Question

Which of the following carboxylic acids undergoes decarboxylation easily?
A) ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{COCOOH}}$
B) ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{COC}}{{\text{H}}_{\text{2}}}{\text{COOH}}$
C) ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{CHOHCOOH}}$
D) ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{CHN}}{{\text{H}}_{\text{2}}}{\text{COOH}}$

Answer 1

Hint: Decarboxylation of acid is the removal of the carboxylic group from acid. \[\beta \]-keto acids undergo decarboxylation very easily. Thermal decomposition of \[\beta\]-keto acids is easy.

Complete step by step answer:
A reaction where there is a loss of carbon dioxide is called a decarboxylation reaction.
The general decarboxylation reaction is as follows:

Simple carboxylic acids do not undergo decarboxylation reactions easily. Carboxylic acids having carbonyl group at \[\beta \] potion that is \[\beta \]-keto acids undergo decarboxylation very easily.
As simple carboxylic acids are stable, they do not undergo decarboxylation reaction at the mild condition. While \[\beta \]-keto acids are unstable they undergo decarboxylation reactions easily at mild conditions also.
Now we can draw the structures of all carboxylic acid given in options and determine the \[\beta \]-keto acid.
The structure of ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{COCOOH}}$ is as follows:

As ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{COCOOH}}$ is not \[\beta \]-keto acid it will not undergo decarboxylation reaction easily. So, option (A) is incorrect.
Structure of
${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{COC}}{{\text{H}}_{\text{2}}}{\text{COOH}}$ is as follows:

As ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{COC}}{{\text{H}}_{\text{2}}}{\text{COOH}}$ is \[\beta \]-keto acid it will undergo decarboxylation reaction easily. So, option (B) is correct.
Structure of ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{CHOHCOOH}}$ is as follows:

As ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{CHOHCOOH}}$ is not \[\beta \]-keto acid it will not undergo decarboxylation reaction easily. So, option (C) is incorrect.
Structure of ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{CHN}}{{\text{H}}_{\text{2}}}{\text{COOH}}$ is as follows:

As ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{CHN}}{{\text{H}}_{\text{2}}}{\text{COOH}}$ is not \[\beta \]-keto acid it will not undergo decarboxylation reaction easily. So, option (D) is incorrect.

Thus, the correct option is (B) ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{COC}}{{\text{H}}_{\text{2}}}{\text{COOH}}$.

Note: In decarboxylation reaction carboxylic acid is converted into ketone after the elimination of carbon dioxide gas. Due to the formation of a cyclic six atom transition state, \[\beta \]-keto acids undergo decarboxylation reaction rapidly under the mild condition as well.