Carboxyl group or carboxyl group –COOH or CO2H is a functional group present in amino acids and carboxylic acids. Its structure is composed of one carbon atom attached to an oxygen atom by a double bond and to a hydroxyl group by a single bond. It is often mentioned that a carboxyl group is a carbonyl group bonded to a hydroxyl group. The carboxyl group has one valence electron in its carbon atom, making it possible to be a part in a larger molecule by bonding through it. Carboxyl groups can only occur at the end of a carbon chain, because the carbon must make 3 bonds (O double bond and single bond to hydroxyl group) in addition to its connection to the R group.
Carboxyl groups will often lend their oh H+ into solution and therefore will act as acids. This is the case with carboxylic acids, such as is the case when acetic acid lends its H+ into solution to become acetate.