Also called carboxy group or carboxyl functional group, carboxyl group is a functional group (in organic chemistry) that comprises an oxygen atom double bonded to a carbon atom (carbonyl group), and the same carbon atom single bonded to a hydroxyl group (covalently bonded hydrogen and oxygen atom). In fact, the terms “hydroxyl” and “carbonyl” contribute to the “carboxyl” name. In other words, carbonyl group is –CO; hydroxyl group is –OH; and carboxyl group is –COOH. Carboxylic acid is a molecule that comprises the carboxyl group. The acid is found in protein-building amino acids and acetic acid.


Carboxyl group chemical structure. Image credit: Flickr
Carboxyl group chemical structure. Image credit: Flickr

A carboxyl group shares specific properties with carbonyl and hydroxyl groups – for instance, the ability to bond with hydrogen. However, a carboxyl group is different from the two as far as chemical reactivity is concerned. Acidity is the most obvious trait of a carboxyl group. The high acidity is thanks to its ability to donate a proton to a solution, becoming ionized in the process. For any substance to become an acid, it should give away a proton.

A carboxyl group comprises an electronegative oxygen having a double bond with a carbon. This oxygen-carbon bond is quite polar, which means the electrons shared between the two atoms are unequal. And the bond polarity only goes up courtesy the double bond. Due to this polarity, compounds having carboxyl groups typically have higher boiling points.

The higher boiling and melting point could also be due to the carboxyl group’s ability to form hydrogen bonds in both the solid and liquid state. A substance boils only if the energy acquired overcomes its intermolecular forces. Stronger intermolecular forces require relatively higher energy to turn a liquid into gas/vapor. A hydrogen bond lends that strong intermolecular force; therefore, the higher melting and boiling points.