9.4 Alkynes 1 2 3 4 5 Like alkenes, alkynes are also unsaturated II. H3C–C≡ C– CH2– CH3 Pent–2-yne hydrocarbons. They contain at least one triple 4 3 2 1 bond between two carbon atoms. The number III. H3C–CH–C≡ CH 3-Methyl but–1-yne |of hydrogen atoms is still less in alkynes as CH3compared to alkenes or alkanes. Their general Structures I and II are position isomers formula is CnH2n–2. and structures I and III or II and III are chain The first stable member of alkyne series isomers. is ethyne which is popularly known as acetylene. Acetylene is used for arc welding Problem 9.13 purposes in the form of oxyacetylene flame Write structures of different isomers obtained by mixing acetylene with oxygen corresponding to the 5 th member of gas. Alkynes are starting materials for a large alkyne series. Also write IUPAC names of number of organic compounds. Hence, it all the isomers. What type of isomerism is interesting to study this class of organic is exhibited by different pairs of isomers? compounds. Solution 9.4.1 Nomenclature and Isomerism th 5 member of alkyne has the molecular In common system, alkynes are named as formula C6H10. The possible isomers are: derivatives of acetylene. In IUPAC system, they Table 9.2 Common and IUPAC Names of Alkynes (CnH2n–2) Value of n Formula Structure Common name IUPAC name 2 C2H2 H-C≡CH Acetylene Ethyne 3 C3H4 CH3-C≡CH Methylacetylene Propyne 4 C4H6 CH3CH2-C≡CH Ethylacetylene But-1-yne 4 C4H6 CH3-C≡C-CH3 Dimethylacetylene But-2-yne Reprint 2025-26 Hydrocarbons 315 (a) HC ≡ C – CH2 – CH2 – CH2 – CH3 Hex-1-yne (b) CH3 – C ≡ C – CH2 – CH2 – CH3 Hex-2-yne (c) CH3 – CH2 – C ≡ C – CH2– CH3 Hex-3-yne 3-Methylpent-1-yne 4-Methylpent-1-yne 4-Methylpent-2-yne Fig. 9.6 Orbital picture of ethyne showing (a) sigma overlaps (b) pi overlaps. orbitals of the other carbon atom, which undergo lateral or sideways overlapping to 3,3-Dimethylbut-1-yne form two pi (π) bonds between two carbon atoms. Thus ethyne molecule consists of one Position and chain isomerism shown by C–C σ bond, two C–H σ bonds and two C–C different pairs. π bonds. The strength of C≡C bond (bond enthalpy 823 kJ mol -1) is more than those 9.4.2 Structure of Triple Bond of C=C bond (bond enthalpy 681 kJ mol –1) Ethyne is the simplest molecule of alkyne and C–C bond (bond enthalpy 348 kJ mol–1). series. Structure of ethyne is shown in The C≡C bond length is shorter (120 pm) Fig. 9.6. than those of C=C (133 pm) and C–C (154 pm). Electron cloud between two carbon Each carbon atom of ethyne has two sp atoms is cylindrically symmetrical about thehybridised orbitals. Carbon-carbon sigma (σ) internuclear axis. Thus, ethyne is a linear bond is obtained by the head-on overlapping molecule. of the two sp hybridised orbitals of the two carbon atoms. The remaining sp hybridised 9.4.3 Preparation orbital of each carbon atom undergoes 1. From calcium carbide: On industrial overlapping along the internuclear axis with scale, ethyne is prepared by treating the 1s orbital of each of the two hydrogen calcium carbide with water. Calcium atoms forming two C-H sigma bonds. carbide is prepared by heating quick lime H-C-C bond angle is of 180°. Each carbon with coke. Quick lime can be obtained byhas two unhybridised p orbitals which are heating limestone as shown in the followingperpendicular to each other as well as to the reactions:plane of the C-C sigma bond. The 2p orbitals of one carbon atom are parallel to the 2p CaCO3 ∆ CaO + O2 (9.55) Reprint 2025-26 316 chemistry CaO + 3C CaC2 + CO (9.56) the sp hybridised carbon2 atoms whereas they are attached to sp hybridised carbon Calcium 3 atoms in ethene and sp hybridised carbons carbide in ethane. Due to the maximum percentage of CaC2 + 2H2O Ca(OH)2 + C2H2 (9.57) s character (50%), the sp hybridised orbitals of carbon atoms in ethyne molecules have2. From vicinal dihalides : Vicinal dihalides highest electronegativity; hence, these attract on treatment with alcoholic potassium the shared electron pair of the C-H bond of hydroxide undergo dehydrohalogenation. ethyne to a greater extent than that of the One molecule of hydrogen halide is 2 sp hybridised orbitals of carbon in ethene eliminated to form alkenyl halide which 3 and the sp hybridised orbital of carbon in on treatment with sodamide gives alkyne. ethane. Thus in ethyne, hydrogen atoms can be liberated as protons more easily as compared to ethene and ethane. Hence, hydrogen atoms of ethyne attached to triply bonded carbon atom are acidic in nature. You may note that the hydrogen atoms attached to the triply bonded carbons are acidic but not all the hydrogen atoms of alkynes. HC ≡ CH + Na → HC ≡ C–Na++ 1/2 H2 9.4.4 Properties Monosodium Physical properties ethynide Physical properties of alkynes follow the same (9.59) trend of alkenes and alkanes. First three HC ≡ C– Na + Na → Na+ C–Na+ ≡ C–Na++ 1/2 H2members are gases, the next eight are liquids and the higher ones are solids. All alkynes Disodium ethynide are colourless. Ethyene has characteristic (9.60)odour. Other members are odourless. Alkynes are weakly polar in nature. They are lighter CH3 – C ≡ C – H + Na+ NH–2 than water and immiscible with water but ↓ soluble in organic solvents like ethers, carbon CH3 – C ≡ C– Na+ + NH3 tetrachloride and benzene. Their melting Sodium propynide (9.61) point, boiling point and density increase with These reactions are