13.17 : Loss of Carboxy Group as CO₂: Decarboxylation of Malonic Acid Derivatives

Recall that β-keto acids undergo thermal decarboxylation to yield a ketone.

Similarly, β-dicarboxylic acids such as malonic acid which bears a β-carboxyl group readily decarboxylate to generate monocarboxylic acids.

The mechanism involves an internal electron transfer via a cyclic six-membered transition state which then forms an enol with the simultaneous release of CO₂ gas. Next, the enol tautomerizes to yield the acetic acid.

Decarboxylation of β-dicarboxylic acids is a crucial step in malonic ester synthesis, wherein haloalkanes are converted to carboxylic acids with the parent chain lengthened by two carbons.

Here, the α-H atom of diethyl malonate is first deprotonated to form a resonance stabilized enolate.

Next, an S_N2 attack on the haloalkane leads to the alkylation of the malonate. This, upon further acidification, hydrolyzes to generate a substituted malonic acid.

Finally, decarboxylation and subsequent tautomerization of the substituted malonic acid yields the monocarboxylic acid.

To synthesize cycloalkanecarboxylic acids, an appropriate dihalide is reacted with diethyl malonate, which undergoes intramolecular cyclization to yield cyclic monocarboxylic acids.