Carboxylic acids are organic compounds that contain one or more carboxyl groups (RC(=O)OH), where R is an organic group. Those acids where R is a straight-chain alkyl group are sometimes known as fatty acids, and are named as alkanoic acids from the corresponding alkane, i.e., their names end in the suffix '-oic' (e.g., ethanoic acid, CH3COOH).
Some carboxylic acids occur free in nature, including formic acid and acetic acid – the two simplest – and citric, lactic, malic, and tartaric acids. These, and others including benzoic, oxalic, and salicylic acids, are found also as their salts and esters. Many of the fatty acids, including oleic, palmitic, and stearic acids, occur in oils and fats as esters of glycerol.
Carboxylic acids are generally weak acids, the exact strength depending on the electronegativity of the group R, and so are often used with their salts as buffers. Many derivatives of carboxylic acids are important in nature or chemical synthesis: they include acid anhydrides, acid chlorides, amides, esters, nitriles, and peroxyacids.
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.