not shown by alkenesincrease in molar mass. and alkanes, hence used for distinction Chemical properties between alkynes, alkenes and alkanes. What Alkynes show acidic nature, addition reactions about the above reactions with but-1-yne and and polymerisation reactions as follows : but-2-yne ? Alkanes, alkenes and alkynes A. Acidic character of alkyne: Sodium follow the following trend in their acidic metal and sodamide (NaNH2) are strong behaviour : bases. They react with ethyne to form sodium i) CH ≡ CH > H2C – CH2 > CH3 –CH3acetylide with the liberation of dihydrogen gas. These reactions have not been observed ii) HC ≡ CH > CH3 –C≡ CH >> CH3 –C≡C–CH3in case of ethene and ethane thus indicating that ethyne is acidic in nature in comparison B. Addition reactions: Alkynes contain a to ethene and ethane. Why is it so ? Has triple bond, so they add up, two molecules of it something to do with their structures dihydrogen, halogen, hydrogen halides etc. and the hybridisation ? You have read that Formation of the addition product takes place hydrogen atoms in ethyne are attached to according to the following steps. Reprint 2025-26 Hydrocarbons 317 The addition product formed depends upon stability of vinylic cation. Addition in unsymmetrical alkynes takes place according to Markovnikov rule. Majority of the reactions of alkynes are the examples of electrophilic addition reactions. A few addition reactions (9.66)are given below: (i) Addition of dihydrogen (iv) Addition of water Pt/Pd/Ni H2 Like alkanes and alkenes, alkynes are alsoHC≡CH+H2 [H2C = CH2] CH3–CH3 immiscible and do not react with water. (9.62) However, one molecule of water adds to alkynes on warming with mercuric sulphate CH3–C≡CH + H2 Pt/Pd/Ni [CH3–CH=CH2] and dilute sulphuric acid at 333 K to form Propyne Propene carbonyl compounds. ↓H2 CH3–CH2–CH3 Propane (9.63) (ii) Addition of halogens (9.67) (9.64) Reddish orange colour of the solution of bromine in carbon tetrachloride is decolourised. This is used as a test for unsaturation. (iii) Addition of hydrogen halides (9.68) Two molecules of hydrogen halides (HCl, HBr, (v) Polymerisation HI) add to alkynes to form gem dihalides (in (a) Linear polymerisation: Under suitable which two halogens are attached to the same conditions, linear polymerisation of ethyne carbon atom) takes place to produce polyacetylene or H–C≡C–H+H–Br [CH2 = CH–Br]→ CHBr2 polyethyne which is a high molecular Bromoethene weight polyene containing repeating units of CH3 (CH = CH – CH = CH ) and can be represented 1,1-Dibromoethane as —(CH = CH – CH = CH)n— Under special (9.65) conditions, this polymer conducts electricity. Reprint 2025-26 318 chemistry Thin film of polyacetylene can be used as but in a majority of reactions of aromatic electrodes in batteries. These films are good compounds, the unsaturation of benzene ring conductors, lighter and cheaper than the is retained. However, there are examples of metal conductors. aromatic hydrocarbons which do not contain a (b) Cyclic polymerisation: Ethyne on benzene ring but instead contain other highly unsaturated ring. Aromatic compoundspassing through red hot iron tube at 873K containing benzene ring are known asundergoes cyclic polymerization. Three benzenoids and those not containing amolecules polymerise to form benzene, which benzene ring are known as non-benzenoids.is the starting molecule for the preparation of Some examples of arenes are givenderivatives of benzene, dyes, drugs and large below:number of other organic compounds. This is the best route for entering from aliphatic to aromatic compounds as discussed below: Benzene Toluene Naphthalene (9.69) Biphenyl Problem 9.14 How will you convert ethanoic acid into 9.5.1 Nomenclature and Isomerism benzene? The nomenclature and isomerism of aromatic Solution hydrocarbons has already been discussed in Unit 8. All six hydrogen atoms in benzene are equivalent; so it forms one and only one type of monosubstituted product. When two hydrogen atoms in benzene are replaced by two similar or different monovalent atoms or groups, three different position isomers are possible. The 1, 2 or 1, 6 is known as the ortho (o–), the 1, 3 or 1, 5 as meta (m–) and the 1, 4 as para (p–) disubstituted compounds. A few examples of derivatives of benzene are given below:

6.21 Primary alkyl halide C4H9Br (a) reacted with alcoholic KOH to give compound (b). Compound (b) is reacted with HBr to give (c) which is an isomer of (a). When (a) is reacted with sodium metal it gives compound (d), C8H18 which is different from the compound formed when n-butyl bromide is reacted with sodium. Give the structural formula of (a) and write the equations for all the reactions.

