phosphate Sugar Phosphate Sugar n Information regarding the sequence of nucleotides in the chain of a nucleic acid is called its primary structure. Nucleic acids have a secondary structure also. James Watson and Francis Crick gave a double strand helix structure for DNA (Fig. 10.7). Two nucleic acid chains are wound about each other and held together by hydrogen bonds between pairs of bases. The two strands are complementary to each other because the hydrogen bonds are formed between specific pairs of bases. Adenine forms hydrogen bonds with thymine whereas cytosine forms hydrogen bonds with guanine. In secondary structure of RNA single stranded helics is present which sometimes foldsback on itself. RNA molecules are of three types and they perform different functions. They are named as messenger RNA (m-RNA), ribosomal RNA (r-RNA) and transfer RNA (t-RNA). Fig. 10.7: Double strand helix structure for DNA 299 Biomolecules Reprint 2025-26 Har Gobind Khorana Har Gobind Khorana, was born in 1922. He obtained his M.Sc. degree from Punjab University in Lahore. He worked with Professor Vladimir Prelog, who moulded Khorana’s thought and philosophy towards science, work and effort. After a brief stay in India in 1949, Khorana went back to England and worked with Professor G.W. Kenner and Professor A.R.Todd. It was at Cambridge, U.K. that he got interested in both proteins and nucleic acids. Dr Khorana shared the Nobel Prize for Medicine and Physiology in 1968 with Marshall Nirenberg and Robert Holley for cracking the genetic code. DNA Fingerprinting It is known that every individual has unique fingerprints. These occur at the tips of the fingers and have been used for identification for a long time but these can be altered by surgery. A sequence of bases on DNA is also unique for a person and information regarding this is called DNA fingerprinting. It is same for every cell and cannot be altered by any known treatment. DNA fingerprinting is now used (i) in forensic laboratories for identification of criminals. (ii) to determine paternity of an individual. (iii) to identify the dead bodies in any accident by comparing the DNA’s of parents or children. (iv) to identify racial groups to rewrite biological evolution. 10.5.3 Biological DNA is the chemical basis of heredity and may be regarded as the reserve Functions of genetic information. DNA is exclusively responsible for maintaining of Nucleic the identity of different species of organisms over millions of years. A Acids DNA molecule is capable of self duplication during cell division and identical DNA strands are transferred to daughter cells. Another important function of nucleic acids is the protein synthesis in the cell. Actually, the proteins are synthesised by various RNA molecules in the cell but the message for the synthesis of a particular protein is present in DNA. 10.610.610.610.610.6 HormonesHormonesHormonesHormonesHormones Hormones are molecules that act as intercellular messengers. These are produced by endocrine glands in the body and are poured directly in the blood stream which transports them to the site of action. In terms of chemical nature, some of these are steroids, e.g., estrogens and androgens; some are poly peptides for example insulin and endorphins and some others are amino acid derivatives such as epinephrine and norepinephrine. Hormones have several functions in the body. They help to maintain the balance of biological activities in the body. The role of insulin in keeping the blood glucose level within the narrow limit is an example of this function. Insulin is released in response to the rapid rise in blood glucose level. On the other hand hormone glucagon tends to increase the glucose level in the blood. The two hormones together regulate the glucose level in the blood. Epinephrine and norepinephrine mediate responses to external stimuli. Growth hormones and sex hormones play role in growth and development. Thyroxine produced in the thyroid gland is an iodinated derivative of amino acid tyrosine. Abnormally low level of thyroxine leads Chemistry 300 Reprint 2025-26 to hypothyroidism which is characterised by lethargyness and obesity. Increased level of thyroxine causes hyperthyroidism. Low level of iodine in the diet may lead to hypothyroidism and enlargement of the thyroid gland. This condition is largely being controlled by adding sodium iodide to commercial table salt (“Iodised” salt). Steroid hormones are produced by adrenal cortex and gonads (testes in males and ovaries in females). Hormones released by the adrenal cortex play very important role in the functions of the body. For example, glucocorticoids control the carbohydrate metabolism, modulate inflammatory reactions, and are involved in reactions to stress. The mineralocorticoids control the level of excretion of water and salt by the kidney. If adrenal cortex does not function properly then one of the results may be Addison’s disease characterised by hypoglycemia, weakness and increased susceptibility to stress. The disease is fatal unless it is treated by glucocorticoids and mineralocorticoids. Hormones released by gonads are responsible for development of secondary sex characters. Testosterone is the major sex hormone produced in males. It is responsible for development of secondary male characteristics (deep voice, facial hair, general physical constitution) and estradiol is the main female sex hormone. It is responsible for development of secondary female characteristics and participates in the control of menstrual cycle. Progesterone is responsible for preparing the uterus for implantation of fertilised egg. IntextIntextIntextIntextIntext QuestionsQuestionsQuestionsQuestionsQuestions 10.6 Why cannot vitamin C be stored in our body? 10.7 What products would be formed when a nucleotide from DNA containing thymine is hydrolysed? 