Carboxyl group

The carboxyl group is composed of two oxygen atoms bound to a carbon, one by a double bond and the other through a single bond. The latter is also covalently bonded to a hydrogen atom (R-COOH). Put simply, it can be seen as a functional group comprising a carbonyl group and a hydroxyl group.

The carboxyl group is the main acidic group in organic chemistry, and therefore can lose an H⁺, resulting in an anionic carboxylate group (R-COO^-). Carboxylate ions are stabilized by electronic resonance, which allows the carboxyl group to present a significant acidic behavior. Carboxyl groups are highly polar and can act as H donors and acceptors in hydrogen bonds.

In biochemistry, the carboxyl groups are sometimes removed from biomolecules in the so-called decarboxylation reactions (they are released in the form of CO₂).
The presence of carboxyl groups in the molecules can be detected by infrared spectroscopy, showing a characteristic vibration between 1680 and 1725 cm^-1. Alternatively, it can be identified by nuclear magnetic resonance, appearing in the region of 10-13 ppm.