After studying this Unit, you will be

able to

• write the common and IUPAC

names of aldehydes, ketones and

carboxylic acids;

• write the structures of the

compounds containing functional

groups namely carbonyl and

carboxyl groups;

• describe the important methods

of preparation and reactions of

these classes of compounds;

• correlate physical properties and

chemical reactions of aldehydes,

ketones and carboxylic acids,

with their structures;

• explain the mechanism of a few

selected reactions of aldehydes

and ketones;

• understand various factors

affecting the acidity of carboxylic

acids and their reactions;

• describe the uses of aldehydes,

ketones and carboxylic acids.

Objectives

Carbonyl compounds are of utmost importance to organic

chemistry. They are constituents of fabrics, flavourings, plastics

and drugs.

Unit

Aldehydes

tonestones

tones

andand

and

CarboxylicCarboxylic

Carboxylic

cidscids

cids

AldehydesAldehydes

Aldehydes

tonestones

tones

andand

and

CarboxylicCarboxylic

Carboxylic

cidscids

cids

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 -NH

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:

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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.

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

12.1

12.112.1

12.1

Nomenclature and Structure of Carbonyl GroupNomenclature and Structure of Carbonyl Group

Nomenclature and Structure of Carbonyl Group

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.

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359 Aldehydes, Ketones and Carboxylic Acids

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.

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360Chemistry

Aldehydes

HCHO Formaldehyde Methanal

CHO Acetaldehyde Ethanal

(CH

)

CHCHO Isobutyraldehyde 2-Methylpropanal

γ-Methylcyclohexanecarbaldehyde 3-Methylcyclohexanecarbaldehyde

CH(OCH

)CHO α-Methoxypropionaldehyde 2-Methoxypropanal

CHO Valeraldehyde Pentanal

=CHCHO Acrolein Prop-2-enal

Phthaldehyde Benzene-1,2-dicarbaldehyde

m-Bromobenzaldehyde 3-Bromobenzaldehyde

Ketones

COCH

Methyl n-propyl ketone Pentan-2-one

(CH

)

CHCOCH(CH

)

Diisopropyl ketone 2,4-Dimethylpentan-3-one

α-Methylcyclohexanone 2-Methylcyclohexanone

(CH

)

C=CHCOCH

Mesityl oxide 4-Methylpent-3-en-2-one

Table 12.1: Common and IUPAC Names of Some Aldehydes and Ketones

Structure Common name IUPAC name

The common and IUPAC names of some aldehydes and ketones are

given in Table 12.1.

3-Bromobenzenecarbaldehyde

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361 Aldehydes, Ketones and Carboxylic Acids

The carbonyl carbon atom is sp

-hybridised and forms three sigma (σ)

bonds. The fourth valence electron of carbon remains in its p-orbital

and forms a π-bond with oxygen by overlap with p-orbital of an oxygen.

In addition, the oxygen atom also has two non bonding electron pairs.

Thus, the carbonyl carbon and the three atoms attached to it lie in the

same plane and the π-electron cloud is above and below this plane. The

bond angles are approximately 120° as expected of a trigonal coplanar

structure (Figure 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 QuestionsIntext Questions

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

Some important methods for the preparation of aldehydes

and ketones are as follows:

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,

12.2.1 Preparation

Aldehydes

and

Ketones

12.212.2

12.2

Preparation of AldehydesPreparation of Aldehydes

Preparation of Aldehydes

and Ketonesand Ketones

and Ketones

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362Chemistry

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 H

and

HgSO

gives acetaldehyde. All other

alkynes give ketones in this reaction (Unit 13, Class XI).

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:

12.2.2 Preparation

Aldehydes

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

3. From hydrocarbons

Aromatic aldehydes (benzaldehyde and its derivatives) are prepared

from aromatic hydrocarbons by the following methods:

(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 (CrO

): Chromyl chloride oxidises

methyl group to a chromium complex, which on hydrolysis

gives corresponding benzaldehyde.

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363 Aldehydes, Ketones and Carboxylic Acids

This reaction is called Etard reaction.

(b) Use of chromic oxide (CrO

): 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.

