8.4 Classification of Organic (homocyclic). Compounds The existing large number of organic compounds and their ever -increasing numbers has made it necessary to classify them on the basis of their structures. Organic Cyclopropane Cyclohexane Cyclohexene compounds are broadly classified as follows: Sometimes atoms other than carbon are also present in the ring (heterocylic). Tetrahydrofuran given below is an example of this type of compound: Tetrahydrofuran These exhibit some of the properties similar to those of aliphatic compounds. (b) Aromatic compounds Aromatic compounds are special types of compounds. You will learn about these compounds in detail in Unit 9. These include benzene and other related ring compounds (benzenoid). Like alicyclic compounds, aromatic comounds may also have hetero atom in the ring. Such compounds are called I. Acyclic or open chain compounds hetrocyclic aromatic compounds. Some of the examples of various types of aromatic These compounds are also called as aliphatic compounds are: compounds and consist of straight or branched chain compounds, for example: Benzenoid aromatic compounds CH3CH3 Ethane Isobutane Benzene Aniline Naphthalene Non-benzenoid compound Acetaldehyde Acetic acid II Cyclic or closed chain or ring compounds (a) Alicyclic compounds Tropone Alicyclic (aliphatic cyclic) compounds contain carbon atoms joined in the form of a ring Reprint 2025-26 262 chemistry Heterocyclic aromatic compounds so because it is found in citrus fruits and the acid found in red ant is named formic acid since the Latin word for ant is formica. These names are traditional and are considered as trivial or common names. Some common Furan Thiophene Pyridine names are followed even today. For example, Organic compounds can also be classified Buckminsterfullerene is a common name on the basis of functional groups, into families given to the newly discovered C60 cluster (a or homologous series. form of carbon) noting its structural similarity to the geodesic domes popularised by the8.4.1 Functional Group famous architect R. Buckminster Fuller. The functional group is an atom or a group Common names are useful and in many of atoms joined to the carbon chain which is cases indispensable, particularly when the responsible for the characteristic chemical alternative systematic names are lengthy and properties of the organic compounds. The complicated. Common names of some organic examples are hydroxyl group (–OH), aldehyde compounds are given in Table 8.1. group (–CHO) and carboxylic acid group (– COOH) etc. Table 8.1 Common or Trivial Names of Some Organic Compounds 8.4.2 Homologous Series A group or a series of organic compounds each containing a characteristic functional group forms a homologous series and the members of the series are called homologues. The members of a homologous series can be represented by general molecular formula and the successive members differ from each other in molecular formula by a –Ch2 unit. There are a number of homologous series of organic compounds. Some of these are alkanes, alkenes, alkynes, haloalkanes, alkanols, alkanals, alkanones, alkanoic acids, amines etc. It is also possible that a compound contains two or more identical or different functional groups. This gives rise to polyfunctional compounds.

8.2 Name the following compounds according to IUPAC system of nomenclature: (i) CH3CH(CH3)CH2CH2CHO (ii) CH3CH2COCH(C2H5)CH2CH2Cl (iii) CH3CH=CHCHO (iv) CH3COCH2COCH3 (v) CH3CH(CH3)CH2C(CH3)2COCH3 (vi) (CH3)3CCH2COOH (vii) OHCC6H4CHO-p

6.10 ACIDS, BASES AND SALTS a very high dielectric constant of 80. Thus, Acids, bases and salts find widespread when sodium chloride is dissolved in water, occurrence in nature. Hydrochloric acid the electrostatic interactions are reduced by present in the gastric juice is secreted by the a factor of 80 and this facilitates the ions to lining of our stomach in a significant amount move freely in the solution. Also, they are of 1.2-1.5 L/day and is essential for digestive well-separated due to hydration with water processes. Acetic acid is known to be the main molecules. constituent of vinegar. Lemon and orange juices contain citric and ascorbic acids, and tartaric acid is found in tamarind paste. As most of the acids taste sour, the word “acid” has been derived from a latin word “acidus” meaning sour. Acids are known to turn blue litmus paper into red and liberate dihydrogen on reacting with some metals. Similarly, bases are known to turn red litmus paper blue, taste bitter and feel soapy. A common example of a base is washing soda used for washing purposes. When acids and bases are mixed in the right proportion they react with each Fig.6.10 Dissolution of sodium chloride in water.other to give salts. Some commonly known Na+ and Cl– ions are stablised by their examples of salts are sodium chloride, barium hydration with polar water molecules. sulphate, sodium nitrate. Sodium chloride (common salt) is an important component of Comparing, the ionization of hydrochloric our diet and is formed by reaction between acid with that of acetic acid in water we find hydrochloric acid and sodium hydroxide. It that though both of them are polar covalent Faraday was born near London into a family of very limited means. At the age of 14 