carboxylate ion
Summary
A carboxylate ion (RCOO⁻) is the conjugate base formed when a carboxylic acid loses its proton. Resonance delocalizes the negative charge over both oxygens, which explains the acidity of carboxylic acids and why carboxylate salts dissolve in water.
Detail
Meaning: A carboxylate ion, RCOO⁻, forms when a carboxylic acid, RCOOH, is deprotonated. The negative charge is shared by both oxygens through resonance, so the two C–O bonds are equivalent and intermediate in length between a single and a double bond. This stabilization makes carboxylic acids (pKa about 4–5) much more acidic than alcohols. Electron-withdrawing groups near the carboxyl group stabilize the carboxylate further and make the acid stronger.
Equation: pH = pKa + log([RCOO⁻]/[RCOOH]), where pH and pKa are dimensionless and the brackets are concentrations in mol/L. When pH is above the pKa, the carboxylate form predominates.
On the MCAT: Extraction passages are the classic setup. A base such as NaOH or NaHCO₃ converts the acid to a charged carboxylate salt, which moves into the aqueous layer while neutral organic compounds stay in the organic layer. Adding strong acid regenerates RCOOH, which can be recovered. Related cases are amino acids at physiological pH, where the carboxyl group is present as COO⁻, and soaps, which are long-chain carboxylate salts.
Common confusion: Students put the charge on one oxygen and treat the two C–O bonds as different. Resonance makes them equivalent. The carboxylate is also the conjugate base, not the acid, so it is the form favored at high pH.
Sources
- Organic Chemistry (Clayden, Greeves, Warren)
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.