6.4 All the hydrogen atoms are equivalent and replacement of any hydrogen will give the same product. The equivalent hydrogens are grouped as a, b and c. The replacement of equivalent hydrogens will give the same product. Similarly the equivalent hydrogens are grouped as a, b, c and d. Thus, four isomeric products are possible. 6.5 6.6 (i) Chloromethane, Bromomethane, Dibromomethane, Bromoform. Boiling point increases with increase in molecular mass. (ii) Isopropylchloride, 1-Chloropropane, 1-Chlorobutane. Isopropylchloride being branched has lower b.p. than 1- Chloropropane. 6.7 (i) CH3CH2CH2CH2Br Being primary halide, there won’t be any steric hindrance. (ii) Secondary halide reacts faster than tertiary halide. (iii) The presence of methyl group closer to the halide group will increase the steric hindrance and decrease the rate. 6.8 (i) Tertiary halide reacts faster than secondary halide because of the greater stability of tert-carbocation. Because of greater stability of secondary carbocation than (ii) primary. 6.9 Chemistry 192 Reprint 2025-26 UnitUnitUnitUnit Unit77 AlcoholsAlcoholsAlcohols,AlcoholsAlcoholsAlcoholsAlcoholsAlcohols,AlcoholsAlcohols PhenolsPhenolsPhenolsPhenolsPhenolsPhenolsPhenolsPhenolsPhenolsPhenolsObjectives After studying this Unit, you will be able to andandandandandandandandandand EtherEthertherthertherstherthertherthersther • name alcohols, phenols and ethers according to the IUPAC system of nomenclature; Alcohols, phenols and ethers are the basic compounds for the • discuss the reactions involved in formation of detergents, antiseptics and fragrances, respectively. the preparation of alcohols from alkenes, aldehydes, ketones and carboxylic acids; You have learnt that substitution of one or more • discuss the reactions involved in hydrogen atom(s) from a hydrocarbon by another atom the preparation of phenols from or a group of atoms result in the formation of an entirely haloarenes, benzene sulphonic new compound having altogether different properties acids, diazonium salts and and applications. Alcohols and phenols are formed cumene; when a hydrogen atom in a hydrocarbon, aliphatic and • discuss the reactions for aromatic respectively, is replaced by –OH group. These preparation of ethers from classes of compounds find wide applications in industry (i) alcohols and (ii) alkyl halides as well as in day-to-day life. For instance, have you and sodium alkoxides/aryloxides; ever noticed that ordinary spirit used for polishing • correlate physical properties of wooden furniture is chiefly a compound containing alcohols, phenols and ethers with their structures; hydroxyl group, ethanol. The sugar we eat, the cotton used for fabrics, the paper we use for writing, are all• discuss chemical reactions of the three classes of compounds on made up of compounds containing –OH groups. Just the basis of their functional think of life without paper; no note-books, books, news- groups. papers, currency notes, cheques, certificates, etc. The magazines carrying beautiful photographs and interesting stories would disappear from our life. It would have been really a different world. An alcohol contains one or more hydroxyl (OH) group(s) directly attached to carbon atom(s), of an aliphatic system (CH3OH) while a phenol contains –OH group(s) directly attached to carbon atom(s) of an aromatic system (C6H5OH). The substitution of a hydrogen atom in a hydrocarbon by an alkoxy or aryloxy group (R–O/Ar–O) yields another class of compounds known as ‘ethers’, for example, CH3OCH3 (dimethyl ether). You may also visualise ethers as compounds formed by Reprint 2025-26 substituting the hydrogen atom of hydroxyl group of an alcohol or phenol by an alkyl or aryl group. In this unit, we shall discuss the chemistry of three classes of compounds, namely — alcohols, phenols and ethers. 7.17.17.17.17.1 ClassificationClassificationClassificationClassificationClassification The classification of compounds makes their study systematic and hence simpler. Therefore, let us first learn how are alcohols, phenols and ethers classified? 7.1.1 Alcohols— Alcohols and phenols may be classified as mono–, di–, tri- or Mono, Di, polyhydric compounds depending on whether they contain one, two, Tri or three or many hydroxyl groups respectively in their structures as Polyhydric given below: alcohols Monohydric Dihydric Trihydric Monohydric alcohols may be further classified according to the hybridisation of the carbon atom to which the hydroxyl group is attached. 3  OH bond: In this class of alcohols, (i) Compounds containing Csp the –OH group is attached to an sp3 hybridised carbon atom of an alkyl group. They are further classified as follows: Primary, secondary and tertiary alcohols: In these three types of alcohols, the –OH group is attached to primary, secondary and tertiary carbon atom, respectively as depicted below: Allylic alcohols: In these alcohols, the —OH group is attached to a sp3 hybridised carbon adjacent to the carbon-carbon double bond, that is to an allylic carbon. For example Benzylic alcohols: In these alcohols, the —OH group is attached to a sp 3—hybridised carbon atom next to an aromatic ring. For example. Chemistry 194 Reprint 2025-26 Allylic and benzylic alcohols may be primary, secondary or tertiary. 2  OH bond: These alcohols contain (ii) Compounds containing Csp —OH group bonded to a carbon-carbon double bond, i.e., to a vinylic carbon or to an aryl carbon. These alcohols are also known as vinylic alcohols. Vinylic alcohol: CH2 = CH – OH 7.1.2 Phenols— Mono, Di and trihydric phenols Monohydric 7.1.3 Ethers Ethers are classified as simple or symmetrical, if the alkyl or aryl groups attached to the oxygen atom are the same, and mixed or unsymmetrical, if the two groups are different. Diethyl ether, C2H5OC2H5, is a symmetrical ether whereas C2H5OCH3 and C2H5OC6H5 are unsymmetrical ethers. IntextIntextIntextIntextIntext QuestionsQuestionsQuestionsQuestionsQuestions 7.1 Classify the following as primary, secondary and tertiary alcohols: CH3 (i) CH3 C CH2OH (ii) H2C CH CH2OH CH3 OH CH CH3 (iii) CH3 CH2 CH2 OH (iv) CH3 CH CH C OH CH2 CH CH3 (vi) (v) CH3 OH 7.2 Identify allylic alcohols in the above examples. 