Carboxylate

In organic chemistry, a carboxylate is the conjugate base of a carboxylic acid, RCOO⁻ (or RCO−2). It is an anion, an ion with negative charge.

Carboxylate salts are salts that have the general formula M(RCOO)_n, where M is a metal and n is 1, 2,.... Carboxylate esters have the general formula RCOOR′ (also written as RCO₂R′), where R and R′ are organic groups.

Synthesis

Carboxylate ions can be formed by deprotonation of carboxylic acids. Such acids typically have pK_a of less than 5, meaning that they can be deprotonated by many bases, such as sodium hydroxide or sodium bicarbonate.^[1]^: 271–2

{\ce {RCOOH + NaOH -> RCOONa + 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.^[1]^: 264–5

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 pK_a values (and are therefore stronger acids) than alcohols. For example, the pK_a value of acetic acid is 4.8, while ethanol has a pK_a 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.^[1]^: 263–7

Reactions

Alkyation

Carboxylic acid salts with a hydrogen atom in the alpha position next to the carboxylate group can be converted to dianions with strong bases like lithium diisopropylamide. These react with alkyl halides to give derivatives:^[1]^: 474

RCH₂COO⁻ + Li⁺[⁻N(CH(CH₃)₂)₂] → RCH⁻COO⁻

RCH⁻COO⁻ + R'X → RR'CHCOO⁻ + X⁻

Nucleophilic substitution

Carboxylate ions are good nucleophiles. They react with alkyl halides to form esters. The following reaction shows the reaction mechanism.^[1]^: 398–9

The S_N2 reaction between sodium acetate and bromoethane. The products are ethyl acetate and sodium bromide.

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).

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.^[1]^: 1212

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.

Formate ion, HCOO⁻
Acetate ion, CH₃COO⁻
Methanetetracarboxylate ion, C(COO⁻)₄
Oxalate ion, (COO)²⁻
₂

References

^ ^a ^b ^c ^d ^e ^f March, Jerry (1992). Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (4th ed.). New York: Wiley. ISBN 0-471-60180-2.

[March4-1] ^ ^a ^b ^c ^d ^e ^f March, Jerry (1992). Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (4th ed.). New York: Wiley. ISBN 0-471-60180-2.

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