Overview

Carboxylic acids possess an acidic –COOH functional group. The acidity can be attributed to the resonance stabilization of their conjugate base, wherein the negative charge is delocalized over both oxygen atoms.

The acyl bond is polar because of the high electronegativity of oxygen, making the carbonyl carbon highly reactive and susceptible to nucleophilic attack. Thus, nucleophilic acyl substitution reactions can convert the –COOH to acid derivatives such as acyl halides, esters, anhydrides, and amides.

Additionally, carboxylic acids can be reduced by strong reducing agents to yield alcohols via aldehyde intermediates.

The α hydrogen in carboxylic acids can also be substituted by halogens to give α-halogenated carboxylic acids. This is the basis of the Hell–Volhard–Zelinsky reaction, where α-halo acids are obtained in the presence of halogen and phosphorus.

Moreover, the silver salt of carboxylic acids, when heated along with halogens such as bromine or iodine, forms alkyl halides with one carbon less than the starting acid via the elimination of carbon dioxide gas. This reaction is known as the Borodin–Hunsdiecker reaction.

Unlike carboxylic acids, β-keto acids are particularly prone to decarboxylation and yield monocarboxylic acids or ketones upon gentle warming.

Procedure

Carboxylic acids feature many reactive sites, including the polar O–H bond, the electron-rich carbonyl oxygen, the electrophilic carbonyl carbon, and the α hydrogens.

Carboxylic acids may react as Br∅nsted–Lowry acids because the polar O–H bond undergoes deprotonation with aqueous bases to form soluble carboxylate salts.

However, they may also act as weak bases in the presence of stronger acids that preferentially protonate the carbonyl oxygen over the hydroxyl oxygen.

Further, the polar C–O bond generates an electrophilic center favoring nucleophilic acyl substitution via a tetrahedral intermediate.

Similarly, the carbonyl group undergoes reduction by strong reducing agents to generate primary alcohols.

Carboxylic acids undergo halogenation at the α-carbons via the Hell-Volhard-Zelinsky reaction. The α-hydrogen atom in the presence of phosphorus, upon hydrolysis, is substituted with a halogen to yield α-halo acids via acyl halides.

Carboxylic acids containing a β-keto group undergo decarboxylation under acidic conditions to generate ketones.

To summarize, carboxylic acids undergo diverse reactions including deprotonation, acyl substitution, reduction, α-substitution, and decarboxylation.