7.27.27.27.27.2 NomenclatureNomenclatureNomenclatureNomenclatureNomenclature (a) Alcohols: The common name of an alcohol is derived from the common name of the alkyl group and adding the word alcohol to it. For example, CH3OH is methyl alcohol. 195 Alcohols, Phenols and Ethers Reprint 2025-26 According to IUPAC system, the name of an alcohol is derived from the name of the alkane from which the alcohol is derived, by substituting ‘e’ of alkane with the suffix ‘ol’. The position of substituents are indicated by numerals. For this, the longest carbon chain (parent chain) is numbered starting at the end nearest to the hydroxyl group. The positions of the –OH group and other substituents are indicated by using the numbers of carbon atoms to which these are attached. For naming polyhydric alcohols, the ‘e’ of alkane is retained and the ending ‘ol’ is added. The number of –OH groups is indicated by adding the multiplicative prefix, di, tri, etc., before ‘ol’. The positions of –OH groups are indicated by appropriate locants, e.g., HO–CH2–CH2–OH is named as ethane–1, 2-diol. Table 7.1 gives common and IUPAC names of a few alcohols as examples. Table 7.1: Common and IUPAC Names of Some Alcohols Compound Common name IUPAC name CH3 – OH Methyl alcohol Methanol CH3 – CH2 – CH2 – OH n-Propyl alcohol Propan-1-ol Isopropyl alcohol Propan-2-ol CH3 – CH2 – CH2 – CH2 – OH n-Butyl alcohol Butan-1-ol sec-Butyl alcohol Butan-2-ol Isobutyl alcohol 2-Methylpropan-1-ol tert-Butyl alcohol 2-Methylpropan-2-ol HO–H2C–CH2–OH Ethylene glycol Ethane-1,2-diol Glycerol Propane -1, 2, 3-triol Cyclic alcohols are named using the prefix cyclo and considering the —OH group attached to C–1. OH OH CH3 Cyclohexanol 2-Methylcyclopentanol (b) Phenols: The simplest hydroxy derivative of benzene is phenol. It is its common name and also an accepted IUPAC name. As structure of phenol involves a benzene ring, in its substituted compounds the terms ortho (1,2- disubstituted), meta (1,3-disubstituted) and para (1,4-disubstituted) are often used in the common names. Chemistry 196 Reprint 2025-26 OH CH3 CH3 CH3 OH OH OH Common name Phenol o-Cresol m-Cresol p-Cresol IUPAC name Phenol 2-Methylphenol 3-Methylphenol 4-Methylphenol Dihydroxy derivatives of benzene are known as 1, 2-, 1, 3- and 1, 4-benzenediol. OH OH OH OH OH OH Common name Catechol Resorcinol Hydroquinone or quinol IUPAC name Benzene-1,2- diol Benzene- 1,3-diol Benzene- 1,4-diol (c) Ethers: Common names of ethers are derived from the names of alkyl/ aryl groups written as separate words in alphabetical order and adding the word ‘ether’ at the end. For example, CH3OC2H5 is ethylmethyl ether. Table 7.2: Common and IUPAC Names of Some Ethers Compound Common name IUPAC name CH3OCH3 Dimethyl ether Methoxymethane C2H5OC2H5 Diethyl ether Ethoxyethane CH3OCH2CH2CH3 Methyl n-propyl ether 1-Methoxypropane C6H5OCH3 Methyl phenyl ether Methoxybenzene (Anisole) (Anisole) C6H5OCH2CH3 Ethyl phenyl ether Ethoxybenzene (Phenetole) C6H5O(CH2)6 – CH3 Heptyl phenyl ether 1-Phenoxyheptane CH3O CH CH3 Methyl isopropyl ether 2-Methoxypropane CH3 Phenyl isopentyl ether 3- Methylbutoxybenzene CH3– O – CH2 – CH2 – OCH3 — 1,2-Dimethoxyethane — 2-Ethoxy- -1,1-dimethylcyclohexane 197 Alcohols, Phenols and Ethers Reprint 2025-26 If both the alkyl groups are the same, the prefix ‘di’ is added before the alkyl group. For example, C2H5OC2H5 is diethyl ether. According to IUPAC system of nomenclature, ethers are regarded as hydrocarbon derivatives in which a hydrogen atom is replaced by an –OR or –OAr group, where R and Ar represent alkyl and aryl groups, respectively. The larger (R) group is chosen as the parent hydrocarbon. The names of a few ethers are given as examples in Table 7.2. ExampleExampleExampleExampleExample 7.17.17.17.17.1 Give IUPAC names of the following compounds: (i) CH3 CH CH CH CH2OH (ii) CH3 CH O CH2CH3 Cl CH3 CH3 CH3 OH NO2 (iii) H3C CH3 (iv) OC2 H5 SolutionSolutionSolutionSolutionSolution (i) 4-Chloro-2,3-dimethylpentan-1-ol (ii) 2-Ethoxypropane (iii) 2,6-Dimethylphenol (iv) 1-Ethoxy-2-nitrocyclohexane IntextIntextIntextIntextIntext QuestionQuestionQuestionQuestionQuestion 7.3 Name the following compounds according to IUPAC system. (i) (ii) (iii) (iv) (v) 7.37.37.37.37.3 StructuresStructuresStructuresStructuresStructures ofofofofof In alcohols, the oxygen of the –OH group is attached to carbon by a FunctionalFunctionalFunctionalFunctionalFunctional sigma (s ) bond formed3 by the overlap of a sp 3 hybridised orbital of carbon with a sp hybridised orbital of oxygen. Fig. 7.1 depicts GroupsGroupsGroupsGroupsGroups structural aspects of methanol, phenol and methoxymethane. Fig. 7.1: Structures of methanol, phenol and methoxymethane Chemistry 198 Reprint 2025-26 The bond angle in alcohols is slightly less than the tetrahedral angle (109°-28¢). It is due to the repulsion between the unshared electron pairs of oxygen. In phenols, the –OH group is attached to sp2 hybridised carbon of an aromatic ring. The carbon– oxygen bond length (136 pm) in phenol is slightly less than that in methanol. This is due to (i) partial double bond character on account of the conjugation of unshared electron pair of oxygen with the aromatic ring (Section 7.4.4) and (ii) sp 2 hybridised state of carbon to which oxygen is attached. In ethers, the four electron pairs, i.e., the two bond pairs and two lone pairs of electrons on oxygen are arranged approximately in a tetrahedral arrangement. The bond angle is slightly greater than the tetrahedral angle due to the repulsive interaction between the two bulky (–R) groups. The C–O bond length (141 pm) is almost the same as in alcohols. 