10.8 When RNA is hydrolysed, there is no relationship among the quantities of different bases obtained. What does this fact suggest about the structure of RNA? SummarySummarySummarySummarySummary Carbohydrates are optically active polyhydroxy aldehydes or ketones or molecules which provide such units on hydrolysis. They are broadly classified into three groups — monosaccharides, disaccharides and polysaccharides. Glucose, the most important source of energy for mammals, is obtained by the digestion of starch. Monosaccharides are held together by glycosidic linkages to form disaccharides or polysaccharides. Proteins are the polymers of about twenty different a-amino acids which are linked by peptide bonds. Ten amino acids are called essential amino acids because they cannot be synthesised by our body, hence must be provided through diet. Proteins perform various structural and dynamic functions in the organisms. Proteins which contain only a-amino acids are called simple proteins. The secondary or tertiary structure of proteins get disturbed on change of pH or temperature and they are not able to perform their functions. This is called denaturation of proteins. Enzymes are biocatalysts which speed up the reactions in biosystems. They are very specific and selective in their action and chemically majority of enzymes are proteins. Vitamins are accessory food factors required in the diet. They are classified as fat soluble (A, D, E and K) and water soluble (B group and C). Deficiency of vitamins leads to many diseases. 301 Biomolecules Reprint 2025-26 Nucleic acids are the polymers of nucleotides which in turn consist of a base, a pentose sugar and phosphate moiety. Nucleic acids are responsible for the transfer of characters from parents to offsprings. There are two types of nucleic acids — DNA and RNA. DNA contains a five carbon sugar molecule called 2-deoxyribose whereas RNA contains ribose. Both DNA and RNA contain adenine, guanine and cytosine. The fourth base is thymine in DNA and uracil in RNA. The structure of DNA is a double strand whereas RNA is a single strand molecule. DNA is the chemical basis of heredity and have the coded message for proteins to be synthesised in the cell. There are three types of RNA — mRNA, rRNA and tRNA which actually carry out the protein synthesis in the cell. ExercisesExercisesExercisesExercisesExercises 10.1 What are monosaccharides? 10.2 What are reducing sugars? 10.3 Write two main functions of carbohydrates in plants. 10.4 Classify the following into monosaccharides and disaccharides. Ribose, 2-deoxyribose, maltose, galactose, fructose and lactose. 10.5 What do you understand by the term glycosidic linkage? 10.6 What is glycogen? How is it different from starch? 10.7 What are the hydrolysis products of (i) sucrose and (ii) lactose? 10.8 What is the basic structural difference between starch and cellulose? 10.9 What happens when D-glucose is treated with the following reagents? (i) HI (ii) Bromine water (iii) HNO3 10.10 Enumerate the reactions of D-glucose which cannot be explained by its open chain structure. 10.11 What are essential and non-essential amino acids? Give two examples of each type. 10.12 Define the following as related to proteins (i) Peptide linkage (ii) Primary structure (iii) Denaturation. 10.13 What are the common types of secondary structure of proteins? 10.14 What type of bonding helps in stabilising the a-helix structure of proteins? 10.15 Differentiate between globular and fibrous proteins. 10.16 How do you explain the amphoteric behaviour of amino acids? 10.17 What are enzymes? 10.18 What is the effect of denaturation on the structure of proteins? 10.19 How are vitamins classified? Name the vitamin responsible for the coagulation of blood. 10.20 Why are vitamin A and vitamin C essential to us? Give their important sources. 10.21 What are nucleic acids? Mention their two important functions. 10.22 What is the difference between a nucleoside and a nucleotide? 10.23 The two strands in DNA are not identical but are complementary. Explain. 10.24 Write the important structural and functional differences between DNA and RNA. 10.25 What are the different types of RNA found in the cell? Chemistry 302 Reprint 2025-26

8.17 Complete each synthesis by giving missing starting material, reagent or products

8.20 Although phenoxide ion has more number of resonating structures than carboxylate ion, carboxylic acid is a stronger acid than phenol. Why? Answers to Some Intext Questions 8.1 (i) (iv) (ii) (v) (iii) (vi) 257 Aldehydes, Ketones and Carboxylic Acids Reprint 2025-26 8.2 (i) (ii) (iii) (iv) 8.3 CH3CH2CH3 < CH3OCH3 < CH3CHO < CH3CH2OH 8.4 (i) Butanone < Propanone < Propanal < Ethanal (ii) Acetophenone < p-Tolualdehyde , Benzaldehyde < p-Nitrobenzaldehyde. 