Unit 12

Aldehydes, Ketones and Carboxylic Acids

 In the previous Unit, you have studied organic compounds with functional groups containing carbon-oxygen single bond. In this Unit, we will study about the organic compounds containing carbon-oxygen double bond (>C=O) called carbonyl group, which is one of the most important functional groups in organic chemistry.

 In aldehydes, the carbonyl group is bonded to a carbon and hydrogen while in the ketones, it is bonded to two carbon atoms. The carbonyl compounds in which carbon of carbonyl group is bonded to carbon or hydrogen and oxygen of hydroxyl moiety (-OH) are known as carboxylic acids, while in compounds where carbon is attached to carbon or hydrogen and nitrogen of -NH2 moiety or to halogens are called amides and acyl halides respectively. Esters and anhydrides are derivatives of carboxylic acids. The general formulas of these classes of compounds are given below:

 

Aldehydes, ketones and carboxylic acids are widespread in plants and animal kingdom. They play an important role in biochemical processes of life. They add fragrance and flavour to nature, for example, vanillin (from vanilla beans), salicylaldehyde (from meadow sweet) and cinnamaldehyde (from cinnamon) have very pleasant fragrances.

 

They are used in many food products and pharmaceuticals to add flavours. Some of these families are manufactured for use as solvents (i.e., acetone) and for preparing materials like adhesives, paints, resins, perfumes, plastics, fabrics, etc.

12.1 Nomenclature and Structure of Carbonyl Group

12.1.1 Nomenclature

I. Aldehydes and ketones

Aldehydes and ketones are the simplest and most important carbonyl compounds.

There are two systems of nomenclature of aldehydes and ketones.

(a) Common names

 Aldehydes and ketones are often called by their common names instead of IUPAC names. The common names of most aldehydes are derived from the common names of the corresponding carboxylic acids [Section 12.6.1] by replacing the ending –ic of acid with aldehyde. At the same time, the names reflect the Latin or Greek term for the original source of the acid or aldehyde. The location of the substituent in the carbon chain is indicated by Greek letters α, β, γ, δ, etc. The α-carbon being the one directly linked to the aldehyde group, β-carbon the next, and so on. For example

 The common names of ketones are derived by naming two alkyl or aryl groups bonded to the carbonyl group. The locations of substituents are indicated by Greek letters, α α′, β β′ and so on beginning with the carbon atoms next to the carbonyl group, indicated as αα′. Some ketones have historical common names, the simplest dimethyl ketone is called acetone. Alkyl phenyl ketones are usually named by adding the name of acyl group as prefix to the word phenone. For example

(b) IUPAC names

 The IUPAC names of open chain aliphatic aldehydes and ketones are derived from the names of the corresponding alkanes by replacing the ending –e with –al and –one respectively. In case of aldehydes the longest carbon chain is numbered starting from the carbon of the aldehyde group while in case of ketones the numbering begins from the end nearer to the carbonyl group. The substituents are prefixed in alphabetical order along with numerals indicating their positions in the carbon chain. The same applies to cyclic ketones, where the carbonyl carbon is numbered one. When the aldehyde group is attached to a ring, the suffix carbaldehyde is added after the full name of the cycloalkane. The numbering of the ring carbon atoms start from the carbon atom attached to the aldehyde group. The name of the simplest aromatic aldehyde carrying the aldehyde group on a benzene ring is benzenecarbaldehyde. However, the common name benzaldehyde is also accepted by IUPAC. Other aromatic aldehydes are hence named as substituted benzaldehydes.

 

 

The common and IUPAC names of some aldehydes and ketones are given in Table 12.1.

12.1.2 Structure of the Carbonyl Group

Fig.12.1 Orbital diagram for the formation of carbonyl group

 The carbon-oxygen double bond is polarised due to higher electronegativity of oxygen relative to carbon. Hence, the carbonyl carbon is an electrophilic (Lewis acid), and carbonyl oxygen, a nucleophilic (Lewis base) centre. Carbonyl compounds have substantial dipole moments and are polar than ethers. The high polarity of the carbonyl group is explained on the basis of resonance involving a neutral (A) and a dipolar (B) structures as shown.

Intext Questions

12.1 Write the structures of the following compounds.

(i) α-Methoxypropionaldehyde (ii) 3-Hydroxybutanal

(iii) 2-Hydroxycyclopentane carbaldehyde (iv) 4-Oxopentanal

(v) Di-sec. butyl ketone (vi) 4-Fluoroacetophenone

12.2 Preparation of Aldehydes and Ketones

Some important methods for the preparation of aldehydes and ketones are as follows:

12.2.1 Preparation of Aldehydes and Ketones

 1. By oxidation of alcohols

Aldehydes and ketones are generally prepared by oxidation of primary and secondary alcohols, respectively (Unit 11, Class XII).

2. By dehydrogenation of alcohols

This method is suitable for volatile alcohols and is of industrial application. In this method alcohol vapours are passed over heavy metal catalysts (Ag or Cu). Primary and secondary alcohols give aldehydes and ketones, respectively (Unit 11, Class XII).

3. From hydrocarbons

(i) By ozonolysis of alkenes: As we know, ozonolysis of alkenes followed by reaction with zinc dust and water gives aldehydes, ketones or a mixture of both depending on the substitution pattern of the alkene (Unit 13, Class XI).

 (ii) By hydration of alkynes: Addition of water to ethyne in the presence of H2SO4 and HgSO4 gives acetaldehyde. All other alkynes give ketones in this reaction (Unit 13, Class XI).

12.2.2 Preparation of Aldehydes

1. From acyl chloride (acid chloride)

 Acyl chloride (acid chloride) is hydrogenated over catalyst, palladium on barium sulphate. This reaction is called Rosenmund reduction.

2. From nitriles and esters

 Nitriles are reduced to corresponding imine with stannous chloride in the presence of hydrochloric acid, which on hydrolysis give corresponding aldehyde.

This reaction is called Stephen reaction.

 Alternatively, nitriles are selectively reduced by diisobutylaluminium hydride, (DIBAL-H) to imines followed by hydrolysis to aldehydes:

Similarly, esters are also reduced to aldehydes with DIBAL-H.

