13.10 : カルボン酸の反応: はじめに

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.