8.5 (i) (ii) (iii) (iv) 8.6 (i) 3-Phenylpropanoic acid (ii) 3-Methylbut-2-enoic acid (iii) 2-Methylcyclopentanecarboxylic acid. (iv) 2,4,6-Trinitrobenzoic acid 8.7 (i) (ii) (iii) (iv) 8.8 Chemistry 258 Reprint 2025-26 UnitUnitUnitUnit Unit9 AminesAminesmines9minesminesminesminesminesminesminesObjectives After studying this Unit, you will be able to “The chief commercial use of amines is as intermediates in the · describe amines as derivatives of synthesis of medicines and fibres” . ammonia having a pyramidal structure; Amines constitute an important class of organic · classify amines as primary, compounds derived by replacing one or more hydrogen secondary and tertiary; atoms of ammonia molecule by alkyl/aryl group(s). In· name amines by common names nature, they occur among proteins, vitamins, alkaloids and IUPAC system; and hormones. Synthetic examples include polymers,· describe some of the important dye stuffs and drugs. Two biologically active methods of preparation of amines; compounds, namely adrenaline and ephedrine, both· explain the properties of amines; containing secondary amino group, are used to increase· distinguish between primary, blood pressure. Novocain, a synthetic amino compound, secondary and tertiary amines; is used as an anaesthetic in dentistry. Benadryl, a well· describe the method of prepara- known antihistaminic drug also contains tertiary amino tion of diazonium salts and their importance in the synthesis of a group. Quaternary ammonium salts are used as series of aromatic compounds surfactants. Diazonium salts are intermediates in the including azo dyes. preparation of a variety of aromatic compounds including dyes. In this Unit, you will learn about amines and diazonium salts. I. AMINES Amines can be considered as derivatives of ammonia, obtained by replacement of one, two or all the three hydrogen atoms by alkyl and/or aryl groups. For example: 9.19.19.19.19.1 StructureStructureStructureStructureStructure ofofofofof AminesAminesAminesAminesAmines Like ammonia, nitrogen atom of amines is trivalent and carries an unshared pair of electrons. Nitrogen orbitals in amines are therefore, sp3 hybridised and the geometry of amines is pyramidal. Each of the three sp3 hybridised orbitals of nitrogen overlap with orbitals of hydrogen or carbon depending upon the composition of the amines. The fourth orbital of nitrogen in all amines contains an unshared pair of electrons. Due to the presence of unshared pair of electrons, the angle C–N–E, (where E is Reprint 2025-26 C or H) is less than 109.5°; for instance, it is 108o in case of trimethylamine as shown in Fig. 9.1. Fig. 9.1 Pyramidal shape of trimethylamine 9.29.29.29.29.2 ClassificationClassificationClassificationClassificationClassification Amines are classified as primary (1o), secondary (2o) and tertiary (3o) depending upon the number of hydrogen atoms replaced by alkyl or aryl groups in ammonia molecule. If one hydrogen atom of ammonia is replaced by R or Ar , we get RNH2 or ArNH2, a primary amine (1o). If two hydrogen atoms of ammonia or one hydrogen atom of R-NH2 are replaced by another alkyl/aryl(R’) group, what would you get? You get R-NHR’, secondary amine. The second alkyl/aryl group may be same or different. Replacement of another hydrogen atom by alkyl/aryl group leads to the formation of tertiary amine. Amines are said to be ‘simple’ when all the alkyl or aryl groups are the same, and ‘mixed’ when they are different. 9.39.39.39.39.3 NomenclatureNomenclatureNomenclatureNomenclatureNomenclature In common system, an aliphatic amine is named by prefixing alkyl group to amine, i.e., alkylamine as one word (e.g., methylamine). In secondary and tertiary amines, when two or more groups are the same, the prefix di or tri is appended before the name of alkyl group. In IUPAC system, primary amines are named as alkanamines. The name is derived by replacement of ‘e’ of alkane by the word amine. For example, CH3NH2 is named as methanamine. In case, more than one amino group is present at different positions in the parent chain, their positions are specified by giving numbers to the carbon atoms bearing –NH2 groups and suitable prefix such as di, tri, etc. is attached to the amine. The letter ‘e’ of the suffix of the hydrocarbon part is retained. For example, H2N–CH2–CH2–NH2 is named as ethane-1, 2-diamine. To name secondary and tertiary amines, we use locant N to designate substituent attached to a nitrogen atom. For example, CH3 NHCH2CH3 is Chemistry 260 Reprint 2025-26 named as N-methylethanamine and (CH3CH2)3N is named as N, N- diethylethanamine. More examples are given in Table 9.1. In arylamines, –NH2 group is directly attached to the benzene ring. C6H5NH2 is the simplest example of arylamine. In common system, it is known as aniline. It is also an accepted IUPAC name. While naming arylamines according to IUPAC system, suffix ‘e’ of arene is replaced by ‘amine’. Thus in IUPAC system, C6H5–NH2 is named as benzenamine. Common and IUPAC names of some alkylamines and arylamines are given in Table 9.1. Table 9.1: Nomenclature of Some Alkylamines and Arylamines Amine Common name IUPAC name CH3-–CH2–NH2 Ethylamine Ethanamine CH3–CH2–CH2–NH2 n-Propylamine Propan-1-amine Isopropylamine Propan-2-amine Ethylmethylamine N-Methylethanamine Trimethylamine N,N-Dimethylmethanamine N,N-Diethylbutylamine N,N-Diethylbutan-1-amine Allylamine Prop-2-en-1-amine Hexamethylenediamine Hexane-1,6-diamine Aniline Aniline or Benzenamine o-Toluidine 2-Methylaniline p-Bromoaniline 4-Bromobenzenamine or 4-Bromoaniline N,N-Dimethylaniline N,N-Dimethylbenzenamine 261 Amines Reprint 2025-26 IntextIntextIntextIntextIntext QuestionsQuestionsQuestionsQuestionsQuestions 9.1 Classify the following amines as primary, secondary or tertiary: 9.2 (i) Write structures of different isomeric amines corresponding to the molecular formula, C4H11N. (ii) Write IUPAC names of all the isomers. (iii) What type of isomerism is exhibited by different pairs of amines? 9.49.49.49.49.4 PreparationPreparationPreparationPreparationPreparation Amines are prepared by the following methods: ofofofofof AminesAminesAminesAminesAmines 1. Reduction of nitro compounds Nitro compounds are reduced to amines by passing hydrogen gas in the presence of finely divided nickel, palladium or platinum and also by reduction with metals in acidic medium. Nitroalkanes can also be similarly reduced to the corresponding alkanamines. Reduction with iron scrap and hydrochloric acid is preferred because FeCl2 formed gets hydrolysed to release hydrochloric acid during the reaction. Thus, only a small amount of hydrochloric acid is required to initiate the reaction. 