(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.

(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.

This reaction is known as Gatterman-Koch reaction.

1. From acyl chlorides

Treatment of acyl chlorides with dialkylcadmium, prepared by the

reaction of cadmium chloride with Grignard reagent, gives ketones.

12.2.3 Preparation

of Ketones

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364Chemistry

2. From nitriles

Treating a nitrile with Grignard reagent followed by hydrolysis yields

a ketone.

Give names of the reagents to bring about the following

transformations:

(i) Hexan-1-ol to hexanal (ii) Cyclohexanol to cyclohexanone

(iii) p-Fluorotoluene to (iv) Ethanenitrile to ethanal

p-fluorobenzaldehyde

(v) Allyl alcohol to propenal (vi) But-2-ene to ethanal

(i) C

CrO

(PCC) (ii) Anhydrous CrO

(iii) CrO

in the presence (iv) (Diisobutyl)aluminium

of acetic anhydride/ hydride (DIBAL-H)

1. CrO

2. HOH

(v) PCC (vi) O

O-Zn dust

Example 12.1Example 12.1

Example 12.1

SolutionSolution

Solution

H CH ) Cd + 2 CH

5 2 2 3

COCl

1.CrO Cl

2 2

2.H

(iii)

C C H

, H SO

2 4

H C

(iv)

Intext QuestionIntext Question

Intext Question

12.2 Write the structures of products of the following reactions;

(i) (ii)

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.

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365 Aldehydes, Ketones and Carboxylic Acids

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.

b.p.(K) Molecular Mass

n-Butane 273 58

Methoxyethane 281 60

Propanal 322 58

Acetone 329 58

Propan-1-ol 370 60

The lower members of aldehydes and ketones such as methanal,

ethanal and propanone are miscible with water in all proportions,

because they form hydrogen bond with water.

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.

12.3 Physical12.3 Physical

12.3 Physical

PropertiesProperties

Properties

Arrange the following compounds in the increasing order of their

boiling points:

CHO, CH

OH, H

-O-C

, CH

The molecular masses of these compounds are in the range of 72 to

74. Since only butan-1-ol molecules are associated due to extensive

intermolecular hydrogen bonding, therefore, the boiling point of

butan-1-ol would be the highest. Butanal is more polar than

ethoxyethane. Therefore, the intermolecular dipole-dipole attraction

is stronger in the former. n-Pentane molecules have only weak van

der Waals forces. Hence increasing order of boiling points of the

given compounds is as follows:

< H

-O-C

< CH

CHO < CH

Example 12.2Example 12.2

Example 12.2

SolutionSolution

Solution

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366Chemistry

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 sp

hybridised orbitals of carbonyl carbon (Fig. 12.2).

The hybridisation of carbon changes from sp

to sp

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

–

and H

across the

carbon oxygen double bond

as shown in Fig. 12.2.

Intext QuestionIntext Question

Intext Question

12.3 Arrange the following compounds in increasing order of

their boiling points.

CHO, CH

OH, CH

OCH

, CH

Fig.12.2: Nucleophilic attack on carbonyl carbon

Would you expect benzaldehyde to be more reactive or less reactive in

nucleophilic addition reactions than propanal? Explain your answer.

The carbon atom of the carbonyl group of benzaldehyde is less

electrophilic than carbon atom of the carbonyl group present in

propanal. The polarity of the carbonyl

group is reduced in benzaldehyde due

to resonance as shown below and

hence it is less reactive than propanal.

Example 12.3Example 12.3

Example 12.3

SolutionSolution

Solution

(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.

12.4 Chemical12.4 Chemical

12.4 Chemical

ReactionsReactions

Reactions

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367 Aldehydes, Ketones and Carboxylic Acids

(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.

(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

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368Chemistry

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 H

N-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, NH

, C

NH, NHCONH

, etc.

Table 12.2: Some N-Substituted Derivatives of Aldehydes and Ketones (>C=N-Z)

-H Ammonia Imine

-R Amine

—OH Hydroxylamine Oxime

—NH

Hydrazine Hydrazone

Phenylhydrazine Phenylhydrazone

Z Reagent name Carbonyl derivative Product name

Substituted imine

(Schiff’s base)

* 2,4-DNP-derivatives are yellow, orange or red solids, useful for characterisation of aldehydes and ketones.