he was an apprentice to a kind bookbinder who allowed Faraday to read the books he was binding. Through a fortunate chance he became laboratory assistant to Davy, and during 1813-4, Faraday accompanied him to the Continent. During this trip he gained much from the experience of coming into contact with many of the leading scientists of the time. In 1825, he succeeded Davy as Director of the Royal Institution laboratories, and in 1833 he also became the first Fullerian Professor of Chemistry. Faraday’s first important work was on analytical chemistry. After 1821 Michael Faraday much of his work was on electricity and magnetism and different electromagnetic (1791–1867) phenomena. His ideas have led to the establishment of modern field theory. He discovered his two laws of electrolysis in 1834. Faraday was a very modest and kind hearted person. He declined all honours and avoided scientific controversies. He preferred to work alone and never had any assistant. He disseminated science in a variety of ways including his Friday evening discourses, which he founded at the Royal Institution. He has been very famous for his Christmas lecture on the ‘Chemical History of a Candle’. He published nearly 450 scientific papers. Reprint 2025-26 190 chemistry molecules, former is completely ionized into its constituent ions, while the latter is only Hydronium and Hydroxyl Ions partially ionized (< 5%). The extent to which Hydrogen ion by itself is a bare proton with very ionization occurs depends upon the strength small size (~10–15 m radius) and intense electric of the bond and the extent of solvation field, binds itself with the water molecule at of ions produced. The terms dissociation one of the two available lone pairs on it giving and ionization have earlier been used with H3O+. This species has been detected in many different meaning. Dissociation refers to the compounds (e.g., H3O+Cl–) in the solid state. In process of separation of ions in water already aqueous solution the hydronium ion is further existing as such in the solid state of the solute, hydrated to give species like H5O2+, H7O3 + and as in sodium chloride. On the other hand, H9O4+. Similarly the hydroxyl ion is hydrated to give several ionic species like H3O2–, H5O3–ionization corresponds to a process in which – and H7O4 etc.a neutral molecule splits into charged ions in the solution. Here, we shall not distinguish between the two and use the two terms interchangeably. 6.10.1 Arrhenius Concept of Acids and Bases According to Arrhenius theory, acids are H9O4+ substances that dissociates in water to give hydrogen ions H+(aq) and bases are 6.10.2 The Brönsted-Lowry Acids and substances that produce hydroxyl ions Bases OH –(aq). The ionization of an acid HX (aq) can The Danish chemist, Johannes Brönsted and be represented by the following equations: the English chemist, Thomas M. Lowry gave HX (aq) → H+(aq) + X– (aq) a more general definition of acids and bases. or According to Brönsted-Lowry theory, acid HX(aq) + H2O(l) → H3O+(aq) + X –(aq) is a substance that is capable of donating a hydrogen ion H+ and bases are substances A bare proton, H+ is very reactive and capable of accepting a hydrogen ion, H+. Incannot exist freely in aqueous solutions. short, acids are proton donors and bases areThus, it bonds to the oxygen atom of a solvent proton acceptors.water molecule to give trigonal pyramidal Consider the example of dissolution of NH3hydronium ion, H3O+ {[H (H2O)]+} (see box). In in H2O represented by the following equation:this chapter we shall use H+(aq) and H3O+(aq) interchangeably to mean the same i.e., a hydrated proton. Similarly, a base molecule like MOH ionizes in aqueous solution according to the equation: MOH(aq) → M+(aq) + OH–(aq) The hydroxyl ion also exists in the hydrated form in the aqueous solution. Arrhenius concept of acid and base, however, suffers The basic solution is formed due to the from the limitation of being applicable only to presence of hydroxyl ions. In this reaction, aqueous solutions and also, does not account water molecule acts as proton donor and for the basicity of substances like, ammonia ammonia molecule acts as proton acceptor which do not possess a hydroxyl group. and are thus, called Lowry-Brönsted acid and Reprint 2025-26 EQUILIBRIUM 191 Arrhenius was born near Uppsala, Sweden. He presented his thesis, on the conductivities of electrolyte solutions, to the University of Uppsala in 1884. For the next five years he travelled extensively and visited a number of research centers in Europe. In 1895 he was appointed professor of physics at the newly formed University of Stockholm, serving its rector from 1897 to 1902. From 1905 until his death he was Director of physical chemistry at the Nobel Institute in Stockholm. He continued to work for many years on electrolytic solutions. In 1899 he discussed the temperature dependence of reaction rates on the basis of an equation, now usually known as Arrhenius equation. He worked in a variety of fields, and made important contributions to Svante Arrhenius immunochemistry, cosmology, the origin of life, and the causes of ice age. He was (1859-1927) the first to discuss the ‘green house effect’ calling by that name. He received Nobel Prize in Chemistry in 1903 for his