2. Ammonolysis of alkyl halides You have read (Unit 6, Class XII) that the carbon - halogen bond in alkyl or benzyl halides can be easily cleaved by a nucleophile. Hence, an alkyl or benzyl halide on reaction with an ethanolic solution of ammonia undergoes nucleophilic substitution reaction in which the halogen atom is replaced by an amino (–NH2) group. This process of cleavage of the C–X bond by ammonia molecule is known as ammonolysis. The reaction is carried out in a sealed tube at 373 K. The primary amine thus obtained behaves as a nucleophile and can further react with alkyl halide to form secondary and tertiary amines, and finally quaternary ammonium salt. Chemistry 262 Reprint 2025-26 The free amine can be obtained from the ammonium salt by treatment with a strong base: Ammonolysis has the disadvantage of yielding a mixture of primary, secondary and tertiary amines and also a quaternary ammonium salt. However, primary amine is obtained as a major product by taking large excess of ammonia. The order of reactivity of halides with amines is RI > RBr >RCl. Write chemical equations for the following reactions: ExampleExampleExampleExampleExample 9.19.19.19.19.1 (i) Reaction of ethanolic NH3 with C2H5Cl. (ii) Ammonolysis of benzyl chloride and reaction of amine so formed with two moles of CH3Cl. SolutionSolutionSolutionSolutionSolution 3. Reduction of nitriles Nitriles on reduction with lithium aluminium hydride (LiAlH4) or catalytic hydrogenation produce primary amines. This reaction is used for ascent of amine series, i.e., for preparation of amines containing one carbon atom more than the starting amine. 4. Reduction of amides The amides on reduction with lithium aluminium hydride yield amines. 263 Amines Reprint 2025-26 5. Gabriel phthalimide synthesis Gabriel synthesis is used for the preparation of primary amines. Phthalimide on treatment with ethanolic potassium hydroxide forms potassium salt of phthalimide which on heating with alkyl halide followed by alkaline hydrolysis produces the corresponding primary amine. Aromatic primary amines cannot be prepared by this method because aryl halides do not undergo nucleophilic substitution with the anion formed by phthalimide. 6. Hoffmann bromamide degradation reaction Hoffmann developed a method for preparation of primary amines by treating an amide with bromine in an aqueous or ethanolic solution of sodium hydroxide. In this degradation reaction, migration of an alkyl or aryl group takes place from carbonyl carbon of the amide to the nitrogen atom. The amine so formed contains one carbon less than that present in the amide. ExampleExampleExampleExampleExample 9.29.29.29.29.2 Write chemical equations for the following conversions: (i) CH3–CH2–Cl into CH3–CH2–CH2–NH2 (ii) C6H5–CH2–Cl into C6H5–CH2–CH2–NH2 SolutionSolutionSolutionSolutionSolution Chemistry 264 Reprint 2025-26 Write structures and IUPAC names of ExampleExampleExampleExampleExample 9.39.39.39.39.3 (i) the amide which gives propanamine by Hoffmann bromamide reaction. (ii) the amine produced by the Hoffmann degradation of benzamide. SolutionSolutionSolutionSolutionSolution (i) Propanamine contains three carbons. Hence, the amide molecule must contain four carbon atoms. Structure and IUPAC name of the starting amide with four carbon atoms are given below: Butanamide (ii) Benzamide is an aromatic amide containing seven carbon atoms. Hence, the amine formed from benzamide is aromatic primary amine containing six carbon atoms. Aniline or benzenamine IntextIntextIntextIntextIntext QuestionQuestionQuestionQuestionQuestion 9.3 How will you convert (i) Benzene into aniline (ii) Benzene into N, N-dimethylaniline (iii) Cl–(CH2)4–Cl into hexan-1,6-diamine? 9.59.59.59.59.5 PhysicalPhysicalPhysicalPhysicalPhysical The lower aliphatic amines are gases with fishy odour. Primary amines with three or more carbon atoms are liquid and still higher ones are PropertiesPropertiesPropertiesPropertiesProperties solid. Aniline and other arylamines are usually colourless but get coloured on storage due to atmospheric oxidation. Lower aliphatic amines are soluble in water because they can form hydrogen bonds with water molecules. However, solubility decreases with increase in molar mass of amines due to increase in size of the hydrophobic alkyl part. Higher amines are essentially insoluble in water. Considering the electronegativity of nitrogen of amine and oxygen of alcohol as 3.0 and 3.5 respectively, you can predict the pattern of solubility of amines and alcohols in water. Out of butan-1-ol and butan-1-amine, which will be more soluble in water and why? Amines are soluble in organic solvents like alcohol, ether and benzene. You may remember that alcohols are more polar than amines and form stronger intermolecular hydrogen bonds than amines. Primary and secondary amines are engaged in intermolecular association due to hydrogen bonding between nitrogen of one and hydrogen of another molecule. This intermolecular association is more in primary amines than in secondary amines as there are two hydrogen atoms available for hydrogen bond formation in it. Tertiary amines do not have intermolecular association due to the absence of hydrogen atom available for hydrogen bond formation. Therefore, the order of boiling points of isomeric amines is as follows: 265 Amines Reprint 2025-26 Primary > Secondary > Tertiary Intermolecular hydrogen bonding in primary amines is shown in Fig. 9.2. Fig. 9.2 Intermolecular hydrogen bonding in primary amines Boiling points of amines, alcohols and alkanes of almost the same molar mass are shown in Table 9.2. Table 9.2: Comparison of Boiling Points of Amines, Alcohols and Alkanes of Similar