Question

Which of the following carboxylic acids undergoes decarboxylation most easily’
(A) $C{H_3}COC{O_2}H$
(B) $C{H_3}COC{H_2}C{O_2}H$
(C) ${C_6}H.C{O_2}H$
(D) $C{H_3}C{H_2}C{O_2}H$

Answer 1

A carbonyl group at the $\beta$position of a carboxylic acid readily undergoes decarboxylation. So students try to draw the structures of the compounds given in the options and find the compound which has a carbonyl group at the $\beta$position.

Complete step by step answer:
Carboxylate ions do not lose $C{O_2}$ , for the same reason that alkanes such as ethane do not lose a proton—because the leaving group would be a carbanion. Carbanions are very strong bases and therefore are very poor leaving groups.
If, however, the $C{O_2}$ group is bonded to a carbon that is adjacent to a carbonyl carbon, the$C{O_2}$ group can be removed because the electrons left behind can be delocalized onto the carbonyl oxygen. Consequently, $\beta$-keto carboxylate ions (carboxylate ions with a keto group at the 3-position) lose when they are heated. Loss of from a molecule is called decarboxylation.
The reaction for decarboxylation for $\beta$ keto carboxylic acid is as follows :
$C{H_3} - \mathop C\limits^{\mathop \parallel \limits^O } - C{H_2} - \mathop C\limits^{\mathop \parallel \limits^O } - OH\xrightarrow[{ - C{O_2}}]{\Delta }C{H_3} - CO - C{H_3} + C{O_2}$

So, the correct answer is Option B.

Additional Information:
It is harder to remove a proton from an α-carbon if the electrons are delocalized onto the carbonyl group of an ester rather than onto the carbonyl group of a ketone. For the same reason, a higher temperature is required to decarboxylate a $\beta$-dicarboxylic acid such as malonic acid than to decarboxylate a $\beta$-keto acid.

Note: Decarboxylation is even easier if the reaction is carried out under acidic conditions, because the reaction is catalyzed by an intramolecular transfer of a proton from the carboxyl group to the carbonyl oxygen. The enol that is formed immediately tautomerizes to a ketone.