7.47.47.47.47.4 AlcoholsAlcoholsAlcoholsAlcoholsAlcohols andandandandand 7.4.1 Preparation of Alcohols PhenolsPhenolsPhenolsPhenolsPhenols Alcohols are prepared by the following methods: 1. From alkenes (i) By acid catalysed hydration: Alkenes react with water in the presence of acid as catalyst to form alcohols. In case of unsymmetrical alkenes, the addition reaction takes place in accordance with Markovnikov’s rule. Mechanism The mechanism of the reaction involves the following three steps: Step 1: Protonation of alkene to form carbocation by electrophilic attack of H3O +. H2O + H+ ® H3O+ Step 2: Nucleophilic attack of water on carbocation. Step 3: Deprotonation to form an alcohol. 199 Alcohols, Phenols and Ethers Reprint 2025-26 Hydroboration - (ii) By hydroboration–oxidation: Diborane (BH3)2 reacts with alkenes oxidation was first to give trialkyl boranes as addition product. This is oxidised to reported by H.C. alcohol by hydrogen peroxide in the presence of aqueous sodium Brown in 1959. For hydroxide.his studies on boron containing organic compounds, Brown shared the 1979 Nobel prize in Chemistry with G. Wittig. The addition of borane to the double bond takes place in such a manner that the boron atom gets attached to the sp2 carbon carrying greater number of hydrogen atoms. The alcohol so formed looks as if it has been formed by the addition of water to the alkene in a way opposite to the Markovnikov’s rule. In this reaction, alcohol is obtained in excellent yield. 2. From carbonyl compounds (i) By reduction of aldehydes and ketones: Aldehydes and ketones are reduced to the corresponding alcohols by addition of hydrogen in the presence of catalysts (catalytic hydrogenation). The usual catalyst is a finely divided metal such as platinum, palladium or nickel. It is also prepared by treating aldehydes and ketones with sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH4). Aldehydes yield primary alcohols whereas ketones give secondary alcohols. The numbers in front (ii) By reduction of carboxylic acids and esters: Carboxylic acids of the reagents along are reduced to primary alcohols in excellent yields by lithium the arrow indicate aluminium hydride, a strong reducing agent. that the second reagent is added only (i) LiAlH4 when the reaction RCOOH RCH2 OH with first is complete. (ii) H2O However, LiAlH4 is an expensive reagent, and therefore, used for preparing special chemicals only. Commercially, acids are reduced to alcohols by converting them to the esters (Section 7.4.4), followed by their reduction using hydrogen in the presence of catalyst (catalytic hydrogenation). R'OH H + Chemistry 200 Reprint 2025-26 3. From Grignard reagents Alcohols are produced by the reaction of Grignard reagents (Unit 6, Class XII) with aldehydes and ketones. The first step of the reaction is the nucleophilic addition of Grignard reagent to the carbonyl group to form an adduct. Hydrolysis of the adduct yields an alcohol. ... (i) ...(ii) The reaction of The overall reactions using different aldehydes and ketones are as Grignard reagents follows: with methanal produces a primary alcohol, with other aldehydes, secondary alcohols and with ketones, tertiary alcohols. You will notice that the reaction produces a primary alcohol with methanal, a secondary alcohol with other aldehydes and tertiary alcohol with ketones. Give the structures and IUPAC names of the products expected from ExampleExampleExampleExampleExample 7.27.27.27.27.2 the following reactions: (a) Catalytic reduction of butanal. (b) Hydration of propene in the presence of dilute sulphuric acid. (c) Reaction of propanone with methylmagnesium bromide followed by hydrolysis. SolutionSolutionSolutionSolutionSolution (a) (b) (c) 7.4.2 Preparation Phenol, also known as carbolic acid, was first isolated in the early of Phenols nineteenth century from coal tar. Nowadays, phenol is commercially produced synthetically. In the laboratory, phenols are prepared from benzene derivatives by any of the following methods: 201 Alcohols, Phenols and Ethers Reprint 2025-26 1. From haloarenes Chlorobenzene is fused with NaOH at 623K and 320 atmospheric pressure. Phenol is obtained by acidification of sodium phenoxide so produced (Unit 6, Class XII). 2. From benzenesulphonic acid Benzene is sulphonated with oleum and benzene sulphonic acid so formed is converted to sodium phenoxide on heating with molten sodium hydroxide. Acidification of the sodium salt gives phenol. 