Molecular Masses Sl. No. Compound Molar mass b.p./K 1. n-C4H9NH2 73 350.8 2. (C2H5)2NH 73 329.3 3. C2H5N(CH3)2 73 310.5 4. C2H5CH(CH3)2 72 300.8 5. n-C4H9OH 74 390.3 9.69.69.69.69.6 ChemicalChemicalChemicalChemicalChemical Difference in electronegativity between nitrogen and hydrogen atoms and the presence of unshared pair of electrons over the nitrogen atom makes ReactionsReactionsReactionsReactionsReactions amines reactive. The number of hydrogen atoms attached to nitrogen atom also decides the course of reaction of amines; that is why primary (–NH2), secondary N H and tertiary amines N differ in many reactions. Moreover, amines behave as nucleophiles due to the presence of unshared electron pair. Some of the reactions of amines are described below: 1. Basic character of amines Amines, being basic in nature, react with acids to form salts. Chemistry 266 Reprint 2025-26 Amine salts on treatment with a base like NaOH, regenerate the parent amine. Amine salts are soluble in water but insoluble in organic solvents like ether. This reaction is the basis for the separation of amines from the non basic organic compounds insoluble in water. The reaction of amines with mineral acids to form ammonium salts shows that these are basic in nature. Amines have an unshared pair of electrons on nitrogen atom due to which they behave as Lewis base. Basic character of amines can be better understood in terms of their Kb and pKb values as explained below:       R  NH 3   OH  K =  R  NH2  H2 O        R  N H3   O H  or K [H 2 O] =  R  NH 2        R  N H 3   O H  or K b =  R  NH2  pKb = –log Kb Larger the value of Kb or smaller the value of pKb, stronger is the base. The pKb values of few amines are given in Table 9.3. pKb value of ammonia is 4.75. Aliphatic amines are stronger bases than ammonia due to +I effect of alkyl groups leading to high electron density on the nitrogen atom. Their pKb values lie in the range of 3 to 4.22. On the other hand, aromatic amines are weaker bases than ammonia due to the electron withdrawing nature of the aryl group. Table 9.3: pKb Values of Amines in Aqueous Phase Name of amine pKb Methanamine 3.38 N-Methylmethanamine 3.27 N,N-Dimethylmethanamine 4.22 Ethanamine 3.29 N-Ethylethanamine 3.00 N,N-Diethylethanamine 3.25 Benzenamine 9.38 Phenylmethanamine 4.70 N-Methylaniline 9.30 N,N-Dimethylaniline 8.92 267 Amines Reprint 2025-26 You may find some discrepancies while trying to interpret the Kb values of amines on the basis of +I or –I effect of the substituents present in amines. Besides inductive effect, there are other effects like solvation effect, steric hinderance, etc., which affect the basic strength of amines. Just ponder over. You may get the answer in the following paragraphs. Structure-basicity relationship of amines Basicity of amines is related to their structure. Basic character of an amine depends upon the ease of formation of the cation by accepting a proton from the acid. The more stable the cation is relative to the amine, more basic is the amine. (a) Alkanamines versus ammonia Let us consider the reaction of an alkanamine and ammonia with a proton to compare their basicity. Due to the electron releasing nature of alkyl group, it pushes electrons towards nitrogen and thus makes the unshared electron pair more available for sharing with the proton of the acid. Moreover, the substituted ammonium ion formed from the amine gets stabilised due to dispersal of the positive charge by the +I effect of the alkyl group. Hence, alkylamines are stronger bases than ammonia. Thus, the basic nature of aliphatic amines should increase with increase in the number of alkyl groups. This trend is followed in the gaseous phase. The order of basicity of amines in the gaseous phase follows the expected order: tertiary amine > secondary amine > primary amine > NH3. The trend is not regular in the aqueous state as evident by their pKb values given in Table 9.3. In the aqueous phase, the substituted ammonium cations get stabilised not only by electron releasing effect of the alkyl group (+I) but also by solvation with water molecules. The greater the size of the ion, lesser will be the solvation and the less stabilised is the ion. The order of stability of ions are as follows: Decreasing order of extent of H-bonding in water and order of stability of ions by solvation. Chemistry 268 Reprint 2025-26 Greater is the stability of the substituted ammonium cation, stronger should be the corresponding amine as a base. Thus, the order of basicity of aliphatic amines should be: primary > secondary > tertiary, which is opposite to the inductive effect based order. Secondly, when the alkyl group is small, like –CH3 group, there is no steric hindrance to H-bonding. In case the alkyl group is bigger than CH3 group, there will be steric hinderance to H-bonding. Therefore, the change of nature of the alkyl group, e.g., from –CH3 to –C2H5 results in change of the order of basic strength. Thus, there is a subtle interplay of the inductive effect, solvation effect and steric hinderance of the alkyl group which decides the basic strength of alkyl amines in the aqueous state. The order of basic strength in case of methyl substituted amines and ethyl substituted amines in aqueous solution is as follows: (C2H5)2NH > (C2H5)3N > C2H5NH2 > NH3 (CH3)2NH > CH3NH2 > (CH3)3N > NH3 (b) Arylamines versus ammonia pKb value of aniline is quite high. Why is it so? It is because in aniline or other arylamines, the -NH2 group is attached directly to the benzene ring. It results in the unshared electron pair on nitrogen atom to be in conjugation with the benzene ring and thus making it less available for protonation. If you write different resonating structures of aniline, you will find that