Carboxyl Group

∞ generated and posted on 2016.12.22 ∞

Common acid-bestowing hydrocarbon addition, presented in shorthand as -COOH.

Unlike most functional groups, and hydrophilic functional groups in particular, the carbon atom is considered to be a component of the carboxyl group itself. More precisely, a carboxyl group consists of a carbon atom to which two oxygen atoms are bound. One of these oxygen atoms is bound via a double bond and this resembles as carbonyl group (C=O). The other oxygen is attached via a single bond and is also bound to a hydrogen atom, thus resembling a hydroxyl group (C-OH). The overall group thus could be represented as O=C-OH where the fourth bond around carbon is attached to an additional carbon atom (thus, C-COOH).

As noted, carboxyl groups bestow an acidity upon organic compounds and thus are commonly described as acids (or with the suffix -ate, meaning the salt of an acid). When a long, linear chained hydrocarbon is capped at an end carbon with an carboxyl group, then the resulting molecule is called a fatty acid. Note that in the ionized form, that is, absent the hydrogen atom (but not absent its electron), the resulting group is possess a full negative charge.

Figure legend: Ionization of a carboxyl group. To the left is the carboxyl group in its un-ionized form, bestowing on its carrier the suffix -ic (acid), as it in acetic acid (vinegar) or pyruvic acid (the three-carbon molecule produced by glycolysis). The loss of the hydrogen ion results in an averaging of bonds, a process known as resonance, which stabilizes the structure. So configured, the group is considered to be a salt rather than acid so is described using the suffix, -ate, as in acetate or pyruvate.

The reason that carboxyl groups are acidic is because their associated hydrogen is easily lost, where hydrogen ion concentration in a solution is the determinant of pH (more H+ = lower pH = greater acidity).

But why does the hydrogen ion tend to dissociate from the carboxyl group? The answer can be found in the potential symmetry of the carboxyl group in the absence of the hydrogen ion. That is, instead of carbon bound to two oxygens, one via a double bond and one via a single bond, without the hydrogen ion the carbon on average can be bound to two one-half bonds, with the greater number of degrees of freedom and therefore greater entropy associated with this configuration resulting in a lower energy state and therefore greater stability. That is, carboxyl groups "want" to lose their hydrogens so that they can become less unstable.


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