3. From diazonium salts A diazonium salt is formed by treating an aromatic primary amine with nitrous acid (NaNO2 + HCl) at 273-278 K. Diazonium salts are hydrolysed to phenols by warming with water or by treating with dilute acids (Unit 9, Class XII). + – NH2 N2 Cl OH NaNO2 H2O + N2 + HCl +HCl Warm Aniline Benzene diazonium chloride Most of the worldwide 4. From cumene production of phenol is from cumene. Phenol is manufactured from the hydrocarbon, cumene. Cumene (isopropylbenzene) is oxidised in the presence of air to cumene hydroperoxide. It is converted to phenol and acetone by treating it with dilute acid. Acetone, a by-product of this reaction, is also obtained in large quantities by this method. Chemistry 202 Reprint 2025-26 IntextIntextIntextIntextIntext QuestionsQuestionsQuestionsQuestionsQuestions 7.4 Show how are the following alcohols prepared by the reaction of a suitable Grignard reagent on methanal ? 7.5 Write structures of the products of the following reactions: (i) (ii) (iii) 7.4.3 Physical Alcohols and phenols consist of two parts, an alkyl/aryl group and a Properties hydroxyl group. The properties of alcohols and phenols are chiefly due to the hydroxyl group. The nature of alkyl and aryl groups simply modify these properties. Boiling Points The boiling points of alcohols and phenols increase with increase in the number of carbon atoms (increase in van der Waals forces). In alcohols, the boiling points decrease with increase of branching in carbon chain (because of decrease in van der Waals forces with decrease in surface area). The –OH group in alcohols and phenols is involved in intermolecular hydrogen bonding as shown below: It is interesting to note that boiling points of alcohols and phenols are higher in comparison to other classes of compounds, namely hydrocarbons, ethers, haloalkanes and haloarenes of comparable molecular masses. For example, ethanol and propane have comparable molecular masses but their boiling points differ widely. The boiling point of methoxymethane is intermediate of the two boiling points. 203 Alcohols, Phenols and Ethers Reprint 2025-26 The high boiling points of alcohols are mainly due to the presence of intermolecular hydrogen bonding in them which is lacking in ethers and hydrocarbons. Solubility Solubility of alcohols and phenols in water is due to their ability to form hydrogen bonds with water molecules as shown. The solubility decreases with increase in size of alkyl/aryl (hydro- phobic) groups. Several of the lower molecular mass alcohols are miscible with water in all proportions. ExampleExampleExampleExampleExample 7.37.37.37.37.3 Arrange the following sets of compounds in order of their increasing boiling points: (a) Pentan-1-ol, butan-1-ol, butan-2-ol, ethanol, propan-1-ol, methanol. (b) Pentan-1-ol, n-butane, pentanal, ethoxyethane. SolutionSolutionSolutionSolutionSolution (a) Methanol, ethanol, propan-1-ol, butan-2-ol, butan-1-ol, pentan-1-ol. (b) n-Butane, ethoxyethane, pentanal and pentan-1-ol. 7.4.4 Chemical Alcohols are versatile compounds. They react both as nucleophiles and Reactions electrophiles. The bond between O–H is broken when alcohols react as nucleophiles. Alcohols as nucleophiles (i) (ii) The bond between C–O is broken when they react as electrophiles. Protonated alcohols react in this manner. Protonated alcohols as electrophiles Based on the cleavage of O–H and C–O bonds, the reactions of alcohols and phenols may be divided into two groups: Chemistry 204 Reprint 2025-26 (a) Reactions involving cleavage of O–H bond 1. Acidity of alcohols and phenols (i) Reaction with metals: Alcohols and phenols react with active metals such as sodium, potassium and aluminium to yield corresponding alkoxides/phenoxides and hydrogen. In addition to this, phenols react with aqueous sodium hydroxide to form sodium phenoxides. OH ONa + NaOH + H 2 O Sodium phenoxide The above reactions show that alcohols and phenols are acidic in nature. In fact, alcohols and phenols are Brönsted acids i.e., they can donate a proton to a stronger base (B:). (ii) Acidity of alcohols: The acidic character of alcohols is due to the polar nature of O–H bond. An electron-releasing group (–CH3, –C2H5) increases electron density on oxygen tending to decrease the polarity of O-H bond. This decreases the acid strength. For this reason, the acid strength of alcohols decreases in the following order: 205 Alcohols, Phenols and Ethers Reprint 2025-26 Alcohols are, however, weaker acids than water. This can be illustrated by the reaction of water with an alkoxide. This reaction shows that water is a better proton donor (i.e., stronger acid) than alcohol. Also, in the above reaction, we note that an alkoxide ion is a better proton acceptor than hydroxide ion, which suggests that alkoxides are stronger bases (sodium ethoxide is a stronger base than sodium hydroxide). Alcohols act as Bronsted bases as well. It is due to the presence of unshared electron pairs on oxygen, which makes them proton acceptors. (iii) Acidity of phenols: The reactions of phenol with metals (e.g., sodium, aluminium) and sodium hydroxide indicate its acidic nature. The hydroxyl group, in phenol is directly attached to the sp2 hybridised carbon of benzene ring which acts as an electron withdrawing group. Due to this, the charge distribution in phenol molecule, as depicted in its resonance structures, causes the oxygen of –OH group to be positive. The reaction of phenol with aqueous sodium hydroxide indicates that phenols are stronger acids than alcohols and water. Let us examine how a compound in which hydroxyl group attached to an aromatic ring is more acidic than the one in which hydroxyl group is attached to an alkyl group. The ionisation of an alcohol and a phenol takes place as follows: Due to the higher electronegativity