3. From hydrocarbons

 (i) By oxidation of methylbenzene

Strong oxidising agents oxidise toluene and its derivatives to benzoic acids. However, it is possible to stop the oxidation at the aldehyde stage with suitable reagents that convert the methyl group to an intermediate that is difficult to oxidise further. The following methods are used for this purpose.

 (a) Use of chromyl chloride (CrO2Cl2): Chromyl chloride oxidises methyl group to a chromium complex, which on hydrolysis gives corresponding benzaldehyde.

This reaction is called Etard reaction.

 (b) Use of chromic oxide (CrO3): Toluene or substituted toluene is converted to benzylidene diacetate on treating with chromic oxide in acetic anhydride. The benzylidene diacetate can be hydrolysed to corresponding benzaldehyde with aqueous acid.

 (ii) By side chain chlorination followed by hydrolysis

 Side chain chlorination of toluene gives benzal chloride, which on hydrolysis gives benzaldehyde. This is a commercial method of manufacture of benzaldehyde.

 (iii) By Gatterman – Koch reaction

When benzene or its derivative is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride or cuprous chloride, it gives benzaldehyde or substituted benzaldehyde.

This reaction is known as Gatterman-Koch reaction.

12.2.3 Preparation of Ketones

1. From acyl chlorides

2. From nitriles

 Treating a nitrile with Grignard reagent followed by hydrolysis yields a ketone.

3. From benzene or substituted benzenes

 When benzene or substituted benzene is treated with acid chloride in the presence of anhydrous aluminium chloride, it affords the corresponding ketone. This reaction is known as Friedel-Crafts acylation reaction.

12.3 Physical Properties

The physical properties of aldehydes and ketones are described as follows.

 Methanal is a gas at room temperature. Ethanal is a volatile liquid. Other aldehydes and ketones are liquid or solid at room temperature. The boiling points of aldehydes and ketones are higher than hydrocarbons and ethers of comparable molecular masses. It is due to weak molecular association in aldehydes and ketones arising out of the dipole-dipole interactions. Also, their boiling points are lower than those of alcohols of similar molecular masses due to absence of intermolecular hydrogen bonding. The following compounds of molecular masses 58 and 60 are ranked in order of increasing boiling points.

However, the solubility of aldehydes and ketones decreases rapidly on increasing the length of alkyl chain. All aldehydes and ketones are fairly soluble in organic solvents like benzene, ether, methanol, chloroform, etc. The lower aldehydes have sharp pungent odours. As the size of the molecule increases, the odour becomes less pungent and more fragrant. In fact, many naturally occurring aldehydes and ketones are used in the blending of perfumes and flavouring agents.

12.4 Chemical Reactions

Since aldehydes and ketones both possess the carbonyl functional group, they undergo similar chemical reactions.

1. Nucleophilic addition reactions

Contrary to electrophilic addition reactions observed in alkenes (refer Unit 13, Class XI), the aldehydes and ketones undergo nucleophilic addition reactions.

 (i) Mechanism of nucleophilic addition reactions

A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of sp2 hybridised orbitals of carbonyl carbon (Fig. 12.2). The hybridisation of carbon changes from sp2 to sp3 in this process, and a tetrahedral alkoxide intermediate is produced. This intermediate captures a proton from the reaction medium to give the electrically neutral product. The net result is addition of Nu– and H+ across the carbon oxygen double bond as shown in Fig. 12.2.

 (ii) Reactivity

Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively than in former.

(iii) Some important examples of nucleophilic addition and nucleophilic addition-elimination reactions:

 (a) Addition of hydrogen cyanide (HCN): Aldehydes and ketones react with hydrogen cyanide (HCN) to yield cyanohydrins. This reaction occurs very slowly with pure HCN. Therefore, it is catalysed by a base and the generated cyanide ion (CN-) being a stronger nucleophile readily adds to carbonyl compounds to yield corresponding cyanohydrin.

Cyanohydrins are useful synthetic intermediates.

 (b) Addition of sodium hydrogensulphite: Sodium hydrogensulphite adds to aldehydes and ketones to form the addition products.

The position of the equilibrium lies largely to the right hand side for most aldehydes and to the left for most ketones due to steric reasons. The hydrogensulphite addition compound is water soluble and can be converted back to the original carbonyl compound by treating it with dilute mineral acid or alkali. Therefore, these are useful for separation and purification of aldehydes.

(c) Addition of Grignard reagents: (refer Unit 11, Class XII).

 (d) Addition of alcohols: Aldehydes react with one equivalent of monohydric alcohol in the presence of dry hydrogen chloride to yield alkoxyalcohol intermediate, known as hemiacetals, which further react with one more molecule of alcohol to give a gem-dialkoxy compound known as acetal as shown in the reaction.

Ketones react with ethylene glycol under similar conditions to form cyclic products known as ethylene glycol ketals.

Dry hydrogen chloride protonates the oxygen of the carbonyl compounds and therefore, increases the electrophilicity of the carbonyl carbon facilitating the nucleophilic attack of ethylene glycol. Acetals and ketals are hydrolysed with aqueous mineral acids to yield corresponding aldehydes and ketones respectively.

 (e) Addition of ammonia and its derivatives: Nucleophiles, such as ammonia and its derivatives H2N-Z add to the carbonyl group of aldehydes and ketones. The reaction is reversible and catalysed by acid. The equilibrium favours the product formation due to rapid dehydration of the intermediate to form >C=N-Z.

Z = Alkyl, aryl, OH, NH2, C6H5NH, NHCONH2, etc.

 (i) Reduction to alcohols: Aldehydes and ketones are reduced to primary and secondary alcohols respectively by sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH4) as well as by catalytic hydrogenation (Unit 11, Class XII).

 (ii) Reduction to hydrocarbons: The carbonyl group of aldehydes and ketones is reduced to CH2 group on treatment with zinc-amalgam and concentrated hydrochloric acid [Clemmensen reduction] or with hydrazine followed by heating with sodium or potassium hydroxide in high boiling solvent such as ethylene glycol (Wolff-Kishner reduction).

Bernhard Tollens (1841-1918) was a Professor of Chemistry at the University of Gottingen, Germany.

3. Oxidation

 Aldehydes differ from ketones in their oxidation reactions. Aldehydes are easily oxidised to carboxylic acids on treatment with common oxidising agents like nitric acid, potassium permanganate, potassium dichromate, etc. Even mild oxidising agents, mainly Tollens’ reagent and Fehlings’ reagent also oxidise aldehydes.

