Carboxylic Acids

Carboxylic Acids

Eg: 

Is 2-Bromo butanoic acid

The carboxylic carbon forms are (sp² - sp²) s - bond with two oxygen atoms. One the oxygen atoms form (sp² - s) s - bond with a hydrogen atom. One half filled unhybridized p-orbital of the carbon and the unhybridized p-orbital of the oxygen atom undergo sideways overlap. This results in the formation of the delocalized p-electron cloud. This is confirmed by the C-O single bond length in formic acid being shorter than the C-O bond length in ethanol.

Physical Propertes

The first three aliphatic acids are colorless liquids with pungent smell. The next six are oily liquids with an odor of rancid butter. The higher members are colorless, odorless, waxy solids. Benzoic acid is a crystalline solid.

Boiling points

Order of b.p. hydrocarbons< aldehydes< ketones< alcohols< carboxylic acids

This is due to more extensive association of carboxylic acid molecules through intermolecular hydrogen bonding.

Most carboxylic acids exists as dimers in the vapor phase and aprotic solvents.

Solubility

Simple aliphatic carboxylic acids having upto four carbon atoms are miscible with water for hydrogen bonds can be formed with water..

The solubility decreases with increasing number of carbon atoms.

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 etc.

Preparation

By oxidation of alchohols and aldehydes.

Readily available aldehydes can also be used to obtain carboxylic acids, which may be oxidised even by milder oxidising agents.

From alkyl benzenes and alkenes

Aromatic carboxylic acids can be prepared by vigorous oxidation of alkyl benzenes with chromic acid or acidic or alkaline KMnO₄, irrespective of its length.

Not possible with tertiary benzylic carbon.

From nitriles

Hydrolysis of nitriles with aqueous acid or alkali give carboxylic acids.

From Grignard's reagent

Grignards reagents react with carbon dioxide to form salts of carboxylic acids which give carboxylic acids on acidification with mineral acids.

carboxylic acid having one carbon atom more than that present in the alkyl halide is obtained.

Chemical Properties

Acidity

The aqueous solution, carboxylic acids dissociate as follows:

Since they liberate hydrogen ions in solution, they are acidic. However they are weaker than mineral acids, but stronger acids than alcohols and phenols.

carboxylate ion is more stabilized by resonance than carboxylic acid, hence carboxylic acid behave as strong acids.

Accordingly, carboxylic acids evolve hydrogen with electropositive metals as alcohols do.

Electron withdrawing groups stabilize the carboxylate anion by dispersal of the negative charge and increase the strength of the acid. Hence,

FCH₂COOH > ClCH₂COOH > Br CH₂COOH > ICH₂COOH > CH₂COOH.

Electron releasing groups like alkyl groups cause concentration of negative charge, destebalize the carboxylate anion and decrease the strength of the acid. Hence

Similarly,

CH₃ClCOOH > Cl₂CHCOOH > ClCH₂COOH > CH₃COOH

Reduction

Carboxylic acids are reduced to primary alcohols on treatment with lithium aluminium hydride or better with diborane.

Sodium borohydride does not reduce the carboxyl group.

Decarboxylation

Sodium or potassium salts of carboxylic acids on heating with soda lime (NaOH and CaO) gives alkanes with one carbon less than the parent acids.

Alkali metal salts of carboxylic acids undergo decarboxylation on electrolysis forms hydrocarbons having twice the number of carbon atoms in the alkyl group of the acid.This is called Kolbe's electrolysis.

Substitution Reactions

• Halogenation

In Carboxylic acids having an a-hydrogen. This reaction is known as Hell - Volhard Zelinsky reaction.

This is an important intermediate able to prepare a variety of compounds.

• Ring substitution in aromatic acids

Aromatic carboxylic acids undergo electrophilic substitution reactions in which the carboxyl group is a deactivating and meta directing group.

Aromatic carboxylic acids, however do not undergo Friedel - Craft's reaction.

Formation of acid derivatives

Acid anhydrides can be prepared by heating carboxylic acid in the presence of P₂O₅ in dehydrating agent.

Asymmetrical and symmetrical anhydrides can be prepared by the reaction of acyl chlorides with sodium salts of carboxylic acids in the presence of pyridine.

Esters are prepared by the acylation of alcohols or phenols.

Amides are generally prepared by the reaction of acyl chlorides or anhydrides with ammonia or amines.

The hydroxyl group of carboxylic acid like that of alcohols are easily replaced by chlorine atom on heating with PCl₅, PCl₃ or SOCl₂.

RCOOH + PCl₅  → RCOCl + POCl₃ + HCl

Comparing acidity of derivatives

The reactivity depends on both the structure of the derivative and also on the nature of the nucleophile.

The reactions of carboxylic acids and their derivatives involve substitution of the group L with nucleophiles and are known as nucleophilic acyl substitution reaction.

………...better leaving groups.................................poorer leaving groups...................

chloride > carboxylate > alkylthiolate > aryloxide > alkoxide > aniline > alkyl amine

poor conjugate base…………..…………………………………..………strong conjugate base

Correspondingly

HCl RCO₂H RSH ArOH ROH ArNH₂ RNH₂

Strong Acid………………………………………………………………..….Weak Acid

Order of reactivity of the acid derivatives is

Examples

Q-1. What are the A, B, C in the following reactions ?

Ans: 3

Sol: Reaction

Q-2. Identify A and B in the following reaction

Ans: 3

Sol: 

A : LiAlH₄ B : HI + Red P

Q-3. 

In the above reactions ‘A’ and ‘B’ respectively are

Ans: 4

Sol: Reaction

Q-4. When compound X is oxidized by acidified potassium dichromate, compound Y is formed. Compound Y on reduction with : LiAlH₄ gives X. X and Y respectively.

Ans: 1

Sol: Reaction

Q-5. In which of the following reactions hydrogen gas is liberated?

Ans: 3

Sol: Reaction