The carboxyl group

Carboxylic acids contain one or more carboxyl groups, –COOH (a carbonyl group C=O and a hydroxyl group –OH on the same carbon). They are the most acidic of the common organic compounds, but they are weak acids compared with mineral acids (HCl, H₂SO₄, HNO₃).

• Aliphatic acids: –COOH attached to an alkyl group (R–COOH). Saturated mono-carboxylic aliphatic acids are called fatty acids because many occur in fats as esters with glycerol.
• Aromatic acids: –COOH attached to a benzene ring (benzoic acid).