2,4-Dinitrophenyl- 2,4 Dinitrophenyl-

Semicarbazide Semicarbazone

2. Reduction

(i) Reduction to alcohols: Aldehydes and ketones are reduced to

primary and secondary alcohols respectively by sodium

borohydride (NaBH

) or lithium aluminium hydride (LiAlH

) as

well as by catalytic hydrogenation (Unit 11, Class XII).

(ii) Reduction to hydrocarbons: The carbonyl group of aldehydes

and ketones is reduced to CH

group on treatment with zinc-

amalgam and concentrated hydrochloric acid [Clemmensen

hydrazonehydrazine

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369 Aldehydes, Ketones and Carboxylic Acids

reduction] or with hydrazine followed by heating with sodium

or potassium hydroxide in high boiling solvent such as ethylene

glycol (Wolff-Kishner reduction).

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.

Bernhard Tollens

(1841-1918) was a

Professor of Chemistry

at the University of

Gottingen, Germany.

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Example 12.4Example 12.4

Example 12.4

An organic compound (A) with molecular formula C

O 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 C

. Identify the compounds (A) and (B) and explain the

reactions involved.

(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:

SolutionSolution

Solution

(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 CH

CO group or CH

CH(OH) group which produces CH

CO group

on oxidation.

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371 Aldehydes, Ketones and Carboxylic Acids

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.

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(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.

∆

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373 Aldehydes, Ketones and Carboxylic Acids

(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.

Intext QuestionsIntext Questions

Intext Questions

12.4 Arrange the following compounds in increasing order of their reactivity in

nucleophilic addition reactions.

(i) Ethanal, Propanal, Propanone, Butanone.

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

Hint: Consider steric effect and electronic effect.

12.5 Predict the products of the following reactions:

(i)

(ii)

(iii)

(iv)

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.

12.512.5

12.5

Uses ofUses of

Uses of

AldehydesAldehydes

Aldehydes

and Ketonesand Ketones

and Ketones

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374Chemistry

Structure Common name IUPAC name

HCOOH Formic acid Methanoic acid

COOH Acetic acid Ethanoic acid

COOH Propionic acid Propanoic acid

COOH Butyric acid Butanoic acid

(CH

)

CHCOOH Isobutyric acid 2-Methylpropanoic acid

HOOC-COOH Oxalic acid Ethanedioic acid

HOOC -CH

-COOH Malonic acid Propanedioic acid

HOOC -(CH

)

-COOH Succinic acid Butanedioic acid

HOOC -(CH

)

-COOH Glutaric acid Pentanedioic acid

HOOC -(CH

)

-COOH Adipic acid Hexanedioic acid

HOOC -CH

-CH(COOH)-CH

-COOH Tricarballylic acid Propane-1, 2, 3-

or carballylic acid tricarboxylic acid

Carboxylic Acids

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 (C

– C

) 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.

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 (CH

COOH) from vinegar (Latin:

acetum, means vinegar), butyric acid (CH

COOH) from rancid

butter (Latin: butyrum, means butter).

In the IUPAC system, aliphatic carboxylic acids are named by

replacing the ending –e in the name of the corresponding alkane with –

oic acid. In numbering the carbon chain, the carboxylic carbon is

numbered one. For naming compounds containing more than one

carboxyl group, the alkyl chain leaving carboxyl groups is numbered

and the number of carboxyl groups is indicated by adding the

multiplicative prefix, dicarboxylic acid, tricarboxylic acid, etc. to the name

of parent alkyl chain. The position of –COOH groups are indicated by the

arabic numeral before the multiplicative prefix. Some of the carboxylic

acids along with their common and IUPAC names are listed in Table 12.3.

