Preview:

Aldehydes are organic compounds containing the group CHO, formed by the oxidation of alcohols; Ketones are organic compounds containing a carbonyl group =C=O bonded to two hydrocarbon groups, formed by oxidizing secondary alcohols and Carboxylic acids are organic acids containing a carboxyl group (COOH).

We shall begin the chapter by defining a commonly used term called, carbonyl group as the functional group in which the carbon atom is linked to oxygen atom through a double bond: >C=O. We will see that, in aldehydes, the carbonyl group is bonded to a carbon and hydrogen while in the ketones; it is bonded to two carbon atoms and in carboxylic acids, these carbonyl groups are attached to their oxygen atom.

The Nomenclature of Aldehydes and Ketones followed by the Structure of Carbonyl Group shall then be studied in some detail. We shall then study some of the methods of Preparation of Aldehydes and Ketones.

The study of Physical and chemical properties of aldehydes and ketones will then be taken up.

We will come across important tests that are used to distinguish aldehydes from ketones in this regard; which include: Tollens test, Fehling s test and Oxidation of methyl ketones by haloform reaction. We shall then discuss the uses of aldehydes and ketones.

We will then take up the study of Carboxylic Acids. We shall look at the Nomenclature of carboxylic acids; followed by the study of Structure of Carboxyl Group.

Subsequently, we shall look at some of the Methods of preparation of Carboxylic acids. Next, we shall discuss the Physical Properties of these carboxylic acids. Then, we will study some interesting Chemical reactions that they undergo. Finally, we shall end the chapter by listing the uses of Carboxylic acids.

12.1 Introduction:

Aldehydes & ketones and carboxylic acids are the group of organic compounds called carbonyl compounds as these contain a carboxyl group.

Definition A carbonyl group is a functional group in which the carbon atom is linked to oxygen atom through a double bond (>C=O).

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 carbonyl group is bonded to oxygen are known as carboxylic acids, and their derivatives while in compounds where carbon is attached to nitrogen and to halogens are called amides and acyl halides respectively.
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.

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 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 acyl group as prefix to 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 below.

12.1.2 Structure of the Carbonyl Group

The carbonyl carbon atom is $sp^2$-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 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.

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 below.

Questions for session 12.1

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
2. What are aldehydes & ketones and carboxylic acids?
3. Mention the importance of aldehydes and ketones.
4. Give a brief note on the common naming system of aldehydes.
5. Write a short note on the IUPAC system of naming of aldehydes.
6. Give a brief note on the common naming system of ketones.
7. Write a short note on the IUPAC system of naming of ketones.
8. With figure, explain the structures and characteristics of structure of carbonyl group.

12.2 Preparation of Aldehydes and Ketones

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.

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.

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.
(ii) By hydration of alkynes: Addition of water to ethyne in the presence of $H_2SO_4$ and $HgSO_4$ gives acetaldehyde. All other alkynes give ketones in this reaction.

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:
$$ \ce { R\textcolor{#ED0891}{CN} ->[{1. AlH(i-Bu)_2}][{2.H_2O}] R – \textcolor{#ED0891}{CHO}}$$
$$ \ce {CH_3 – CH \textcolor{#ED0891}{=} CH – CH_2CH_2 – \textcolor{#ED0891}{CN} ->[{1. AlH(i-Bu)_2}][{2. H_2O}] CH_3 – CH \textcolor{red}{=} CH – CH_2CH_2-\textcolor{#ED0891}{CHO}}$$
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 ($CrO_2Cl_2$)
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 ($CrO_3$)
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.

12.2.3 Preparation of Ketones

1. From acyl chlorides
   Treatment of acyl chlorides with dialkylcadmium, prepared by the reaction of cadmium chloride with Grignard reagent, gives ketones.

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.

Questions for session 12.2

1. Write the structures of products of the following reactions;

2. Explain the preparation of ketones by:
   I. Acyl chlorides
   II. Nitriles
   III. Benzene or substituted benzenes

12.3 Physical Properties

Methanal is a gas at room temperature. Ethanal is a volatile liquid. Other aldehydes and ketones are liquid or solid at room temperature.
The lower aldehydes have sharp pungent odours. As the size of the moleculincreases, 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.
The lower members of aldehydes and ketones such as methanal, ethanol 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 increasing the length of alkyl chain.