Ketones are generally oxidised under vigorous conditions, i.e., strong oxidising agents and at elevated temperatures. Their oxidation involves carbon-carbon bond cleavage to afford a mixture of carboxylic acids having lesser number of carbon atoms than the parent ketone.

The mild oxidising agents given below are used to distinguish aldehydes from ketones:

 (i) Tollens’ test: On warming an aldehyde with freshly prepared ammoniacal silver nitrate solution (Tollens’ reagent), a bright silver mirror is produced due to the formation of silver metal. The aldehydes are oxidised to corresponding carboxylate anion. The reaction occurs in alkaline medium.

 (ii) Fehling’s test: Fehling reagent comprises of two solutions, Fehling solution A and Fehling solution B. Fehling solution A is aqueous copper sulphate and Fehling solution B is alkaline sodium potassium tartarate (Rochelle salt). These two solutions are mixed in equal amounts before test. On heating an aldehyde with Fehling’s reagent, a reddish brown precipitate is obtained. Aldehydes are oxidised to corresponding carboxylate anion. Aromatic aldehydes do not respond to this test.

 (iii) Oxidation of methyl ketones by haloform reaction: Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom (methyl ketones) are oxidised by sodium hypohalite to sodium salts of corresponding carboxylic acids having one carbon atom less than that of carbonyl compound. The methyl group is converted to haloform. This oxidation does not affect a carbon-carbon double bond, if present in the molecule.

Iodoform reaction with sodium hypoiodite is also used for detection of CH3CO group or CH3CH(OH) group which produces CH3CO group on oxidation.

Example 12.4

An organic compound (A) with molecular formula C8H8O forms an orange-red precipitate with 2,4-DNP reagent and gives yellow precipitate on heating with iodine in the presence of sodium hydroxide. It neither reduces Tollens’ or Fehlings’ reagent, nor does it decolourise bromine water or Baeyer’s reagent. On drastic oxidation with chromic acid, it gives a carboxylic acid (B) having molecular formula C7H6O2. Identify the compounds (A) and (B) and explain the reactions involved. 

Solution

(A) forms 2,4-DNP derivative. Therefore, it is an aldehyde or a ketone. Since it does not reduce Tollens’ or Fehling reagent, (A) must be a ketone. (A) responds to iodoform test. Therefore, it should be a methyl ketone. The molecular formula of (A) indicates high degree of unsaturation, yet it does not decolourise bromine water or Baeyer’s reagent. This indicates the presence of unsaturation due to an aromatic ring.

Compound (B), being an oxidation product of a ketone should be a carboxylic acid. The molecular formula of (B) indicates that it should be benzoic acid and compound (A) should, therefore, be a monosubstituted aromatic methyl ketone. The molecular formula of (A) indicates that it should be phenyl methyl ketone (acetophenone). Reactions are as follows:

 

 

4. Reactions due to a-hydrogen

 Acidity of α-hydrogens of aldehydes and ketones: The aldehydes and ketones undergo a number of reactions due to the acidic nature of α-hydrogen. The acidity of α-hydrogen atoms of carbonyl compounds is due to the strong electron withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base.

 (i) Aldol condensation: Aldehydes and ketones having at least one α-hydrogen undergo a reaction in the presence of dilute alkali as catalyst to form β-hydroxy aldehydes (aldol) or β-hydroxy ketones (ketol), respectively. This is known as Aldol reaction.

The name aldol is derived from the names of the two functional groups, aldehyde and alcohol, present in the products. The aldol and ketol readily lose water to give α,β-unsaturated carbonyl compounds which are aldol condensation products and the reaction is called Aldol condensation. Though ketones give ketols (compounds containing a keto and alcohol groups), the general name aldol condensation still applies to the reactions of ketones due to their similarity with aldehydes.

 (ii) Cross aldol condensation: When aldol condensation is carried out between two different aldehydes and / or ketones, it is called cross aldol condensation. If both of them contain α-hydrogen atoms, it gives a mixture of four products. This is illustrated below by aldol reaction of a mixture of ethanal and propanal.

Ketones can also be used as one component in the cross aldol reactions.

5. Other reactions

 (i) Cannizzaro reaction: Aldehydes which do not have an α-hydrogen atom, undergo self oxidation and reduction (disproportionation) reaction on heating with concentrated alkali. In this reaction, one molecule of the aldehyde is reduced to alcohol while another is oxidised to carboxylic acid salt.

 

 

 

 (ii) Electrophilic substitution reaction: Aromatic aldehydes and ketones undergo electrophilic substitution at the ring in which the carbonyl group acts as a deactivating and meta-directing group.

12.4 Chemical Reactions

Since aldehydes and ketones both possess the carbonyl functional group, they undergo similar chemical reactions.

1. Nucleophilic addition reactions

Contrary to electrophilic addition reactions observed in alkenes (refer Unit 13, Class XI), the aldehydes and ketones undergo nucleophilic addition reactions.

 (i) Mechanism of nucleophilic addition reactions

A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of sp2 hybridised orbitals of carbonyl carbon (Fig. 12.2). The hybridisation of carbon changes from sp2 to sp3 in this process, and a tetrahedral alkoxide intermediate is produced. This intermediate captures a proton from the reaction medium to give the electrically neutral product. The net result is addition of Nu– and H+ across the carbon oxygen double bond as shown in Fig. 12.2.

 (ii) Reactivity

Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively than in former.

(iii) Some important examples of nucleophilic addition and nucleophilic addition-elimination reactions:

 (a) Addition of hydrogen cyanide (HCN): Aldehydes and ketones react with hydrogen cyanide (HCN) to yield cyanohydrins. This reaction occurs very slowly with pure HCN. Therefore, it is catalysed by a base and the generated cyanide ion (CN-) being a stronger nucleophile readily adds to carbonyl compounds to yield corresponding cyanohydrin.

Cyanohydrins are useful synthetic intermediates.

 (b) Addition of sodium hydrogensulphite: Sodium hydrogensulphite adds to aldehydes and ketones to form the addition products.

