Chemistry Preparation of Carboxylic Acids

Topics Covered :

● Methods of Preparation of Carboxylic Acids
● From Primary Alcohols and Aldehydes
● From Alkylbenzenes
● From Nitriles and Amides
● From Grignard Reagents
● From Acyl Halides and Anhydrides
● From Esters

Methods of Preparation of Carboxylic Acids :

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

From primary alcohols and aldehydes :

`=>` Primary alcohols are readily oxidised to carboxylic acids with common oxidising agents such as potassium permanganate `color{red}((KMnO_4))` in neutral, acidic or alkaline media or by potassium dichromate `color{red}((K_2Cr_2O_7))` and chromium trioxide `color{red}((CrO_3))` in acidic media.

`color{red}(RCH_2OH underset(2.H_3overset(+)O) overset(1. alkali n e KMnO_4)→ RCOOH)`

`color{red}(undersettext(1- Decanol)(CH_3(CH_2)_8 CH_2OH) overset(CrO_3 . H_2SO_4)→ undersettext(Decanoic acid)(CH_3 (CH_2)_3COOH))`

Carboxylic acids are also prepared from aldehydes by the use of mild oxidising agents (See Chemical Reactions of Carbonyl Compounds : Reduction and Oxidation Notes).

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.

From nitriles and amides :

`=>` Nitriles are hydrolysed to amides and then to acids in the presence of `color{red}(H^+)` or `color{red}(OH^−)` as catalyst.

● Mild reaction conditions are used to stop the reaction at the amide stage.

`color{red}(R- CN underset(H_2O) overset(H^+ or barOH)→ R - overset ( overset (O)(||))C-NH_2 underset(Delta) overset(H^+ "or" barOH)→ RCOOH)`

`color{red}(undersettext(Ethanamide)(CH_3CONH_2) underset(Delta) overset(H_3 overset(+)O)→ undersettext(Ethanoic acid)(CH_3COOH)+NH_3)`

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.

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

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.

`color{red}(RCOCl overset(H_2O)→ RCOOH + Cbarl)`

`color{red}(RCOCl overset(barOH//H_2O)→ RCOO^(-) + barCl overset(H_3 overset(+)O)→ RCOOH)`

`color{red}(undersettext(Benzoic anhydride)((C_6H_5CO)_2O) overset(H_2O)→ undersettext(Benzoic acid)(2C_6H_5COOH))`

`color{red}(undersettext(Benzoic ethanoic anhydride)(C_6H_5 COOCOCH_3) overset(H_2O)→ undersettext(Benzoic acid)(C_6H_5COOH) +undersettext(Ethanoic acid)(CH_3COOH))`

From esters :

Acidic hydrolysis of esters gives directly carboxylic acids while basic hydrolysis gives carboxylates, which on acidification give corresponding carboxylic acids.
Q 3082267137

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.