Carboxyl group
A carboxyl group is a set of four atoms bonded together and present in carboxylic acids, including amino acids. Usually abbreviated as either CO2H or COOH, this set of atoms constitutes a functional group. In every carboxyl group the carbon atom is attached to an oxygen atom by a double bond and to a hydroxyl group (OH) by a single bond. In this way a carboxyl group is equivalent to a carbonyl group bonded to a hydroxyl group.
The acidic nature of a carboxyl group arises from its H atom. Specifically, in aqueous solution the H atom can be transferred as H+ to H2O molecules, forming H3O+ and leaving behind a COO– group. For example, the H atom in the carboxyl group of acetic acid (CH3COOH) reacts with H2O to make H3O+ and the acetate ion: CH3COOH + H2O → H3O+ + CH3COO–.
The drawings on this page show a carboxyl group bonding to an atom, or a groups of atoms, through its own carbon. In these representations, "R" symbolizes the atom or atoms to which bonding occurs. As examples, R = H for HCOOH, formic acid, but R = CH3 for acetic acid, CH3COOH.
The COOH group has only a separate fleeting existence when not bonded to a fifth atom. In such cases it is called the carboxyl radical and symbolized as •COOH. See, for example, the study by Milligan and Jacox.[1] The acid dissociation constant of •COOH has been measured using electron paramagnetic resonance spectrocopy.[2]
References
- ^ Milligan, D. E. (1971). "Infrared Spectrum and Structure of Intermediates in Reaction of OH with CO". Journal of Chemical Physics. 54 (3): 927–942.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ The value is pKa = -0.2 ± 0.1 according to the work of Jeevarajan, A. S. (1990). "ESR Measurement of the pKa of Carboxyl Radical and Ab Initio Calculation of the C-13 Hyperfine Constant". Journal of Physical Chemistry. 94 (4): 1372–1376.
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