The position of the equilibrium lies largely to the right hand side for most aldehydes and to the left for most ketones due to steric reasons. The hydrogensulphite addition compound is water soluble and can be converted back to the original carbonyl compound by treating it with dilute mineral acid or alkali. Therefore, these are useful for separation and purification of aldehydes.

(c) Addition of Grignard reagents: (refer Unit 11, Class XII).

 (d) Addition of alcohols: Aldehydes react with one equivalent of monohydric alcohol in the presence of dry hydrogen chloride to yield alkoxyalcohol intermediate, known as hemiacetals, which further react with one more molecule of alcohol to give a gem-dialkoxy compound known as acetal as shown in the reaction.

Ketones react with ethylene glycol under similar conditions to form cyclic products known as ethylene glycol ketals.

Dry hydrogen chloride protonates the oxygen of the carbonyl compounds and therefore, increases the electrophilicity of the carbonyl carbon facilitating the nucleophilic attack of ethylene glycol. Acetals and ketals are hydrolysed with aqueous mineral acids to yield corresponding aldehydes and ketones respectively.

 (e) Addition of ammonia and its derivatives: Nucleophiles, such as ammonia and its derivatives H2N-Z add to the carbonyl group of aldehydes and ketones. The reaction is reversible and catalysed by acid. The equilibrium favours the product formation due to rapid dehydration of the intermediate to form >C=N-Z.

Z = Alkyl, aryl, OH, NH2, C6H5NH, NHCONH2, etc.

 (i) Reduction to alcohols: Aldehydes and ketones are reduced to primary and secondary alcohols respectively by sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH4) as well as by catalytic hydrogenation (Unit 11, Class XII).

 (ii) Reduction to hydrocarbons: The carbonyl group of aldehydes and ketones is reduced to CH2 group on treatment with zinc-amalgam and concentrated hydrochloric acid [Clemmensen reduction] or with hydrazine followed by heating with sodium or potassium hydroxide in high boiling solvent such as ethylene glycol (Wolff-Kishner reduction).

Bernhard Tollens (1841-1918) was a Professor of Chemistry at the University of Gottingen, Germany.

3. Oxidation

 Aldehydes differ from ketones in their oxidation reactions. Aldehydes are easily oxidised to carboxylic acids on treatment with common oxidising agents like nitric acid, potassium permanganate, potassium dichromate, etc. Even mild oxidising agents, mainly Tollens’ reagent and Fehlings’ reagent also oxidise aldehydes.

Ketones are generally oxidised under vigorous conditions, i.e., strong oxidising agents and at elevated temperatures. Their oxidation involves carbon-carbon bond cleavage to afford a mixture of carboxylic acids having lesser number of carbon atoms than the parent ketone.

The mild oxidising agents given below are used to distinguish aldehydes from ketones:

 (i) Tollens’ test: On warming an aldehyde with freshly prepared ammoniacal silver nitrate solution (Tollens’ reagent), a bright silver mirror is produced due to the formation of silver metal. The aldehydes are oxidised to corresponding carboxylate anion. The reaction occurs in alkaline medium.

 (ii) Fehling’s test: Fehling reagent comprises of two solutions, Fehling solution A and Fehling solution B. Fehling solution A is aqueous copper sulphate and Fehling solution B is alkaline sodium potassium tartarate (Rochelle salt). These two solutions are mixed in equal amounts before test. On heating an aldehyde with Fehling’s reagent, a reddish brown precipitate is obtained. Aldehydes are oxidised to corresponding carboxylate anion. Aromatic aldehydes do not respond to this test.

 (iii) Oxidation of methyl ketones by haloform reaction: Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom (methyl ketones) are oxidised by sodium hypohalite to sodium salts of corresponding carboxylic acids having one carbon atom less than that of carbonyl compound. The methyl group is converted to haloform. This oxidation does not affect a carbon-carbon double bond, if present in the molecule.

Iodoform reaction with sodium hypoiodite is also used for detection of CH3CO group or CH3CH(OH) group which produces CH3CO group on oxidation.

Example 12.4

An organic compound (A) with molecular formula C8H8O forms an orange-red precipitate with 2,4-DNP reagent and gives yellow precipitate on heating with iodine in the presence of sodium hydroxide. It neither reduces Tollens’ or Fehlings’ reagent, nor does it decolourise bromine water or Baeyer’s reagent. On drastic oxidation with chromic acid, it gives a carboxylic acid (B) having molecular formula C7H6O2. Identify the compounds (A) and (B) and explain the reactions involved. 

Solution

(A) forms 2,4-DNP derivative. Therefore, it is an aldehyde or a ketone. Since it does not reduce Tollens’ or Fehling reagent, (A) must be a ketone. (A) responds to iodoform test. Therefore, it should be a methyl ketone. The molecular formula of (A) indicates high degree of unsaturation, yet it does not decolourise bromine water or Baeyer’s reagent. This indicates the presence of unsaturation due to an aromatic ring.

Compound (B), being an oxidation product of a ketone should be a carboxylic acid. The molecular formula of (B) indicates that it should be benzoic acid and compound (A) should, therefore, be a monosubstituted aromatic methyl ketone. The molecular formula of (A) indicates that it should be phenyl methyl ketone (acetophenone). Reactions are as follows:

 

 

4. Reactions due to a-hydrogen

 Acidity of α-hydrogens of aldehydes and ketones: The aldehydes and ketones undergo a number of reactions due to the acidic nature of α-hydrogen. The acidity of α-hydrogen atoms of carbonyl compounds is due to the strong electron withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base.

 (i) Aldol condensation: Aldehydes and ketones having at least one α-hydrogen undergo a reaction in the presence of dilute alkali as catalyst to form β-hydroxy aldehydes (aldol) or β-hydroxy ketones (ketol), respectively. This is known as Aldol reaction.

The name aldol is derived from the names of the two functional groups, aldehyde and alcohol, present in the products. The aldol and ketol readily lose water to give α,β-unsaturated carbonyl compounds which are aldol condensation products and the reaction is called Aldol condensation. Though ketones give ketols (compounds containing a keto and alcohol groups), the general name aldol condensation still applies to the reactions of ketones due to their similarity with aldehydes.

 (ii) Cross aldol condensation: When aldol condensation is carried out between two different aldehydes and / or ketones, it is called cross aldol condensation. If both of them contain α-hydrogen atoms, it gives a mixture of four products. This is illustrated below by aldol reaction of a mixture of ethanal and propanal.

