A dicarboxylic acid is a carboxylic acid with two carboxyl groups (–COOH). Examples include ethanedioic acid (also known as oxalic acid, (COOH)2), hexanedoic acid (adipic acid, (CH2)4(COOH)2), and pthalic acid (C6H4(COOH)2).
Dicarboxylic acids form both acidic and neutral salts and esters; some give anhydrides by loss of a molecule of water between the two carboxyl groups. Many occur naturally as the free acid or an ester. They are generally prepared by the oxidation of a glycol, hydroxy-acid or hydroxyaldehyde, or by hydrolysis of a dinitrile or cyano-acid.