of sp2 hybridised carbon of phenol to which –OH is attached, electron density decreases on oxygen. This increases the polarity of O–H bond and results in an increase in ionisation of phenols than that of alcohols. Now let us examine the stabilities of alkoxide and phenoxide ions. In alkoxide ion, the negative charge is localised on oxygen while in phenoxide ion, the charge is delocalised. The delocalisation of negative charge (structures I-V) makes Chemistry 206 Reprint 2025-26 phenoxide ion more stable and favours the ionisation of phenol. Although there is also charge delocalisation in phenol, its resonance structures have charge separation due to which the phenol molecule is less stable than phenoxide ion. In substituted phenols, the presence of electron withdrawing groups such as nitro group, enhances the acidic strength of phenol. This effect is more pronounced when such a group is present at ortho and para positions. It is due to the effective delocalisation of negative charge in phenoxide ion when substituent is at ortho or para position. On the other hand, electron releasing groups, such as alkyl groups, in general, do not favour the formation of phenoxide ion resulting in decrease in acid strength. Cresols, for example, are less acidic than phenol. The greater the pKa Table 7.3: pKa Values of some Phenols and Ethanol value, the weaker the acid. Compound Formula pKa o-Nitrophenol o–O2N–C6H4–OH 7.2 m-Nitrophenol m–O2N–C6H4–OH 8.3 p-Nitrophenol p-O2N–C6H4–OH 7.1 Phenol C6H5–OH 10.0 o-Cresol o-CH3–C6H4–OH 10.2 m-Cresol m-CH3C6H4–OH 10.1 p-Cresol p-CH3–C6H4–OH 10.2 Ethanol C2H5OH 15.9 From the above data, you will note that phenol is million times more acidic than ethanol. Arrange the following compounds in increasing order of their acid strength: ExampleExampleExampleExampleExample 7.47.47.47.47.4 Propan-1-ol, 2,4,6-trinitrophenol, 3-nitrophenol, 3,5-dinitrophenol, phenol, 4-methylphenol. Propan-1-ol, 4-methylphenol, phenol, 3-nitrophenol, 3,5-dinitrophenol, SolutionSolutionSolutionSolutionSolution 2,4, 6-trinitrophenol. 2. Esterification Alcohols and phenols react with carboxylic acids, acid chlorides and acid anhydrides to form esters. 207 Alcohols, Phenols and Ethers Reprint 2025-26 Pyridine R/Ar OH+R’COCl R/ArOCOR’ + HCl The reaction with carboxylic acid and acid anhydride is carried Aspirin possesses out in the presence of a small amount of concentrated sulphuric analgesic, anti- acid. The reaction is reversible, and therefore, water is removed as inflammatory and soon as it is formed. The reaction with acid chloride is carried out in antipyretic properties. the presence of a base (pyridine) so as to neutralise HCl which is formed during the reaction. It shifts the equilibrium to the right hand side. The introduction of acetyl (CH3CO) group in alcohols or phenols is known as acetylation. Acetylation of salicylic acid produces aspirin. (b) Reactions involving cleavage of carbon – oxygen (C–O) bond in alcohols The reactions involving cleavage of C–O bond take place only in alcohols. Phenols show this type of reaction only with zinc. 1. Reaction with hydrogen halides: Alcohols react with hydrogen halides to form alkyl halides (Refer Unit 6, Class XII). ROH + HX ® R–X + H2O The difference in reactivity of three classes of alcohols with HCl distinguishes them from one another (Lucas test). Alcohols are soluble in Lucas reagent (conc. HCl and ZnCl2) while their halides are immiscible and produce turbidity in solution. In case of tertiary alcohols, turbidity is produced immediately as they form the halides easily. Primary alcohols do not produce turbidity at room temperature. 2. Reaction with phosphorus trihalides: Alcohols are converted to alkyl bromides by reaction with phosphorus tribromide (Refer Unit 6, Class XII). 3. Dehydration: Alcohols undergo dehydration (removal of a molecule of water) to form alkenes on treating with a protic acid e.g., concentrated H2SO4 or H3PO4, or catalysts such as anhydrous zinc chloride or alumina. Ethanol undergoes dehydration by heating it with concentrated H2SO4 at 443 K. Chemistry 208 Reprint 2025-26 Secondary and tertiary alcohols are dehydrated under milder conditions. For example Thus, the relative ease of dehydration of alcohols follows the following order: Tertiary > Secondary > Primary The mechanism of dehydration of ethanol involves the following steps: Tertiary carbocations Mechanism are more stable and Step 1: Formation of protonated alcohol. therefore are easier to form than secondary and primary carbocations; tertiary alcohols are the easiest to dehydrate. Step 2: Formation of carbocation: It is the slowest step and hence, the rate determining step of the reaction. Step 3: Formation of ethene by elimination of a proton. The acid used in step 1 is released in step 3. To drive the equilibrium to the right, ethene is removed as it is formed. 