Ketones can also be used as one component in the cross aldol reactions.

5. Other reactions

 (i) Cannizzaro reaction: Aldehydes which do not have an α-hydrogen atom, undergo self oxidation and reduction (disproportionation) reaction on heating with concentrated alkali. In this reaction, one molecule of the aldehyde is reduced to alcohol while another is oxidised to carboxylic acid salt.

 

 

 

 (ii) Electrophilic substitution reaction: Aromatic aldehydes and ketones undergo electrophilic substitution at the ring in which the carbonyl group acts as a deactivating and meta-directing group.

12.5 Uses of Aldehydes and Ketones

 In chemical industry aldehydes and ketones are used as solvents, starting materials and reagents for the synthesis of other products. Formaldehyde is well known as formalin (40%) solution used to preserve biological specimens and to prepare bakelite (a phenol-formaldehyde resin), urea-formaldehyde glues and other polymeric products. Acetaldehyde is used primarily as a starting material in the manufacture of acetic acid, ethyl acetate, vinyl acetate, polymers and drugs. Benzaldehyde is used in perfumery and in dye industries. Acetone and ethyl methyl ketone are common industrial solvents. Many aldehydes and ketones, e.g., butyraldehyde, vanillin, acetophenone, camphor, etc. are well known for their odours and flavours.

 Carbon compounds containing a carboxyl functional group, –COOH are called carboxylic acids. The carboxyl group, consists of a carbonyl group attached to a hydroxyl group, hence its name carboxyl. Carboxylic acids may be aliphatic (RCOOH) or aromatic (ArCOOH) depending on the group, alkyl or aryl, attached to carboxylic carbon. Large number of carboxylic acids are found in nature. Some higher members of aliphatic carboxylic acids (C12 – C18) known as fatty acids, occur in natural fats as esters of glycerol. Carboxylic acids serve as starting material for several other important organic compounds such as anhydrides, esters, acid chlorides, amides, etc.

12.6 Nomenclature and Structure of Carboxyl Group

12.6.1 Nomenclature

 Since carboxylic acids are amongst the earliest organic compounds to be isolated from nature, a large number of them are known by their common names. The common names end with the suffix –ic acid and have been derived from Latin or Greek names of their natural sources. For example, formic acid (HCOOH) was first obtained from red ants (Latin: formica means ant), acetic acid (CH3COOH) from vinegar (Latin: acetum, means vinegar), butyric acid (CH3CH2CH2COOH) from rancid butter (Latin: butyrum, means butter).

12.6.2 Structure of Carboxyl Group

 In carboxylic acids, the bonds to the carboxyl carbon lie in one plane and are separated by about 120°. The carboxylic carbon is less electrophilic than carbonyl carbon because of the possible resonance structure shown below:

12.7 Methods of Preparation of Carboxylic Acids

Some important methods of preparation of carboxylic acids are as follows.

1. From primary alcohols and aldehydes

 Primary alcohols are readily oxidised to carboxylic acids with common oxidising agents such as potassium permanganate (KMnO4) in neutral, acidic or alkaline media or by potassium dichromate (K2Cr2O7) and chromium trioxide (CrO3) in acidic media (Jones reagent).

 

Carboxylic acids are also prepared from aldehydes by the use of mild oxidising agents (Section 12.4).

2. From alkylbenzenes

 Aromatic carboxylic acids can be prepared by vigorous oxidation of alkyl benzenes with chromic acid or acidic or alkaline potassium permanganate. The entire side chain is oxidised to the carboxyl group irrespective of length of the side chain. Primary and secondary alkyl groups are oxidised in this manner while tertiary group is not affected. Suitably substituted alkenes are also oxidised to carboxylic acids with these oxidising reagents (refer Unit 13, Class XI).

3. From nitriles and amides

 Nitriles are hydrolysed to amides and then to acids in the presence of H+ or as catalyst. Mild reaction conditions are used to stop the reaction at the amide stage.

4. From Grignard reagents

 Grignard reagents react with carbon dioxide (dry ice) to form salts of carboxylic acids which in turn give corresponding carboxylic acids after acidification with mineral acid.

As we know, the Grignard reagents and nitriles can be prepared from alkyl halides (refer Unit 10, Class XII). The above methods
(3 and 4) are useful for converting alkyl halides into corresponding carboxylic acids having one carbon atom more than that present in alkyl halides (ascending the series).

5. From acyl halides and anhydrides

 Acid chlorides when hydrolysed with water give carboxylic acids or more readily hydrolysed with aqueous base to give carboxylate ions which on acidification provide corresponding carboxylic acids. Anhydrides on the other hand are hydrolysed to corresponding acid(s) with water.

6. From esters

 Acidic hydrolysis of esters gives directly carboxylic acids while basic hydrolysis gives carboxylates, which on acidification give corresponding carboxylic acids.

Example 12.5

 Write chemical reactions to affect the following transformations:

(i) Butan-1-ol to butanoic acid

(ii) Benzyl alcohol to phenylethanoic acid

(iii) 3-Nitrobromobenzene to 3-nitrobenzoic acid

(iv) 4-Methylacetophenone to benzene-1,4-dicarboxylic acid

(v) Cyclohexene to hexane-1,6-dioic acid

 

(vi) Butanal to butanoic acid.

Solution

(i)

(ii)

 

(iii)

(iv)

 

(v)

(vi)

12.8 Physical Properties

 Aliphatic carboxylic acids upto nine carbon atoms are colourless liquids at room temperature with unpleasant odours. The higher acids are wax like solids and are practically odourless due to their low volatility. Carboxylic acids are higher boiling liquids than aldehydes, ketones and even alcohols of comparable molecular masses. This is due to more extensive association of carboxylic acid molecules through intermolecular hydrogen bonding. The hydrogen bonds are not broken completely even in the vapour phase. In fact, most carboxylic acids exist as dimer in the vapour phase or in the aprotic solvents.

In vapour state or in aprotic solvent

Simple aliphatic carboxylic acids having upto four carbon atoms are miscible in water due to the formation of hydrogen bonds with water. The solubility decreases with increasing number of carbon atoms. Higher carboxylic acids are practically insoluble in water due to the increased hydrophobic interaction of hydrocarbon part. Benzoic acid, the simplest aromatic carboxylic acid is nearly insoluble in cold water. Carboxylic acids are also soluble in less polar organic solvents like benzene, ether, alcohol, chloroform, etc.

