carboxylate ion
/ kar-BOK-sih-layt /
When a carboxylic acid hands its acidic proton to a base, what is left behind is the carboxylate ion. If you have ever made soap, neutralized vinegar with baking soda, or dissolved aspirin's salt form, you have made carboxylate ions. They are the deprotonated, negatively charged version of the acid, and they are remarkably stable — which is the whole reason carboxylic acids are as acidic as they are.
A carboxylate ion is written R-COO- (or R-CO2-). Here is the key picture: after the O-H proton leaves, the negative charge does not sit on just one oxygen. Instead it is shared equally over both oxygens by resonance. You can draw two resonance structures — charge on the left oxygen with a C=O on the right, or charge on the right oxygen with a C=O on the left — but the real ion is a single hybrid in which both carbon-oxygen bonds are identical, each about one-and-a-half bonds, and each oxygen carries half a negative charge. Spreading the charge over two electronegative oxygens lowers the energy enormously.
This resonance stabilization is why a carboxylic acid (pKa around 4–5) is millions of times more acidic than an alcohol (pKa around 16). An alkoxide ion from an alcohol must hold its negative charge on one lonely oxygen with no resonance help, so it is far less stable and far less willing to form. The lesson generalizes across organic chemistry: an acid is strong when its conjugate base is stabilized, and resonance delocalization is one of the most powerful stabilizers there is.
Acetic acid plus hydroxide gives acetate ion, CH3-COO-, plus water. In acetate, both C-O bonds measured by experiment are the same length — neither a pure single nor a pure double bond — exactly as resonance predicts.
The two oxygens of a carboxylate are equivalent; the charge is delocalized, not bouncing between them.
The two resonance structures are not the ion flickering back and forth between two real forms; they are two drawings of one averaged hybrid. The carboxylate is a single, symmetric species at all times.