4. Oxidation: Oxidation of alcohols involves the formation of a carbon- oxygen double bond with cleavage of an O-H and C-H bonds. Such a cleavage and formation of bonds occur in oxidation reactions. These are also known as dehydrogenation reactions as these involve loss of dihydrogen from an alcohol molecule. Depending on the oxidising agent used, a primary alcohol is oxidised to an aldehyde which in turn is oxidised to a carboxylic acid. 209 Alcohols, Phenols and Ethers Reprint 2025-26 Strong oxidising agents such as acidified potassium permanganate are used for getting carboxylic acids from alcohols directly. CrO3 in anhydrous medium is used as the oxidising agent for the isolation of aldehydes. CrO3 RCH 2 OH  RCHO A better reagent for oxidation of primary alcohols to aldehydes in good yield is pyridinium chlorochromate (PCC), a complex of chromium trioxide with pyridine and HCl. PCC CH 3  CH  CH  CH 2 O H  CH 3  CH  CH  CH O Secondary alcohols are oxidised to ketones by chromic anhyride (CrO3). Tertiary alcohols do not undergo oxidation reaction. Under strong reaction conditions such as strong oxidising agents (KMnO4) and elevated temperatures, cleavage of various C-C bonds takes place and a mixture of carboxylic acids containing lesser number of carbon atoms is formed. When the vapours of a primary or a secondary alcohol are passed over heated copper at 573 K, dehydrogenation takes place and an aldehyde or a ketone is formed while tertiary alcohols undergo dehydration. Biological oxidation of methanol and ethanol in the body produces the corresponding aldehyde followed by the acid. At times the alcoholics, by mistake, drink ethanol, mixed with methanol also called denatured alcohol. In the body, methanol is oxidised first to methanal and then to methanoic acid, which may cause blindness and death. A methanol poisoned patient is treated by giving intravenous infusions of diluted ethanol. The enzyme responsible for oxidation of aldehyde (HCHO) to acid is swamped allowing time for kidneys to excrete methanol. (c) Reactions of phenols Following reactions are shown by phenols only. Chemistry 210 Reprint 2025-26 1. Electrophilic aromatic substitution In phenols, the reactions that take place on the aromatic ring are electrophilic substitution reactions (Unit 9, Class XI). The –OH group attached to the benzene ring activates it towards electrophilic substitution. Also, it directs the incoming group to ortho and para positions in the ring as these positions become electron rich due to the resonance effect caused by –OH group. The resonance structures are shown under acidity of phenols. Common electrophilic aromatic substitution reactions taking place in phenol are as follows: (i) Nitration: With dilute nitric acid at low temperature (298 K), phenol yields a mixture of ortho and para nitrophenols. The ortho and para isomers can be separated by steam distillation. o-Nitrophenol is steam volatile due to intramolecular hydrogen bonding while p-nitrophenol is less volatile due to intermolecular hydrogen bonding which causes the association of molecules. 2, 4, 6 - Trinitrophenol is a strong acid due to the presence of three With concentrated nitric acid, phenol is converted to electron withdrawing 2,4,6-trinitrophenol. The product is commonly known as picric –NO2 groups which acid. The yield of the reaction product is poor. facilitate the release of hydrogen ion. Nowadays picric acid is prepared by treating phenol first with concentrated sulphuric acid which converts it to phenol-2,4-disulphonic acid, and then with concentrated nitric acid to get 2,4,6-trinitrophenol. Can you write the equations of the reactions involved? 211 Alcohols, Phenols and Ethers Reprint 2025-26 (ii) Halogenation: On treating phenol with bromine, different reaction products are formed under different experimental conditions. (a) When the reaction is carried out in solvents of low polarity such as CHCl3 or CS2 and at low temperature, monobromophenols are formed. The usual halogenation of benzene takes place in the presence of a Lewis acid, such as FeBr3 (Unit 6, Class XII), which polarises the halogen molecule. In case of phenol, the polarisation of bromine molecule takes place even in the absence of Lewis acid. It is due to the highly activating effect of –OH group attached to the benzene ring. (b) When phenol is treated with bromine water, 2,4,6-tribromophenol is formed as white precipitate. ExampleExampleExampleExampleExample 7.57.57.57.57.5 Write the structures of the major products expected from the following reactions: (a) Mononitration of 3-methylphenol (b) Dinitration of 3-methylphenol (c) Mononitration of phenyl methanoate. SolutionSolutionSolutionSolutionSolution The combined influence of –OH and –CH3 groups determine the position of the incoming group. 2. Kolbe’s reaction Phenoxide ion generated by treating phenol with sodium hydroxide is even more reactive than phenol towards electrophilic aromatic substitution. Hence, it undergoes electrophilic substitution with carbon dioxide, a weak electrophile. Ortho hydroxybenzoic acid is formed as the main reaction product. Chemistry 212 Reprint 2025-26 3. Reimer-Tiemann reaction On treating phenol with chloroform in the presence of sodium hydroxide, a –CHO group is introduced at ortho position of benzene ring. This reaction is known as Reimer - Tiemann reaction. The intermediate substituted benzal chloride is hydrolysed in the presence of alkali to produce salicylaldehyde. 4. Reaction of phenol with zinc dust Phenol is converted to benzene on heating with zinc dust. 5. Oxidation Oxidation of phenol with chromic acid produces a conjugated diketone known as benzoquinone. In the presence of air, phenols are slowly oxidised to dark coloured mixtures containing quinones. IntextIntextIntextIntextIntext QuestionsQuestionsQuestionsQuestionsQuestions