Reactions of carboxylic acids and their salts include nucleophilic substitution and decarboxylation to leave enols, free radicals, or alkyl halides. A review of the IR spectroscopy of acid derivatives includes the use of vibrational coupling in the structure determination of anhydrides and imides. Many acid derivatives can be interconverted by substitution through a tetrahedral intermediate, and differences in acidity can be used to drive such reactions toward completion. Reduction of acid derivatives illustrates the challenge of designing selective reactions. Acidic and basic mechanisms allow conversion of nitriles to carboxylic acids. Ketenes provide routes to several acid derivatives. The Baeyer-Villiger oxidation of ketones to esters illustrates atom insertion into acyl-R bonds.