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In organic chemistry, a carboxylate is the conjugate base of a carboxylic acid, (or ). It is an anion, an ion with negative charge.
Carboxylate salts are salts that have the general formula , where M is a metal and n is 1, 2,.... Carboxylate esters have the general formula (also written as ), where R and R′ are organic groups.
This delocalization of the electron means that both of the oxygen atoms are less strongly negatively charged: the positive proton is therefore less strongly attracted back to the carboxylate group once it has left; hence, the carboxylate ion is more stable and less basic as a result of resonance stabilization of the negative charge. In contrast, an alkoxide ion, once formed, would have a strong negative charge localized on its lone oxygen atom, which would strongly attract any nearby protons (indeed, alkoxides are very strong bases). Because of resonance stabilization, carboxylic acids have much lower pKa values (and are therefore stronger acids) than alcohols. For example, the p Ka value of acetic acid is 4.8, while ethanol has a p Ka of 16. Hence acetic acid is a much stronger acid than ethanol. This in turn means that for equimolar solutions of a carboxylic acid or an alcohol in water, the carboxylic acid would have a much lower pH.
The nucleophilicity of carboxylate ions is much weaker than that of hydroxide and alkoxide ions, but stronger than that of halide anions (in a polar aprotic solvent, though there are other effects such as solubility of the ion).
Reactions
Alkyation
Carboxylic acid salts with a hydrogen atom in the alpha position next to the carboxylate group can be converted to with strong bases like lithium diisopropylamide. These react with alkyl halides to give derivatives:
Nucleophilic substitution
Carboxylate ions are good nucleophile. They react with alkyl halides to form . The following reaction shows the reaction mechanism.
Reduction
Unlike the reduction of ester, the reduction of carboxylate is different, due to the lack of the leaving group and the relatively electron-rich carbon atom (due to the negative charge on the oxygen atoms). With a small amount of acid, the reaction occurs with lithium aluminium hydride by changing the LAH into the Lewis acid AlH3 in the process, converting the oxyanion to 4 Al–O bonds.
Examples
This list is for cases where there is a separate article for the anion or its derivatives. All other organic acids should be found at their parent carboxylic acid.
See also
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