Hydrogen bonding of RCOOH with H₂O

12.7 Methods of Preparation of Carboxylic Acids

Some important methods of preparation of carboxylic acids are as follows.

1. From primary alcohols and aldehydes

 Primary alcohols are readily oxidised to carboxylic acids with common oxidising agents such as potassium permanganate (KMnO4) in neutral, acidic or alkaline media or by potassium dichromate (K2Cr2O7) and chromium trioxide (CrO3) in acidic media (Jones reagent).

 

Carboxylic acids are also prepared from aldehydes by the use of mild oxidising agents (Section 12.4).

2. From alkylbenzenes

 Aromatic carboxylic acids can be prepared by vigorous oxidation of alkyl benzenes with chromic acid or acidic or alkaline potassium permanganate. The entire side chain is oxidised to the carboxyl group irrespective of length of the side chain. Primary and secondary alkyl groups are oxidised in this manner while tertiary group is not affected. Suitably substituted alkenes are also oxidised to carboxylic acids with these oxidising reagents (refer Unit 13, Class XI).

3. From nitriles and amides

 Nitriles are hydrolysed to amides and then to acids in the presence of H+ or as catalyst. Mild reaction conditions are used to stop the reaction at the amide stage.

4. From Grignard reagents

 Grignard reagents react with carbon dioxide (dry ice) to form salts of carboxylic acids which in turn give corresponding carboxylic acids after acidification with mineral acid.

As we know, the Grignard reagents and nitriles can be prepared from alkyl halides (refer Unit 10, Class XII). The above methods
(3 and 4) are useful for converting alkyl halides into corresponding carboxylic acids having one carbon atom more than that present in alkyl halides (ascending the series).

5. From acyl halides and anhydrides

 Acid chlorides when hydrolysed with water give carboxylic acids or more readily hydrolysed with aqueous base to give carboxylate ions which on acidification provide corresponding carboxylic acids. Anhydrides on the other hand are hydrolysed to corresponding acid(s) with water.

6. From esters

 Acidic hydrolysis of esters gives directly carboxylic acids while basic hydrolysis gives carboxylates, which on acidification give corresponding carboxylic acids.

Example 12.5

 Write chemical reactions to affect the following transformations:

(i) Butan-1-ol to butanoic acid

(ii) Benzyl alcohol to phenylethanoic acid

(iii) 3-Nitrobromobenzene to 3-nitrobenzoic acid

(iv) 4-Methylacetophenone to benzene-1,4-dicarboxylic acid

(v) Cyclohexene to hexane-1,6-dioic acid

 

(vi) Butanal to butanoic acid.

Solution

(i)

(ii)

 

(iii)

(iv)

 

(v)

(vi)

12.8 Physical Properties

 Aliphatic carboxylic acids upto nine carbon atoms are colourless liquids at room temperature with unpleasant odours. The higher acids are wax like solids and are practically odourless due to their low volatility. Carboxylic acids are higher boiling liquids than aldehydes, ketones and even alcohols of comparable molecular masses. This is due to more extensive association of carboxylic acid molecules through intermolecular hydrogen bonding. The hydrogen bonds are not broken completely even in the vapour phase. In fact, most carboxylic acids exist as dimer in the vapour phase or in the aprotic solvents.

In vapour state or in aprotic solvent

Simple aliphatic carboxylic acids having upto four carbon atoms are miscible in water due to the formation of hydrogen bonds with water. The solubility decreases with increasing number of carbon atoms. Higher carboxylic acids are practically insoluble in water due to the increased hydrophobic interaction of hydrocarbon part. Benzoic acid, the simplest aromatic carboxylic acid is nearly insoluble in cold water. Carboxylic acids are also soluble in less polar organic solvents like benzene, ether, alcohol, chloroform, etc.

Hydrogen bonding of RCOOH with H₂O

12.9 Chemical Reactions

The reaction of carboxylic acids are classified as follows:

12.9.1 Reactions Involving Cleavage of O–H Bond

Acidity

 The carboxylic acids like alcohols evolve hydrogen with electropositive metals and form salts with alkalies similar to phenols. However, unlike phenols they react with weaker bases such as carbonates and hydrogencarbonates to evolve carbon dioxide. This reaction is used to detect the presence of carboxyl group in an organic compound.

where Keq, is equilibrium constant and Ka is the acid dissociation constant.

For convenience, the strength of an acid is generally indicated by its pka value rather than its Ka value.

pKa = – log Ka

The pKa of hydrochloric acid is –7.0, where as pKa of trifluoroacetic acid (the strongest carboxylic acid), benzoic acid and acetic acid are 0.23, 4.19 and 4.76, respectively.

Smaller the pKa, the stronger the acid ( the better it is as a proton donor). Strong acids have pKa values < 1, the acids with pKa values between 1 and 5 are considered to be moderately strong acids, weak acids have pKa values between 5 and 15, and extremely weak acids have pKa values >15.

 

 Effect of substituents on the acidity of carboxylic acids: Substituents may affect the stability of the conjugate base and thus, also affect the acidity of the carboxylic acids. Electron withdrawing groups increase the acidity of carboxylic acids by stabilising the conjugate base through delocalisation of the negative charge by inductive and/or resonance effects. Conversely, electron donating groups decrease the acidity by destabilising the conjugate base.

 

 

 

Electron withdrawing group (EWG) stabilises the carboxylate anion and strengthens the acid

 

Electron donating group (EDG) destabilises the carboxylate anion and weakens the acid

 The effect of the following groups in increasing acidity order is

Ph < I < Br < Cl < F < CN < NO2 < CF3

Thus, the following acids are arranged in order of increasing acidity (based on pKa values):

CF3COOH > CCl3COOH > CHCl2COOH > NO2CH2COOH > NC-CH2COOH >

FCH2COOH > ClCH2COOH > BrCH2COOH > HCOOH > ClCH2CH2COOH >

(continue)

C6H5COOH > C6H5CH2COOH > CH3COOH > CH3CH2COOH

(continue )

 Direct attachment of groups such as phenyl or vinyl to the carboxylic acid, increases the acidity of corresponding carboxylic acid, contrary to the decrease expected due to resonance effect shown below:

 This is because of greater electronegativity of sp2 hybridised carbon to which carboxyl carbon is attached. The presence of electron withdrawing group on the phenyl of aromatic carboxylic acid increases their acidity while electron donating groups decrease their acidity.