aniline is a resonance hybrid of the following five structures. On the other hand, anilinium ion obtained by accepting a proton can have only two resonating structures (kekule). We know that greater the number of resonating structures, greater is the stability. Thus you can infer that aniline (five resonating structures) is more stable than anilinium ion. Hence, the proton acceptability or the basic nature of aniline or other aromatic amines would be less than that of ammonia. In case of substituted aniline, it is observed that electron releasing groups like –OCH3, –CH3 increase basic strength whereas electron withdrawing groups like –NO2, –SO3H, –COOH, –X decrease it. 269 Amines Reprint 2025-26 ExampleExampleExampleExampleExample 9.49.49.49.49.4 Arrange the following in decreasing order of their basic strength: C6H5NH2, C2H5NH2, (C2H5)2NH, NH3 SolutionSolutionSolutionSolutionSolution The decreasing order of basic strength of the above amines and ammonia follows the following order: (C2H5)2NH > C2H5NH2 > NH3 > C6H5NH2 2. Alkylation Amines undergo alkylation on reaction with alkyl halides (refer Unit 6, Class XII). 3. Acylation Aliphatic and aromatic primary and secondary amines react with acid chlorides, anhydrides and esters by nucleophilic substitution reaction. This reaction is known as acylation. You can consider this reaction as the replacement of hydrogen atom of –NH2 or >N–H group by the acyl group. The products obtained by acylation reaction are known as amides. The reaction is carried out in the presence of a base stronger than the amine, like pyridine, which removes HCl so formed and shifts the equilibrium to the right hand side. Amines also react with benzoyl chloride (C6H5COCl). This reaction is known as benzoylation. CH 3 NH 2  C 6 H 5 CO Cl  CH 3 NH CO C 6 H 5  H Cl Methanamine Benzoyl chloride N  Methylbenzamide What do you think is the product of the reaction of amines with carboxylic acids ? They form salts with amines at room temperature. Chemistry 270 Reprint 2025-26 4. Carbylamine reaction Aliphatic and aromatic primary amines on heating with chloroform and ethanolic potassium hydroxide form isocyanides or carbylamines which are foul smelling substances. Secondary and tertiary amines do not show this reaction. This reaction is known as carbylamine reaction or isocyanide test and is used as a test for primary amines. 5. Reaction with nitrous acid Three classes of amines react differently with nitrous acid which is prepared in situ from a mineral acid and sodium nitrite. (a) Primary aliphatic amines react with nitrous acid to form aliphatic diazonium salts which being unstable, liberate nitrogen gas quantitatively and alcohols. Quantitative evolution of nitrogen is used in estimation of amino acids and proteins. (b) Aromatic amines react with nitrous acid at low temperatures (273-278 K) to form diazonium salts, a very important class of compounds used for synthesis of a variety of aromatic compounds discussed in Section 9.7. Secondary and tertiary amines react with nitrous acid in a different manner. 6. Reaction with arylsulphonyl chloride Benzenesulphonyl chloride (C6H5SO2Cl), which is also known as Hinsberg’s reagent, reacts with primary and secondary amines to form sulphonamides. (a) The reaction of benzenesulphonyl chloride with primary amine yields N-ethylbenzenesulphonyl amide. The hydrogen attached to nitrogen in sulphonamide is strongly acidic due to the presence of strong electron withdrawing sulphonyl group. Hence, it is soluble in alkali. (b) In the reaction with secondary amine, N,N-diethyl- benzenesulphonamide is formed. O O S Cl + H N C 2H 5 S N C 2H 5 + HCl O C 2H 5 O C 2H 5 N,N-Diethylbenzenesulphonamide 271 Amines Reprint 2025-26 Since N, N-diethylbenzene sulphonamide does not contain any hydrogen atom attached to nitrogen atom, it is not acidic and hence insoluble in alkali. (c) Tertiary amines do not react with benzenesulphonyl chloride. This property of amines reacting with benzenesulphonyl chloride in a different manner is used for the distinction of primary, secondary and tertiary amines and also for the separation of a mixture of amines. However, these days benzenesulphonyl chloride is replaced by p-toluenesulphonyl chloride. 7. Electrophilic substitution You have read earlier that aniline is a resonance hybrid of five structures. Where do you find the maximum electron density in these structures? Ortho- and para-positions to the –NH2 group become centres of high electron density. Thus –NH2 group is ortho and para directing and a powerful activating group. (a) Bromination: Aniline reacts with bromine water at room temperature to give a white precipitate of 2,4,6-tribromoaniline. The main problem encountered during electrophilic substitution reactions of aromatic amines is that of their very high reactivity. Substitution tends to occur at ortho- and para-positions. If we have to prepare monosubstituted aniline derivative, how can the activating effect of –NH2 group be controlled ? This can be done by protecting the -NH2 group by acetylation with acetic anhydride, then carrying out the desired substitution followed by hydrolysis of the substituted amide to the substituted amine. The lone pair of electrons on nitrogen of acetanilide interacts with oxygen atom due to resonance as shown below: Chemistry 272 Reprint 2025-26 Hence, the lone pair of electrons on nitrogen is less available for donation to benzene ring by resonance. Therefore, activating effect of –NHCOCH3 group is less than that of amino group. (b) Nitration: Direct nitration of aniline yields tarry oxidation products in addition to the nitro derivatives. Moreover, in the strongly acidic medium, aniline is protonated to form the anilinium ion which is meta directing. That is why besides the ortho and para derivatives, significant amount of meta derivative is also formed. However, by protecting the –NH2 group by acetylation reaction with acetic anhydride, the nitration reaction can be controlled and the p-nitro derivative can be obtained as the major product. (c) Sulphonation: Aniline reacts with concentrated sulphuric acid to form anilinium hydrogensulphate which on heating with sulphuric acid at 453-473K produces p-aminobenzene sulphonic acid, commonly known as sulphanilic acid, as the major product. Aniline does not undergo Friedel-Crafts reaction (alkylation and acetylation) due to salt formation with aluminium chloride, the Lewis acid, which is used as a catalyst. Due to this, nitrogen of aniline acquires positive charge and hence acts as a strong deactivating group for further reaction. 