The following carboxylic acids undergo decarboxylation upon heating. The ease of decarboxylation is in the order

Concept:

Decarboxylation:

• Decarboxylation is the process of removal of a molecule of carbon dioxide from a compound.
• The rate of decarboxylation may be different for different molecules depending upon the nature of the substituent present in the molecule.
• The decarboxylation reaction is a kind of reverse Friedel craft reaction, in which a proton (provided by the carboxylic acid) itself acts as an electrophile. The protonation may occur anywhere, but it leads to decarboxylation only if it occurs where there is a -CO2H group.

Explanation:

• The electrophilic aromatic substitution reaction works best for indole in 3-position. The -CO2H group is present at the 3-position in indole.
• The electrophilic aromatic substitution reaction works best for pyridine in the 3-position. The -CO2H group is present at the 2-position in pyridine. This concludes the decarboxylation reaction will be fast for indol than for pyridine.
• The N atom present at the beta position facilitates the decarboxylation reaction.
• Due to the absence of an N atom at the beta position, benzoic acid will undergo decarboxylation at the slowest rate.
• The mechanism of the decarboxylation reaction is

Conclusion:

• Hence, the ease of decarboxylation is in the order 

C > B > A.