12.612.6

12.6

Nomenclature and Structure of Carboxyl GroupNomenclature and Structure of Carboxyl Group

Nomenclature and Structure of Carboxyl Group

Table 12.3 Names and Structures of Some Carboxylic Acids

12.6.1

Nomenclature

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375 Aldehydes, Ketones and Carboxylic Acids

Benzoic acid Benzenecarboxylic acid

(Benzoic acid)

Phenylacetic acid 2-Phenylethanoic acid

Phthalic acid Benzene-1, 2-dicarboxylic

acid

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:

12.6.2 Structure

of Carboxyl

Group

Intext QuestionIntext Question

Intext Question

12.6 Give the IUPAC names of the following compounds:

(i) Ph CH

COOH (ii) (CH

)

C=CHCOOH

(iii)

COOH

(iv)

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 (KMnO

) in

neutral, acidic or alkaline media or by potassium dichromate (K

)

and chromium trioxide (CrO

) in acidic media (Jones reagent).

12.712.7

12.7

Methods ofMethods of

Methods of

PreparationPreparation

Preparation

of Carboxylicof Carboxylic

of Carboxylic

AcidsAcids

Acids

Jones reagent

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376Chemistry

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

−

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

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377 Aldehydes, Ketones and Carboxylic Acids

(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 12.5Example 12.5

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.

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378Chemistry

Intext QuestionIntext Question

Intext Question

12.7 Show how each of the following compounds can be

converted to benzoic acid.

(i) Ethylbenzene (ii) Acetophenone

(iii) Bromobenzene (iv) Phenylethene (Styrene)

SolutionSolution

Solution

(i)

(ii)

(iii)

(iv)

(v)

(vi)

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379 Aldehydes, Ketones and Carboxylic Acids

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.

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.

The reaction of carboxylic acids are classified as follows:

Acidity

Reactions with metals and alkalies

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.

Carboxylic acids dissociate in water to give resonance stabilised

carboxylate anions and hydronium ion.

12.9.1 Reactions

Involving

Cleavage of

O–H Bond

12.812.8

12.8

PhysicalPhysical

Physical

PropertiesProperties

Properties

12.912.9

12.9

Chemical ReactionsChemical Reactions

Chemical Reactions

In vapour state or in

aprotic solvent

Hydrogen bonding of

RCOOH with H

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380Chemistry

where K

, is equilibrium constant and K

is the acid dissociation

constant.

For convenience, the strength of an acid is generally indicated by

its pK

value rather than its K

value.

= – log K

The pK

of hydrochloric acid is –7.0, where as pK

of trifluoroacetic

acid (the strongest carboxylic acid), benzoic acid and acetic acid are

0.23, 4.19 and 4.76, respectively.

Smaller the pK

, the stronger the acid ( the better it is as a proton

donor). Strong acids have pK

values < 1, the acids with pK

values

between 1 and 5 are considered to be moderately strong acids, weak

acids have pK

values between 5 and 15, and extremely weak acids

have pK

values >15.

Carboxylic acids are weaker than mineral acids, but they are stronger

acids than alcohols and many simple phenols (pK

is ~16 for ethanol

and 10 for phenol). In fact, carboxylic acids are amongst the most acidic

organic compounds you have studied so far. You already know why

phenols are more acidic than alcohols. The higher acidity of carboxylic

acids as compared to phenols can be understood similarly. The conjugate

base of carboxylic acid, a carboxylate ion, is stabilised by two equivalent

resonance structures in which the negative charge is at the more

electronegative oxygen atom. The conjugate base of phenol, a phenoxide

ion, has non-equivalent resonance structures in which the negative charge

is at the less electronegative carbon atom. Therefore, resonance in

phenoxide ion is not as important as it is in carboxylate ion. Further, the

negative charge is delocalised over two electronegative oxygen atoms in

carboxylate ion whereas it is less effectively delocalised over one oxygen

atom and less electronegative carbon atoms in phenoxide ion (Unit 11,

Class XII). Thus, the carboxylate ion is more stabilised than phenoxide

ion, so carboxylic acids are more acidic than phenols.