12.9.2 Reactions Involving Cleavage of C–OH Bond

1. Formation of anhydride

 Carboxylic acids on heating with mineral acids such as H2SO4 or with P2O5 give corresponding anhydride.

2. Esterification

Carboxylic acids are esterified with alcohols or phenols in the presence of a mineral acid such as concentrated H2SO4 or HCl gas as a catalyst.

3. Reactions with PCl5, PCl3 and SOCl2

 The hydroxyl group of carboxylic acids, behaves like that of alcohols and is easily replaced by chlorine atom on treating with PCl5, PCl3 or SOCl2. Thionyl chloride (SOCl2) is preferred because the other two products are gaseous and escape the reaction mixture making the purification of the products easier.

4. Reaction with ammonia

 Carboxylic acids react with ammonia to give ammonium salt which on further heating at high temperature give amides. For example:

12.9.3 Reactions Involving –COOH Group

1. Reduction

 Carboxylic acids are reduced to primary alcohols by lithium aluminium hydride or better with diborane. Diborane does not easily reduce functional groups such as ester, nitro, halo, etc. Sodium borohydride does not reduce the carboxyl group.

 

2. Decarboxylation

 Carboxylic acids lose carbon dioxide to form hydrocarbons when their sodium salts are heated with sodalime (NaOH and CaO in the ratio of 3 : 1). The reaction is known as decarboxylation.

Alkali metal salts of carboxylic acids also undergo decarboxylation on electrolysis of their aqueous solutions and form hydrocarbons having twice the number of carbon atoms present in the alkyl group of the acid. The reaction is known as Kolbe electrolysis (Unit 13, Class XI).

12.9.4. Substitution Reactions in the Hydrocarbon Part

1. Halogenation

 Carboxylic acids having an α-hydrogen are halogenated at the α-position on treatment with chlorine or bromine in the presence of small amount of red phosphorus to give α-halocarboxylic acids. The reaction is known as Hell-Volhard-Zelinsky reaction.

 

 Aromatic carboxylic acids undergo electrophilic substitution reactions in which the carboxyl group acts as a deactivating and meta-directing group. They however, do not undergo Friedel-Crafts reaction (because the carboxyl group is deactivating and the catalyst aluminium chloride (Lewis acid) gets bonded to the carboxyl group).

12.10 Uses of Carboxylic Acids

 Methanoic acid is used in rubber, textile, dyeing, leather and electroplating industries. Ethanoic acid is used as solvent and as vinegar in food industry. Hexanedioic acid is used in the manufacture of nylon-6, 6. Esters of benzoic acid are used in perfumery. Sodium benzoate is used as a food preservative. Higher fatty acids are used for the manufacture of soaps and detergents.

Exercises

Answers to Some Intext Questions

12.1

12.3 CH3CH2CH3 < CH3OCH3 < CH3CHO < CH3CH2OH

12.4 (i) Butanone < Propanone < Propanal < Ethanal

(ii) Acetophenone < p-Tolualdehyde , Benzaldehyde < p-Nitrobenzaldehyde.

12.5

12.6 (i) 3-Phenylpropanoic acid (ii) 3-Methylbut-2-enoic acid

(iii) 2-Methylcyclopentanecarboxylic acid. (iv) 2,4,6-Trinitrobenzoic acid

12.7 

12.8

Exemplar Questions

Q. 1 Addition of water to alkynes occurs in acidic medium and in the presence of ${\mathrm{Hg}}^{2+}$ ions as a catalyst. Which of the following products will be formed on addition of water to but-1-yne under these conditions?

$$\begin{gathered} \left(\mathrm{a}\right) {\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2-\stackrel{\mathrm{O} \\ \left|\right|}{\mathrm{C}}-\mathrm{H} \end{gathered}$$

$$\begin{gathered} \left(\mathrm{b}\right) {\mathrm{CH}}_3-{\mathrm{CH}}_2-\stackrel{\mathrm{O} \\ \left|\right|}{\mathrm{C}}-{\mathrm{CH}}_3 \end{gathered}$$

$$\begin{gathered} \left(\mathrm{c}\right) {\mathrm{CH}}_3-{\mathrm{CH}}_2-\stackrel{\mathrm{O} \\ \left|\right|}{\mathrm{C}}-\mathrm{OH} + {\mathrm{CO}}_2 \end{gathered}$$

$$\begin{gathered} \left(\mathrm{d}\right) {\mathrm{CH}}_3-\stackrel{\mathrm{O} \\ \left|\right|}{\mathrm{C}}-\mathrm{OH} + \mathrm{H}-\stackrel{\mathrm{O} \\ \left|\right|}{\mathrm{C}}-\mathrm{H} \end{gathered}$$

Q. 2 Which of the following compounds is most reactive towards nucleophilic addition reactions?
$$\begin{gathered} \left(\mathrm{a}\right) C{H}_3-\stackrel{\mathrm{O} \\ \left|\right|}{\mathrm{C}}-\mathrm{H} \left(\mathrm{b}\right) {\mathrm{CH}}_3-\stackrel{\mathrm{O} \\ \left|\right|}{\mathrm{C}}-{\mathrm{CH}}_3 \end{gathered}$$

Q. 3 The correct order of increasing acidic strength is
(a) phenol < ethanol < chloroacetic acid < acetic acid
(b) ethanol < phenol < chloroacetic acid < acetic acid
(c) ethanol < phenol < acetic acid < chloroacetic acid
(d) chloroacetic acid < acetic acid < phenol < ethanol

Q.4 Compound$$\begin{gathered} \mathrm{O} \\ \left|\right| \\ \mathrm{Ph}-\mathrm{O}-\mathrm{C}-\mathrm{Ph} \end{gathered}$$can  be prepared by the reaction of ..
(a) phenol and benzoic acid in the presence of NaOH
(b) phenol and benzoyl chloride in the presence ot pyridine
(c) phenol and benzoyl chloride in the presence of ${\mathrm{ZnCl}}_2$
(d) phenol and benzaldehyde in the presence of palladium

Q. 5 The reagent which does not react with both, acetone and benzaldehyde?
(a) Sodium hydrogen sulphite     (b) Phenyl hydrazine
(c)  Fehling's solution                  (d) Grignard reagent

Q. 6 Cannizzaro’s reaction is not given by ?

(c) HCHO                             (d) ${\mathrm{CH}}_3\mathrm{CHO}$.

