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The carboxyl group (-COOH) is made up of one carbon, two oxygen, and one hydrogen atom. Organic compounds with carboxyl groups are called carboxylic acids. The carboxylate anion (-COO-) is stabilized by resonance, which makes carboxylic acids stronger than expected.
A carboxyl group, or carboxylic acid group, is the combination of four atoms that act as a unit: one carbon (C), two oxygens (O), and one hydrogen (H). Organic chemists usually write the structure of the carboxyl group simply -COOH, or -CO2H. For the uninitiated, this suggests that the two oxygen atoms are connected or bonded to each other, even though they aren’t. Oxygen attracted to the immediate right of carbon shares both of its valence electrons with that atom, forming a carbonyl group (-C=O). Other oxygen bonds to carbon itself and hydrogen only via single bonds, resulting in a hydroxyl group bonded to the carbon (-C-OH).
Organic compounds containing one or more carboxyl groups are called carboxylic acids. Two common examples of single-carboxylic group carboxylic acids are formic acid (HCOOH), first prepared by distillation of ants, and acetic acid (CH3COOH), the fermenting vinegar. The powerful oxalic acid is the simplest of those acids with two carboxyl groups. Its chemical structure can be drawn as HOOC-COOH or (COOH)2. Oxygen-containing carboxylic acids are generally stronger than one might assume.
This is because certain factors favor the ionized form, or carboxylate anion, -COO-, over the joined carboxyl group. When hydrogen leaves, its electron is left behind. While it is a phenomenon in nature that the charge “wishes” to be neutralized, other factors, such as resonance, can stabilize a charged chemical species considerably. To visualize this, it is necessary to once again consider the structure of the carboxyl group at a more detailed level.
In the carboxylate, the hydroxyl group bonded to the carbon, -C-OH, changes to -CO-. A free electron – here, the minuscule drawn top right of oxygen, but by itself, written as e- – has some freedom of movement. It would seem to be able to start through the reaction mechanism -CO- → -C=O + e-.
Conversely, the other oxygen should be able to capture that electron -C=O + e- → -CO-. The point is that both oxygens are equivalent in this environment, where neither is encumbered by a hydrogen atom. At least on paper, the electron should be able to resonate, or travel back and forth, between the two oxygen atoms.
Logically, this resonance should stabilize the carboxylate due to electron delocalization. Also, neither oxygen should bond to carbon with either a single bond or a double bond. The length of the ties should be equal and be something like a “one and a half” tie. In fact, they are. For acetic acid, the carbon-oxygen-carbon bond length is 1.21Å and the hydroxyl attached to the carbon has a length of 1.36Å, while for the carboxylate, both carbon-oxygen bond lengths are 1.26Å.
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