Question

Which of the following compound will have the smallest $p{K}_a$ value?

• A. Benzoic acid
• B. Formic acid
• C. Acetic acid
• D. Phenyl acetic acid

Answer 1

Answer: (B)

The stronger is the acid, the lower is the $p{K}_a$ value. Since, an electron releasing group such as $-C{H}_3,-C_2{H}_5$ destabilises the carboxylate ion, thus decreases the acidic strength of the acid.
Phenyl group is however electron withdrawing but benzoic acid is a weaker acid as compared to formic acid.
This is because of $+R$ effect of phenyl group, which on the other hand destabilises the carboxylate ion.
Hence, formic acid $(HCOOH)$ is the strongest acid.
Thus, its $p{K}_a$ value will be lowest.