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 SN2 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.
13.17:
Loss of Carboxy Group as CO2: Decarboxylation of Malonic Acid Derivatives
Just like β-keto acids—which upon thermal decarboxylation form ketones—β-dicarboxylic acids undergo decarboxylation to generate monocarboxylic acids with the liberation of carbon dioxide.
The mechanism initiates with an internal electronic redistribution, resulting in a cyclic six-membered transition state. This is followed by a C–C bond cleavage to produce an enol by releasing carbon dioxide gas. Next, the enol rapidly tautomerizes under the acidic conditions to yield a more stable monocarboxylic acid.
In malonic ester synthesis, the key step is the decarboxylation of malonic acid, wherein alkyl halides are converted to monocarboxylic acids.
