Carboxylic acid refers to any of a class of organic compounds in which a carbon (C) atom is bonded to an oxygen (O) atom by a double bond and to a hydroxyl group (?OH) by a single bond. It contains a carboxyl group (C(= O)OH). Vitamin C (ascorbic acid) is an organic acid without exhibiting a carboxylic group. It has no carboxyl group, only hydroxyl groups and a carbonyl group.