273 Amines Reprint 2025-26 IntextIntextIntextIntextIntext QuestionsQuestionsQuestionsQuestionsQuestions 9.4 Arrange the following in increasing order of their basic strength: (i) C2H5NH2, C6H5NH2, NH3, C6H5CH2NH2 and (C2H5)2NH (ii) C2H5NH2, (C2H5)2NH, (C2H5)3N, C6H5NH2 (iii) CH3NH2, (CH3)2NH, (CH3)3N, C6H5NH2, C6H5CH2NH2. 9.5 Complete the following acid-base reactions and name the products: (i) CH3CH2CH2NH2 + HCl ® (ii) (C2H5)3N + HCl ® 9.6 Write reactions of the final alkylation product of aniline with excess of methyl iodide in the presence of sodium carbonate solution. 9.7 Write chemical reaction of aniline with benzoyl chloride and write the name of the product obtained. 9.8 Write structures of different isomers corresponding to the molecular formula, C3H9N. Write IUPAC names of the isomers which will liberate nitrogen gas on treatment with nitrous acid. II. DIAZONIUM SALTS  – The diazonium salts have the general formula R N 2 X where R stands – – – for an aryl group and X ion may be Cl Br, HSO,4 BF,4 etc. They are named by suffixing diazonium to the name of the parent hydrocarbon from which they are formed, followed by the name of anion such as  chloride, hydrogensulphate, etc. The N 2 group is called diazonium  – group. For example, C 6 H 5 N 2 Cl is named as benzenediazonium chloride and C6H5N2+HSO4– is known as benzenediazonium hydrogensulphate. Primary aliphatic amines form highly unstable alkyldiazonium salts (refer to Section 9.6). Primary aromatic amines form arenediazonium salts which are stable for a short time in solution at low temperatures (273-278 K). The stability of arenediazonium ion is explained on the basis of resonance. 9.79.79.79.79.7 MethodMethodMethodMethodMethod ofofofofof Benzenediazonium chloride is prepared by the reaction of aniline with PreparationPreparationPreparationPreparationPreparation nitrous acid at 273-278K. Nitrous acid is produced in the reaction mixture by the reaction of sodium nitrite with hydrochloric acid. The ofofofofof DiazoniunDiazoniunDiazoniunDiazoniunDiazoniun conversion of primary aromatic amines into diazonium salts is known SaltsSaltsSaltsSaltsSalts as diazotisation. Due to its instability, the diazonium salt is not generally stored and is used immediately after its preparation.  – C 6 H 5 N H 2  NaNO 2  2H Cl 273  278K C 6 H 5 N 2 Cl  Na Cl  2H 2 O Chemistry 274 Reprint 2025-26 9.89.89.89.89.8 PhysicalPhysicalPhysicalPhysicalPhysical Benzenediazonium chloride is a colourless crystalline solid. It is readily soluble in water and is stable in cold but reacts with water when PropertiesPropertiesPropertiesPropertiesProperties warmed. It decomposes easily in the dry state. Benzenediazonium fluoroborate is water insoluble and stable at room temperature. 9.99.99.99.99.9 ChemicalChemicalChemicalChemicalChemical The reactions of diazonium salts can be broadly divided into two ReactionsReactionsReactionsReactionsReactions categories, namely (A) reactions involving displacement of nitrogen and (B) reactions involving retention of diazo group. A. Reactions involving displacement of nitrogen Diazonium group being a very good leaving group, is substituted by other groups such as Cl–, Br –, I – , CN – and OH– which displace nitrogen from the aromatic ring. The nitrogen formed escapes from the reaction mixture as a gas. 1. Replacement by halide or cyanide ion: The Cl –, Br– and CN – nucleophiles can easily be introduced in the benzene ring in the presence of Cu(I) ion. This reaction is called Sandmeyer reaction. 2 2 2 2 Alternatively, chlorine or bromine can also be introduced in the benzene ring by treating the diazonium salt solution with corresponding halogen acid in the presence of copper powder. This is referred as Gatterman reaction. The yield in Sandmeyer reaction is found to be better than Gattermann reaction. 2. Replacement by iodide ion: Iodine is not easily introduced into the benzene ring directly, but, when the diazonium salt solution is treated with potassium iodide, iodobenzene is formed. 3. Replacement by fluoride ion: When arenediazonium chloride is treated with fluoroboric acid, arene diazonium fluoroborate is precipitated which on heating decomposes to yield aryl fluoride. 4. Replacement by H: Certain mild reducing agents like hypophosphorous acid (phosphinic acid) or ethanol reduce diazonium salts to arenes and themselves get oxidised to phosphorous acid and ethanal, respectively. 275 Amines Reprint 2025-26 5. Replacement by hydroxyl group: If the temperature of the diazonium salt solution is allowed to rise upto 283 K, the salt gets hydrolysed to phenol. 