In molecules , any of numerous combinations of atoms that undergo characteristic chemical reactions themselves and in many cases influence the reactivity of the rest of the molecule.
Organic compounds are often classified according to the functional groups they contain. Common functional groups include hydroxyl (―OH), in alcohols and phenols ; carboxyl (―COOH), in carboxylic acids ; carbonyl (―C=O), in aldehydes , ketones , amides , carboxylic acids , esters , and quinones ; and nitro (―NO 2 ) and amino (―NH 2 ), in certain organic nitrogen compounds.