a carboxylic acid. In some cases, an epoxide is used instead of the alcohol; or an acid halide or anhydride replaces the acid. Less often, an alkyl or aryl halide is treated with a carboxylate salt.

an 18-carbon saturated fatty acid. The normal numbering system, starting with the carboxylic acid, is shown in blue. The carbons and oxygens of the carboxylic acid functional group are red.