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

For the above reaction:

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381 Aldehydes, Ketones and Carboxylic Acids

The effect of the following groups in increasing acidity order is

Ph < I < Br < Cl < F < CN < NO

< CF

Thus, the following acids are arranged in order of increasing acidity

(based on pK

values):

COOH > CCl

COOH > CHCl

COOH > NO

COOH > NC-CH

COOH >

FCH

COOH > ClCH

COOH > BrCH

COOH > HCOOH > ClCH

COOH >

(continue)

COOH > C

COOH > CH

COOH

(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 sp

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.

COOH

OCH

COOH

4-Methoxy

benzoic acid

= 4.46)K

Benzoic acid

(p = 4.19)K

4-Nitrobenzoic

acid

(p = 3.41)K

1. Formation of anhydride

Carboxylic acids on heating with mineral acids such as H

or with

give corresponding anhydride.

2. Esterification

Carboxylic acids are esterified with alcohols or phenols in the presence

of a mineral acid such as concentrated H

or HCl gas as a catalyst.

12.9.2 Reactions

Involving

Cleavage of

C–OH Bond

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382Chemistry

Mechanism of esterification of carboxylic acids: The esterification of carboxylic

acids with alcohols is a kind of nucleophilic acyl substitution. Protonation of the

carbonyl oxygen activates the carbonyl group towards nucleophilic addition of the

alcohol. Proton transfer in the tetrahedral intermediate converts the hydroxyl group

into –

group, which, being a better leaving group, is eliminated as neutral water

molecule. The protonated ester so formed finally loses a proton to give the ester.

3. Reactions with PCl

, PCl

and SOCl

The hydroxyl group of carboxylic acids, behaves like that of alcohols

and is easily replaced by chlorine atom on treating with PCl

, PCl

SOCl

. Thionyl chloride (SOCl

) 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:

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383 Aldehydes, Ketones and Carboxylic Acids

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).

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.

12.9.4

Substitution

Reactions in the

Hydrocarbon Part

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384Chemistry

12.1012.10

12.10

Uses ofUses of

Uses of

Carboxylic

CarboxylicCarboxylic

Carboxylic

AcidsAcids

Acids

SummarySummary

Summary

Aldehydes, ketones and carboxylic acids are some of the important classes of

organic compounds containing carbonyl group. These are highly polar molecules.

Therefore, they boil at higher temperatures than the hydrocarbons and weakly

polar compounds such as ethers of comparable molecular masses. The lower

members are more soluble in water because they form hydrogen bonds with water.

The higher members, because of large size of hydrophobic chain of carbon atoms,

are insoluble in water but soluble in common organic solvents. Aldehydes are

prepared by dehydrogenation or controlled oxidation of primary alcohols and

controlled or selective reduction of acyl halides. Aromatic aldehydes may also be

prepared by oxidation of (i) methylbenzene with chromyl chloride or CrO

in the

presence of acetic anhydride, (ii) formylation of arenes with carbon monoxide and

hydrochloric acid in the presence of anhydrous aluminium chloride, and (iii) cuprous

chloride or by hydrolysis of benzal chloride. Ketones are prepared by oxidation of

secondary alcohols and hydration of alkynes. Ketones are also prepared by reaction

of acyl chloride with dialkylcadmium. A good method for the preparation of aromatic

ketones is the Friedel-Crafts acylation of aromatic hydrocarbons with acyl chlorides

or anhydrides. Both aldehydes and ketones can be prepared by ozonolysis of alkenes.

Aldehydes and ketones undergo nucleophilic addition reactions onto the carbonyl

group with a number of nucleophiles such as, HCN, NaHSO

, alcohols (or diols),

2. Ring substitution

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).

Intext QuestionIntext Question

Intext Question

12.8 Which acid of each pair shown here would you expect to be stronger?

(i) CH

H or CH

FCO

H (ii) CH

FCO

H or CH

ClCO

(iii) CH

FCH

H or CH

CHFCH

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.

