13.12: Reactions of Carboxylic Acids: Introduction
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.