A carboxylate is the conjugate base of a carboxylic acid. Carboxylate salts have the general formula M(RCOO)n, where M is a metal and n is 1, 2,...; carboxylate esters have the general formula RCOOR′. R and R′ are organic groups; R′ ≠ H.
A carboxylate can be easily made by deprotonation of carboxylic acids. Most of them has a pKa of approximately 5, which means that they can be deprotonated by many bases, such as sodium hydroxide.
Resonance stabilization of the carboxylate ionEdit
Carboxylic acids easily dissociate into a carboxylate anion and a positively charged hydrogen ion (proton), much more readily than alcohols do (into an alkoxide ion and a proton), because the carboxylate ion is stabilized by resonance. The negative charge that is left after deprotonation of the carboxyl group is delocalized between the two electronegative oxygen atoms in a resonance structure.
This delocalization of the electron cloud 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. In contrast, an alkoxide ion, once formed, would have a strong negative charge on the oxygen atom, which would make it difficult for the proton to escape. Carboxylic acids thus have a lower pKa values than alcohols. For example, the pKa value of ethanol is 16 while acetic acid has a pKa of 4.9. Hence acetic acid is a much stronger acid than ethanol. the higher the number of protons in solution, the lower the pH.