theory of electrolytic dissociation and its use in the development of chemistry. base, respectively. In the reverse reaction, in case of ammonia it acts as an acid by H+ is transferred from NH4+ to OH–. In this donating a proton. case, NH4+ acts as a Bronsted acid while Problem 6.12OH– acted as a Brönsted base. The acid-base What will be the conjugate bases for thepair that differs only by one proton is called following Brönsted acids: HF, H2SO4 anda conjugate acid-base pair. Therefore, OH– – HCO3 ?is called the conjugate base of an acid H2O and NH4+ is called conjugate acid of the base Solution NH3. If Brönsted acid is a strong acid then The conjugate bases should have one its conjugate base is a weak base and vice- proton less in each case and therefore the versa. It may be noted that conjugate acid corresponding conjugate bases are: F –, has one extra proton and each conjugate base HSO4– and CO32– respectively. has one less proton. Problem 6.13 Consider the example of ionization of Write the conjugate acids for the following hydrochloric acid in water. HCl(aq) acts as Brönsted bases: NH2–, NH3 and HCOO–. an acid by donating a proton to H2O molecule which acts as a base. Solution The conjugate acid should have one extra proton in each case and therefore the corresponding conjugate acids are: NH3, NH4+ and HCOOH respectively. Problem 6.14 The species: H2O, HCO3–, HSO4– and NH3 can act both as Bronsted acids and bases. For each case give the corresponding conjugate It can be seen in the above equation, that acid and conjugate base. water acts as a base because it accepts the Solution proton. The species H3O+ is produced when The answer is given in the following Table:water accepts a proton from HCl. Therefore, Cl– is a conjugate base of HCl and HCl is the Species Conjugate Conjugate conjugate acid of base Cl –. Similarly, H2O is acid base a conjugate base of an acid H3O+ and H3O+ is H2O H3O+ OH–a conjugate acid of base H2O. – 2– HCO3 H2CO3 CO3 It is interesting to observe the dual role – 2– HSO4 H2SO4 SO4of water as an acid and a base. In case of + – reaction with HCl water acts as a base while NH3 NH4 NH2 Reprint 2025-26 192 chemistry 6.10.3 Lewis Acids and Bases (HClO4), hydrochloric acid (HCl), hydrobromic G.N. Lewis in 1923 defined an acid as a acid (HBr), hyrdoiodic acid (HI), nitric acid species which accepts electron pair and base (HNO3) and sulphuric acid (H2SO4) are termed which donates an electron pair. As far as bases strong because they are almost completely are concerned, there is not much difference dissociated into their constituent ions in an between Brönsted-Lowry and Lewis concepts, aqueous medium, thereby acting as proton as the base provides a lone pair in both the (H+) donors. Similarly, strong bases like cases. However, in Lewis concept many lithium hydroxide (LiOH), sodium hydroxide acids do not have proton. A typical example (NaOH), potassium hydroxide (KOH), caesium is reaction of electron deficient species BF3 hydroxide (CsOH) and barium hydroxide with NH3. Ba(OH)2 are almost completely dissociated into ions in an aqueous medium giving BF3 does not have a proton but still acts hydroxyl ions, OH–. According to Arrhenius as an acid and reacts with NH3 by accepting concept they are strong acids and bases asits lone pair of electrons. The reaction can be they are able to completely dissociate andrepresented by, produce H3O+ and OH– ions respectively in BF3 + :NH3 → BF3:NH3 the medium. Alternatively, the strength of an Electron deficient species like AlCl3, Co3+, acid or base may also be gauged in terms of Mg2+, etc. can act as Lewis acids while species Brönsted-Lowry concept of acids and bases, like H2O, NH3, OH– etc. which can donate a wherein a strong acid means a good proton pair of electrons, can act as Lewis bases. donor and a strong base implies a good proton acceptor. Consider, the acid-base dissociation Problem 6.15 equilibrium of a weak acid HA, Classify the following species into Lewis HA(aq) + H2O(l)   H3O+(aq) + A–(aq) acids and Lewis bases and show how these conjugate conjugate act as such: acid base acid base (a) HO– (b) F – (c) H+ (d) BCl3 In section 6.10.2 we saw that acid (or Solution base) dissociation equilibrium is dynamic involving a transfer of proton in forward and (a) Hydroxyl ion is a Lewis base as it can reverse directions. Now, the question arises donate an electron lone pair (:OH– ). that if the equilibrium is dynamic then with (b) Flouride ion acts as a Lewis base passage of time which direction is favoured? as it can donate any one of its four What is the driving force behind it? In order electron lone pairs. to answer these questions we shall deal (c) A proton is a Lewis acid as it can into the issue of comparing the strengths accept a lone pair of electrons from of the two acids (or bases) involved in the bases like hydroxyl ion and fluoride dissociation equilibrium. Consider the two ion. acids HA and H3O+ present in the above mentioned acid-dissociation equilibrium. (d) BCl3 acts as a Lewis acid as it can We have to see which amongst them is a accept a lone pair of electrons from stronger proton donor. Whichever exceeds species like ammonia or amine in its tendency of donating a proton over the molecules. other shall be termed as the stronger acid