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in organic chemistry, a divalent chemical unit consisting of a carbon (C) and an oxygen (O) atom connected by a double bond. The group is a constituent of carboxylic acids, esters, anhydrides, acyl halides, amides, and quinones, and it is the characteristic functional group (reactive group) of aldehydes and ketones. Carboxylic acids (and their derivatives), aldehydes, ketones, and quinones are also known collectively as carbonyl compounds.