(iv)

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385 Aldehydes, Ketones and Carboxylic Acids

ammonia derivatives, and Grignard reagents. The α-hydrogens in aldehydes and

ketones are acidic. Therefore, aldehydes and ketones having at least one α-hydrogen,

undergo Aldol condensation in the presence of a base to give α-hydroxyaldehydes

(aldol) and α-hydroxyketones(ketol), respectively. Aldehydes having no α-hydrogen

undergo Cannizzaro reaction in the presence of concentrated alkali. Aldehydes

and ketones are reduced to alcohols with NaBH

, LiAlH

, or by catalytic hydrogenation.

The carbonyl group of aldehydes and ketones can be reduced to a methylene group

by Clemmensen reduction or Wolff-Kishner reduction. Aldehydes are easily

oxidised to carboxylic acids by mild oxidising reagents such as Tollens’ reagent and

Fehling’s reagent. These oxidation reactions are used to distinguish aldehydes from

ketones. Carboxylic acids are prepared by the oxidation of primary alcohols, aldehydes

and alkenes by hydrolysis of nitriles, and by treatment of Grignard reagents with

carbon dioxide. Aromatic carboxylic acids are also prepared by side-chain oxidation

of alkylbenzenes. Carboxylic acids are considerably more acidic than alcohols and

most of simple phenols. Carboxylic acids are reduced to primary alcohols with LiAlH

or better with diborane in ether solution and also undergo α-halogenation with Cl

and Br

in the presence of red phosphorus (Hell-Volhard Zelinsky reaction).

Methanal, ethanal, propanone, benzaldehyde, formic acid, acetic acid and benzoic

acid are highly useful compounds in industry.

Exercises

12.1 What is meant by the following terms ? Give an example of the reaction in

each case.

(i) Cyanohydrin (ii) Acetal (iii) Semicarbazone

(iv) Aldol (v) Hemiacetal (vi) Oxime

(vii) Ketal (vii) Imine (ix) 2,4-DNP-derivative

(x) Schiff’s base

12.2 Name the following compounds according to IUPAC system of nomenclature:

(i) CH

CH(CH

)CH

CHO (ii) CH

COCH(C

)CH

(iii) CH

CH=CHCHO (iv) CH

COCH

(v) CH

CH(CH

)CH

C(CH

)

COCH

(vi) (CH

)

CCH

COOH

(vii) OHCC

CHO-p

12.3 Draw the structures of the following compounds.

(i) 3-Methylbutanal (ii) p-Nitropropiophenone

(iii) p-Methylbenzaldehyde (iv) 4-Methylpent-3-en-2-one

(v) 4-Chloropentan-2-one (vi) 3-Bromo-4-phenylpentanoic acid

(vii) p,p’-Dihydroxybenzophenone (viii) Hex-2-en-4-ynoic acid

12.4 Write the IUPAC names of the following ketones and aldehydes. Wherever

possible, give also common names.

(i) CH

CO(CH

)

(ii) CH

CHBrCH

CH(CH

)CHO

(iii) CH

(CH

)

CHO (iv) Ph-CH=CH-CHO

(v)

CHO

(vi) PhCOPh

12.5 Draw structures of the following derivatives.

(i) The 2,4-dinitrophenylhydrazone of benzaldehyde

(ii) Cyclopropanone oxime

(iii) Acetaldehydedimethylacetal

(iv) The semicarbazone of cyclobutanone

(v) The ethylene ketal of hexan-3-one

(vi) The methyl hemiacetal of formaldehyde

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386Chemistry

12.6 Predict the products formed when cyclohexanecarbaldehyde reacts with

following reagents.

(i) PhMgBr and then H

(ii) Tollens’ reagent

(iii) Semicarbazide and weak acid (iv) Excess ethanol and acid

(v) Zinc amalgam and dilute hydrochloric acid

12.7 Which of the following compounds would undergo aldol condensation, which

the Cannizzaro reaction and which neither? Write the structures of the expected

products of aldol condensation and Cannizzaro reaction.

(i) Methanal (ii) 2-Methylpentanal (iii) Benzaldehyde

(iv) Benzophenone (v) Cyclohexanone (vi) 1-Phenylpropanone

(vii) Phenylacetaldehyde (viii) Butan-1-ol (ix) 2,2-Dimethylbutanal

12.8 How will you convert ethanal into the following compounds?

(i) Butane-1,3-diol (ii) But-2-enal (iii) But-2-enoic acid

12.9 Write structural formulas and names of four possible aldol condensation

products from propanal and butanal. In each case, indicate which aldehyde

acts as nucleophile and which as electrophile.