Q. 7 Which products is formed when the compound is treated with concentrated aqueous KOH solution?

(a) 

(b) 

(c) 

(d) 

Q.8.  $$\begin{gathered} {\mathrm{CH}}_3-\mathrm{C}\equiv \mathrm{CH} \underset{1\%{\mathrm{HgSO}}_4}{\overset{40\%{\mathrm{H}}_2{\mathrm{SO}}_4}{\to }}\mathrm{A} \stackrel{\mathrm{Isomerisation}}{\to }{\mathrm{CH}}_3-\underset{\left|\right| \\ \mathrm{O}}{\mathrm{C}}-{\mathrm{CH}}_3 \end{gathered}$$

Structure of ‘A’ and type of isomerism in the above reaction are respectively
(a) Prop-1-en-2-ol, metamerism
(b) Prop-1-en-1-ol, tautomerism
(c) Prop-2-en-2-ol, geometrical isomerism
(d) Prop-1-en-2-ol, tautomerism

Q. 9 Compounds A and C in the following reaction are ?
 $C{H}_{3}CHO\underset{\left(ii\right) H_{2}0}{\overset{\left(i\right) C{H}_{3}MgBr}{\to }} \left(A\right) \stackrel{H_{2}S{0}_4}{\to }\left(B\right) \stackrel{Hydroboration oxidation}{\to }\left(C\right)$

(a) identical                                (b) positional isomers
(c) functional isomers                (d) optical isomers

Q. 10 Which is the most suitable reagent for the following conversion?

$$\begin{gathered} {\mathrm{CH}}_3—\mathrm{CH}=\mathrm{CH}—{\mathrm{CH}}_2—\stackrel{\mathrm{O} \\ \left|\right|}{\mathrm{C}}—{\mathrm{CH}}_3—\mathrm{CH}=\mathrm{CH}—{\mathrm{CH}}_2—\stackrel{\mathrm{O} \\ \left|\right|}{\mathrm{C}}—\mathrm{OH} \end{gathered}$$

(a) Tollen’s reagent 
(b) Benzoyl peroxide

(c) ${\mathrm{I}}_2$ and NaOH solution

(d) Sn and NaOH solution

Exemplar Questions

Q. 11 Which of the following compounds will give butanone on oxidation with alkaline ${\mathrm{KMnO}}_4$ solution?

(a) Butan-1-o1                        (b) Butan-2-ol
(c) Both (a) and (b)                (d) None of these

Q. 12 In Clemmensen reduction, carbonyl compound is treated with ....
(a) zinc amalgam + HCL
(b) sodium amalgam + HCI
(c) zinc amalgam + nitric acid
(d) sodium amalgam + ${\mathrm{HNO}}_3$

Q. 13 Which of the following compounds do not undergo aldol condensation?

Q. 14 Treatment of compound $$\begin{gathered} 0 \\ \left|\right| \\ Ph—0—C—PH \end{gathered}$$with NaOH solution yields
(a) phenol ‘
(b) sodium phenoxide
(c) sodium benzoate
(d) benzophenone

Q. 15 Which of the following conversions can be carried out by Clemmensen reduction?
(a) Benzaldehyde into benzyl alcohol
(b) Cyclohexanone into cyclohexane
(c) Benzoyl chloride into benzaldehyde
(d) Benzophenone into diphenyl methane

Q. 16 Through which of the following reactions number of carbon atoms can be increased in the chain?
(a) Grignard reaction                (b) Cannizzaro’s reaction
(c) Aldol condensation             (d) HVZ reaction

Q. 17 Benzophenone can be obtained by .......... .
(a) benzoyl chloride + benzene + 
(b) benzoyl chloride + diphenyl cadmium
(c) benzoyl chloride + phenyl magnesium chloride
(d) benzene + carbon monoxide + 

Q. 18 Which of the following is the correct representation for  intermediate of nucleophilic addition reaction to the given carbonyl compound (A)?                                                    

Q. 19 Why is there a large difference in the boiling points of butanal and butan-1-ol?

Q. 20 Write a test to differentiate between pentan-2-one and pentan-3-one.

Exemplar Questions

Q. 21 Give the IUPAC names of the following compounds.

Q. 22 Give the structure of the following compounds.
(a) 4-nitropropiophenone
(b) 2-hydroxycyclopentanecarbaldehyde
(c) Phenyl acetaldehyde

Q. 23 Write IUPAC names of the following structures

Q. 24 Benzaldehyde can be obtained from benzal chloride, Write reactions for obtaining benzal chloride and then benzaldehyde from it.

Q. 25 Name the electrophile produced in the reaction of benzene with benzoyl chloride in the presence of anhydrous . Name the reaction also.

Q. 26 Oxidation of ketones involves carbon-carbon bond cleavage. Name the products formed on oxidation of 2, 5-dimethylhexan-3-one.

Q. 27 Arrange the following in decreasing order of their acidic strength and give reason for your answer.

$C{H}_{3}C{H}_{3}OH, C{H}_{3}COO{H}_{2}ClC{H}_{2}COOH,FC{H}_{2}COOH, C_6{H}_{5}C{H}_{2}COOH$

Q. 28 What product will be formed on reaction of propanal with 2-methylpropanal in the presence of NaOH? What products will be formed? Write the name of the reaction also.

Q. 29 Compound ‘A’ was prepared by oxidation of compound ‘B’ with alkaline KMn0, Compound ‘A’ on reduction with lithium aluminium hydride gets converted back to compound ‘B'. When compound ‘A’ is heated with compound 'B' in the presence of, it produces a fruity smell of compound C to which family the compounds ‘A, ‘B’ and ‘C’ belongs to?

Q. 30 Arrange the following in decreasing order of their acidic strength. Give an explanation for the arrangement.

Exemplar Questions

Exemplar Question