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Carboxylic acids are stabilized after losing a proton because the carboxylate anion delocalizes its negative charge across two oxygen atoms. Electron-withdrawing groups increase acidity, with a weaker effect farther from the carboxyl group; electron-donating groups decrease it. In dicarboxylic acids, the second proton is less acidic than the first. Acyl chlorides, anhydrides, esters, acids, and amides differ in substitution reactivity, reflecting the basicity of their leaving groups. In nucleophilic acyl substitution, addition to the carbonyl forms a tetrahedral intermediate, followed by leaving-group elimination. The topic also introduces routes to derivatives and acids, including oxidation, carboxylation with carbon dioxide, and nitrile hydrolysis.