Carboxylate

Carboxylate ion
Acrylate ion

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′ (or RCO2R′). R and R′ are organic groups; R′ ≠ H.

A carboxylate ion is the conjugate base of a carboxylic acid, RCOO (or (or RCO2). It is an ion with negative charge.

Synthesis

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 or sodium bicarbonate.

${\displaystyle {\ce {RCO2H + NaOH -> RCO2Na + H2O}}}$

Resonance stabilization of the carboxylate ion

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. If the R group is an electron-withdrawing group (such as -CF3), the basicity of the carboxylate will be further weakened.

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.[1]

Reactions

Nucleophilic Substitution

Carboxylate ions are good nucleophiles. They react with alkyl halides to form ester. The following reaction shows the reaction  mechanism.

The SN2 reaction between sodium acetate and bromoethane. The products are ethyl acetate and sodium bromide.

The nucleophilicity of carboxylate ions are much weaker than that of hydroxide and alkoxide ions, but stronger than halide anions (in a polar aprotic solvent, though there are other effects such as solubility of the ion).

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 small amount of acid, the reaction occurs with lithium aluminium hydride by changing the LAH into a Lewis acid, AlH3 in the process, converting the oxyanion to 4 Al-O bonds.