All aldehydes and ketones are fairly soluble in organic solvents like benzene, ether, methanol, chloroform, etc.

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.

Questions for session 12.3

1. Write a short note on the physical properties of ketones.

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, the aldehydes and ketones undergo nucleophilic addition reactions.
(i) Mechanism of nucleophilic addition reactions
Step 1: A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of $sp^2$ hybridised orbitals of carbonyl carbon. The hybridisation of carbon changes from $sp^2$ to $sp^3$ in this process, and a tetrahedral alkoxide intermediate is produced.
Step 2: 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 figure below.

(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 suc 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, $NH_2, C_6H_5NH, NHCONH_2$, etc.

Some N-Substituted Derivatives of Aldehydes and Ketones

2. Reduction
(i) Reduction to alcohols
Aldehydes and ketones are reduced to primary and secondary alcohols respectively by sodium borohydride ($NaBH_4$) or lithium aluminium hydride ($LiAlH_4$) as well as by catalytic hydrogenation.

(ii) Reduction to hydrocarbons
The carbonyl group of aldehydes and ketones is reduced to $CH_2$ group on treatment with zincamalgam 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).

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.
$$ \ce { R-\textcolor{#ED0891}{CHO} ->[{[O]}] R-\textcolor{#ED0891}{COOH}}$$
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 $CH_3CO$ group or $CH_3CH(OH)$ group which produces $CH_3CO$ group on oxidation.

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 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.

Questions for session 12.4

1. 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.
2. What are nucleophilic addition reactions? Explain in brief the mechanism.
3. Give a brief note on the reactivity of ketones.
4. Explain with reactions w.r.t ketones:
   I. Addition of hydrogen cyanide
   II. Addition of sodium hydrogensulphite
   III. Addition of grigard reagents
   IV. Addition of alcohols
   V. Addition of ammonia and its derivatives
5. Predict the products of the following reactions:
   
6. Explain the reduction reaction w.r.t ketones:
                 I. Reduction to alcohols
                II. Reduction to hydrocarbons
7. Explain in brief the oxidation reactions of ketones.
8. Explain with reactions the following tests:
                 I. Tollen s test
                II. Fehling s test
               III. Oxidation of methyl ketones by haloform reaction
9. Explain in brief the acidity of α-hydrogens of aldehydes and ketones.
10. Explain the following reactions w.r.t ketones:
                  I. Aldol condensation
                 II. Cross aldol condensation
                III. Cannozaro reaction
                IV. Electrophilic substitution reaction

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.

Questions for session 12.5

1. What are the uses of aldehydes and ketones?

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 ($C_{12} – C_{18}$) 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

Trivial system:
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 ($CH_3COOH$) from vinegar (Latin: acetum, means vinegar)
Butyric acid ($CH_3CH_2CH_2COOH$) from rancid butter (Latin: butyrum, means butter).

IUPAC system
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 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 below.

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:

Questions for session 12.6

1. Give the IUPAC names of the following compounds:

2. What are carboxylic acids?

3. Write a short note on the trivial system of nomenclature of carboxylic acids.

4. Write a short note on the IUPAC system of nomenclature of carboxylic acids.

5. Draw and explain the structure of carboxyl.

12.7 Methods of Preparation of Carboxylic Acids

1. From primary alcohols and aldehydes
Primary alcohols are readily oxidised to carboxylic acids with common oxidising agents such as potassium permanganate ($KMnO_4$) in neutral, acidic or alkaline media or by potassium dichromate ($K_2Cr_2O_7$) and chromium trioxide ($CrO_3$) in acidic media (Jones reagent).

Carboxylic acids are also prepared from aldehydes by the use of mild oxidising agents.

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

3. From nitriles and amides
Nitriles are hydrolysed to amides and then to acids in the presence of H+ or OH− 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.

The Grignard reagents and nitriles can be prepared from alkyl halides.
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.

