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 ordinarily write the structure of the carboxyl group simply -COOH, or -CO2H. To the uninitiated, this suggests the two oxygen atoms are connected or bound to each other, though they are not. Oxygen drawn to the immediate right of carbon shares both its valence electrons with that atom, forming a carbonyl group (-C=O). The other oxygen attaches to that same carbon as well as to hydrogen by single bonds only, resulting in a carbon-attached hydroxyl group (-C-OH).
Organic compounds containing one or more carboxyl groups are called carboxylic acids. Two common examples of single carboxyl group carboxylic acids are formic acid (HCOOH), first prepared from the distillation of ants, and acetic acid (CH3COOH), the vinegar of fermentation. 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 usually stronger than might be supposed.
This is because certain factors favor the ionized form, or carboxylate anion, -COO-, over the united carboxyl group. When hydrogen departs, its electron remains behind. Although it a phenomenon in nature that charge "desires" 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 carboxylate, the carbon-attached hydroxyl group, -C-OH changes to -C-O-. A free electron — here, the tiny minus drawn at the upper right of the oxygen, but alone, written as e- — has some freedom to move about. It would seem to be able to depart via the reaction mechanism -C-O- → -C=O + e-.
Conversely, the other oxygen should be able to pick up that electron -C=O + e- → -C-O-. The point is, both oxygens are equivalent in this environment, in which neither is encumbered with 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 carboxylate because of the electron delocalization. In addition, neither oxygen should bond to carbon with either a single or a double bond. The length of the bonds should be equal and be something like a "one-and-a-half" bond. In fact, they are. For acetic acid, carbonyl oxygen-to-carbon bond length is 1.21Å and carbon-attached hydroxyl has a length of 1.36Å, whereas for carboxylate, both carbon-to-oxygen bond lengths are 1.26Å.