Aldehydes, Ketones and Carboxylic Acids - Preparation of Carboxylic Acids

Carboxylic acids can be prepared from primary alcohols and aldehydes using strong oxidizing agents such as potassium permanganate ($KMnO_4$) in acidic, alkaline, or neutral media, or Jones reagent ($CrO_3 - H_2SO_4$).

Aromatic carboxylic acids are prepared by vigorous oxidation of alkylbenzenes with chromic acid or alkaline $KMnO_4$. The entire side chain is oxidized to a carboxyl group regardless of the length of the chain, provided there is at least one benzylic hydrogen.

Nitriles are hydrolyzed to amides and then to carboxylic acids in the presence of $H^+$ or $OH^-$ as a catalyst. The reaction is: $R-CN \xrightarrow{H_2O/H^+} R-CONH_2 \xrightarrow{H_2O/H^+, \Delta} R-COOH$.

Grignard reagents react with dry ice (solid $CO_2$) in ethereal solution to form salts of carboxylic acids, which upon acidification with mineral acids yield corresponding carboxylic acids. This method is useful for increasing the carbon chain length by one unit.

Acid chlorides (acyl halides) are hydrolyzed with water to give carboxylic acids, or more readily with aqueous base to give carboxylate ions, which on acidification provide carboxylic acids.

Esters undergo nucleophilic substitution (hydrolysis) with dilute mineral acids to yield carboxylic acids and alcohols directly, or with aqueous alkalis (saponification) to yield carboxylates and alcohols.

Acid anhydrides are hydrolyzed to carboxylic acids upon reaction with water. Symmetrical anhydrides yield two molecules of the same acid, while unsymmetrical ones yield two different acids.