Questions for session 12.7

1. Show how each of the following compounds can be converted to benzoic acid.
   (i) Ethylbenzene
   (ii) Acetophenone
   (iii) Bromobenzene
   (iv) Phenylethene (Styrene)
2. Explain the method of preparation of carboxylic acids by:
   I. From primary alcohols and aldehydes
   II. From alkylbenzenes
   III. From nitriles and amides
   IV. From grigard reagents
   V. From acyl halides and anhydrides
   VI. From easter

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 up to 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 $H_2O$

Questions for session 12.8

1. Write a brief note on the physical properties of carboxylic acids.

12.9 Chemical Reactions

The reactions of carboxylic acids are classified as follows:

12.9.1 Reactions Involving Cleavage of O H Bond

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.

For the above reaction:

$k_{eq}$, is equilibrium constant and $K_a$ is the acid dissociation constant.
For convenience, the strength of an acid is generally indicated by its $pK_a$ value rather than its $K_a$
$$ pK_a = – log K_a$$
Stronger the $ pK_a$ values, stronger are the acids.
Carboxylic acids are weaker than mineral acids, but they are stronger acids than alcohols and many simple phenols ( $ pK_a$ is ~16 for ethanol and 10 for phenol).
In fact, carboxylic acids are amongst the most acidic organic compounds you have studied so far.
The higher acidity of carboxylic acids as compared to phenols.
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.
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.
The effect of the following groups in increasing acidity order is
Ph < I < Br < Cl < F < CN < $NO_2$ < $CF_3$ Thus, the following acids are arranged in order of increasing acidity: $$ CF_3COOH > CCl_3COOH > CHCl_2COOH > NO_2CH_2COOH > NC-CH_2COOH > FCH_2COOH > ClCH_2COOH > BrCH_2COOH > HCOOH > ClCH_2CH_2COOH > C_6H_5COOH > C_6H_5CH_2COOH > CH_3COOH > CH_3CH_2COOH$$
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 $sp^2$ 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 $H_2SO_4$ or with $P_2O_5$ give corresponding anhydride.

2. Esterification

3. Reactions with $PCl_5, PCl_3$ and $SOCl_2$

The hydroxyl group of carboxylic acids, behaves like that of alcohols and is easily replaced by chlorine atom on treating with $PCl_5, PCl_3$ or $SOCl_2$. Thionyl chloride ($SOCl_2$) 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 gives amides.
For example

12.9.3 Reactions involving COOH groups

1. Reduction
Carboxylic acids are reduced to primary alcohols by lithium aluminium hydride or better with diborane. Diborane does not easily reduce functionl groups such as ester, nitro, halo etc. Sodium borohydride does not reduce the carboxyl group.
$$ \ce { R-\textcolor{#ED0891}{COOH} ->[{(i) LiAlH_4/ether \space or \space B_2H_6}][{ii) H_3O^+}]R-\textcolor{#ED0891}{CH_2OH}}$$

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.
$$ \ce { R- \textcolor{#ED0891}{COONa} ->[{NaOH & CaO}][{Heat}] R-H + Na_2CO_3}$$
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.

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.

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).

Methanoic acid is used in rubber, textile, dyeing, leather and electroplating industries. Ethanoic acid is used as solvent and as vinegar in food industry.

Questions for session 12.9

1. Which acid of each pair shown here would you expect to be stronger?
   I. $CH_3CO_2H$ or $CH_2FCO_2H$
   II. $CH_2FCO_2H$ or $CH_2ClCO_2H$
   III. $CH_2FCH_2CH_2CO_2H$ or $CH_3CHFCH_2CO_2H$
2. Write a brief note on the acidity of carboxylic acids with reactions.
3. What are the effects of substituents on the acidity of carboxylic acids? Explain with reactions.
4. Explain the reactions involving cleavage of C-OH bond w.r.t:
   I. Formation of anhydride
   II. Esterification
   III. Reactions with $PCl_5, PCl_3$ and$ SOCl_2$
   IV. Reaction with ammonia
5. Explain the reactions involving COOH group w.r.t:
   I. Reduction
   II. Decarboxylation
6. Explain with equations the following reactions w.r.t carboxylic acids:
   I. Halogenation
   II. Ring substitution

12.10 Uses of Carboxylic Acids

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 tezhe manufacture of soaps and detergents.
Methanoic acid is used in rubber, textile, dying, leather and electroplating industries.
Ethanoic acid is used as solvent and as vinegar in food industry.

Questions for session 12.10

1. What are the uses of carboxylic acids?