Inorganic chemistry, acarboxylate is theconjugate base of acarboxylic acid,RCOO− (orRCO−2). It is an anion, anion withnegative charge.
Carboxylate salts aresalts that have the general formulaM(RCOO)n, where M is a metal andn is 1, 2,....Carboxylate esters have the general formulaRCOOR′ (also written asRCO2R′), where R and R′ are organic groups.
Carboxylate ions can be formed by deprotonation of carboxylic acids. Such acids typically havepKa of less than 5, meaning that they can be deprotonated by many bases, such assodium hydroxide orsodium bicarbonate.[1]: 271–2
Carboxylic acids easily dissociate into a carboxylate anion and a positively charged hydrogen ion (proton), much more readily than alcohols do (into analkoxide ion and a proton), because the carboxylate ion is stabilized byresonance. The negative charge that is left afterdeprotonation of the carboxyl group is delocalized between the twoelectronegative oxygen atoms in a resonance structure. If the R group is an electron-withdrawing group (such as–CF3), the basicity of the carboxylate will be further weakened.[1]: 264–5
Thisdelocalization 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 ofresonance stabilization of the negative charge. In contrast, analkoxide 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 lowerpKa values (and are therefore stronger acids) thanalcohols. For example, the pKa value ofacetic acid is 4.8, whileethanol has a pKa 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 lowerpH.[1]: 263–7
Carboxylic acid salts with a hydrogen atom in thealpha position next to the carboxylate group can be converted todianions with strong bases likelithium diisopropylamide. These react withalkyl halides to give derivatives:[1]: 474
Carboxylate ions are goodnucleophiles. They react withalkyl halides to formesters. The following reaction shows the reaction mechanism.[1]: 398–9
The nucleophilicity of carboxylate ions is much weaker than that ofhydroxide andalkoxide ions, but stronger than that ofhalide anions (in apolar aprotic solvent, though there are other effects such assolubility of the ion).
Unlike the reduction of ester, the reduction of carboxylate is different, due to the lack of theleaving 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 withlithium aluminium hydride by changing the LAH into theLewis acidAlH3 in the process, converting the oxyanion to 4 Al–O bonds.[1]: 1212
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.