6. Replacement by –NO2 group: When diazonium fluoroborate is heated with aqueous sodium nitrite solution in the presence of copper, the diazonium group is replaced by –NO2 group. B. Reactions involving retention of diazo group coupling reactions The azo products obtained have an extended conjugate system having both the aromatic rings joined through the –N=N– bond. These compounds are often coloured and are used as dyes. Benzene diazonium chloride reacts with phenol in which the phenol molecule at its para position is coupled with the diazonium salt to form p-hydroxyazobenzene. This type of reaction is known as coupling reaction. Similarly the reaction of diazonium salt with aniline yields p-aminoazobenzene. This is an example of electrophilic substitution reaction. 9.10.10.10.10.10 ImportanceImportanceImportanceImportanceImportance From the above reactions, it is clear that the diazonium salts are very ofofofofof DiazoniumDiazoniumDiazoniumDiazoniumDiazonium good intermediates for the introduction of –F, –Cl, –Br, –I, –CN, –OH, –NO2 groups into the aromatic ring. SaltsSaltsSaltsSaltsSalts ininininin Aryl fluorides and iodides cannot be prepared by direct halogenation. SynthesisSynthesisSynthesisSynthesisSynthesis The cyano group cannot be introduced by nucleophilic substitution of ofofofofof AromaticAromaticAromaticAromaticAromatic chlorine in chlorobenzene but cyanobenzene can be easily obtained from diazonium salt. CompoundsCompoundsCompoundsCompoundsCompounds Thus, the replacement of diazo group by other groups is helpful in Chemistry 276 Reprint 2025-26 preparing those substituted aromatic compounds which cannot be prepared by direct substitution in benzene or substituted benzene. How will you convert 4-nitrotoluene to 2-bromobenzoic acid ? ExampleExampleExampleExampleExample 9.59.59.59.59.5 SolutionSolutionSolutionSolutionSolution IntextIntextIntextIntextIntext QuestionQuestionQuestionQuestionQuestion 9.9 Convert (i) 3-Methylaniline into 3-nitrotoluene. (ii) Aniline into 1,3,5 - tribromobenzene. SummarySummarySummarySummarySummary Amines can be considered as derivatives of ammonia obtained by replacement of hydrogen atoms with alkyl or aryl groups. Replacement of one hydrogen atom of ammonia gives rise to structure of the type R-NH2, known as primary amine. Secondary amines are characterised by the structure R2NH or R-NHR¢ and tertiary amines by R3N, RNR¢R¢¢¢¢¢¢¢¢¢¢ or R2NR¢.¢.¢.¢.¢. Secondary and tertiary amines are known as simple amines if the alkyl or aryl groups are the same and mixed amines if the groups are different. Like ammonia, all the three types of amines have one unshared electron pair on nitrogen atom due to which they behave as Lewis bases. Amines are usually formed from nitro compounds, halides, amides, imides, etc. They exhibit hydrogen bonding which influence their physical properties. In alkylamines, a combination of electron releasing, steric and H-bonding factors influence the stability of the substituted ammonium cations in protic polar solvents and thus affect the basic nature of amines. Alkyl amines are found to be stronger bases than ammonia. In aromatic amines, electron releasing and withdrawing groups, respectively increase and decrease their basic character. Aniline is a weaker base 277 Amines Reprint 2025-26 than ammonia. Reactions of amines are governed by availability of the unshared pair of electrons on nitrogen. Influence of the number of hydrogen atoms at nitrogen atom on the type of reactions and nature of products is responsible for identification and distinction between primary, secondary and tertiary amines. p-Toluenesulphonyl chloride is used for the identification of primary, secondary and tertiary amines. Presence of amino group in aromatic ring enhances reactivity of the aromatic amines. Reactivity of aromatic amines can be controlled by acylation process, i.e., by treating with acetyl chloride or acetic anhydride. Tertiary amines like trimethylamine are used as insect attractants. Aryldiazonium salts, usually obtained from arylamines, undergo replacement of the diazonium group with a variety of nucleophiles to provide advantageous methods for producing aryl halides, cyanides, phenols and arenes by reductive removal of the diazo group. Coupling reaction of aryldiazonium salts with phenols or arylamines give rise to the formation of azo dyes. Exercises 9.1 Write IUPAC names of the following compounds and classify them into primary, secondary and tertiary amines. (i) (CH3)2CHNH2 (ii) CH3(CH2)2NH2 (iii) CH3NHCH(CH3)2 (iv) (CH3)3CNH2 (v) C6H5NHCH3 (vi) (CH3CH2)2NCH3 (vii) m–BrC6H4NH2 9.2 Give one chemical test to distinguish between the following pairs of compounds. (i) Methylamine and dimethylamine (ii) Secondary and tertiary amines (iii) Ethylamine and aniline (iv) Aniline and benzylamine (v) Aniline and N-methylaniline. 9.3 Account for the following: (i) pKb of aniline is more than that of methylamine. (ii) Ethylamine is soluble in water whereas aniline is not. (iii) Methylamine in water reacts with ferric chloride to precipitate hydrated ferric oxide. (iv) Although amino group is o– and p– directing in aromatic electrophilic substitution reactions, aniline on nitration gives a substantial amount of m-nitroaniline. (v) Aniline does not undergo Friedel-Crafts reaction. (vi) Diazonium salts of aromatic amines are more stable than those of aliphatic amines. (vii) Gabriel phthalimide synthesis is preferred for synthesising primary amines. 9.4 Arrange the following: (i) In decreasing order of the pKb values: C2H5NH2, C6H5NHCH3, (C2H5)2NH and C6H5NH2 (ii) In increasing order of basic strength: C6H5NH2, C6H5N(CH3)2, (C2H5)2NH and CH3NH2 (iii) In increasing order of basic strength: (a) Aniline, p-nitroaniline and p-toluidine Chemistry 278 Reprint 2025-26 (b) C6H5NH2, C6H5NHCH3, C6H5CH2NH2. (iv) In decreasing order of basic strength in gas phase: C2H5NH2, (C2H5)2NH, (C2H5)3N and NH3 (v) In increasing order of boiling point: C2H5OH, (CH3)2NH, C2H5NH2 (vi) In increasing order of solubility in water: C6H5NH2, (C2H5)2NH, C2H5NH2.