12.10 An organic compound with the molecular formula C

O forms 2,4-DNP derivative,

reduces Tollens’ reagent and undergoes Cannizzaro reaction. On vigorous oxidation,

it gives 1,2-benzenedicarboxylic acid. Identify the compound.

12.11 An organic compound (A) (molecular formula C

) was hydrolysed with

dilute sulphuric acid to give a carboxylic acid (B) and an alcohol (C). Oxidation

of (C) with chromic acid produced (B). (C) on dehydration gives but-1-ene.

Write equations for the reactions involved.

12.12 Arrange the following compounds in increasing order of their property as indicated:

(i) Acetaldehyde, Acetone, Di-tert-butyl ketone, Methyl tert-butyl ketone

(reactivity towards HCN)

(ii) CH

CH(Br)COOH, CH

CH(Br)CH

COOH, (CH

)

CHCOOH,

COOH (acid strength)

(iii) Benzoic acid, 4-Nitrobenzoic acid, 3,4-Dinitrobenzoic acid,

4-Methoxybenzoic acid (acid strength)

12.13 Give simple chemical tests to distinguish between the following pairs of compounds.

(i) Propanal and Propanone (ii) Acetophenone and Benzophenone

(iii) Phenol and Benzoic acid (iv) Benzoic acid and Ethyl benzoate

(v) Pentan-2-one and Pentan-3-one (vi) Benzaldehyde and Acetophenone

(vii) Ethanal and Propanal

12.14 How will you prepare the following compounds from benzene? You may use

any inorganic reagent and any organic reagent having not more than one

carbon atom

(i) Methyl benzoate (ii) m-Nitrobenzoic acid

(iii) p-Nitrobenzoic acid (iv) Phenylacetic acid

(v) p-Nitrobenzaldehyde.

12.15 How will you bring about the following conversions in not more than two steps?

(i) Propanone to Propene (ii) Benzoic acid to Benzaldehyde

(iii) Ethanol to 3-Hydroxybutanal (iv) Benzene to m-Nitroacetophenone

(v) Benzaldehyde to Benzophenone (vi) Bromobenzene to 1-Phenylethanol

(vii) Benzaldehyde to 3-Phenylpropan-1-ol

(viii) Benazaldehyde to α-Hydroxyphenylacetic acid

(ix) Benzoic acid to m- Nitrobenzyl alcohol

12.16 Describe the following:

(i) Acetylation (ii) Cannizzaro reaction

(iii) Cross aldol condensation (iv) Decarboxylation

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387 Aldehydes, Ketones and Carboxylic Acids

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

12.18 Give plausible explanation for each of the following:

(i) Cyclohexanone forms cyanohydrin in good yield but 2,2,6-trimethylcyclo-

hexanone does not.

(ii) There are two –NH

groups in semicarbazide. However, only one is involved

in the formation of semicarbazones.

(iii) During the preparation of esters from a carboxylic acid and an alcohol in

the presence of an acid catalyst, the water or the ester should be removed

as soon as it is formed.

12.19 An organic compound contains 69.77% carbon, 11.63% hydrogen and rest oxygen.

The molecular mass of the compound is 86. It does not reduce Tollens’ reagent

but forms an addition compound with sodium hydrogensulphite and give positive

iodoform test. On vigorous oxidation it gives ethanoic and propanoic acid. Write

the possible structure of the compound.

12.20 Although phenoxide ion has more number of resonating structures than

carboxylate ion, carboxylic acid is a stronger acid than phenol. Why?

Answers to Some Intext Questions

12.1

(i)

(iv)

(ii)

(v)

(iii)

(vi)

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388Chemistry

12.3 CH

< CH

OCH

< CH

CHO < CH

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

12.2

(i)

(iii)

(ii)

(iv)

(i)

(iii)

(ii)

(iv)

(i)

(ii)

(iii)

(iv)

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