Compare the acidity of the following: (A) ${\text{CH}}_3{\text{COCH}}_3$, (B) ${\text{CH}}_3{\text{NO}}_2$, (C) ${\text{CH}}_3\text{CN}$, (D) ${\text{CH}}_3\text{CHO}$.

Solution

1. The conjugate base of ${\text{CH}}_3{\text{NO}}_2$ is ${\text{CH}}_2^{-}{\text{NO}}_2$, where the negative charge is delocalized over two highly electronegative oxygen atoms.
2. The conjugate base of ${\text{CH}}_3{\text{COCH}}_3$ is ${\text{CH}}_2^{-}{\text{COCH}}_3$, where the charge is delocalized over one oxygen.
3. The conjugate base of ${\text{CH}}_3\text{CN}$ is ${\text{CH}}_2^{-}\text{CN}$, where the charge is delocalized over nitrogen.
4. The conjugate base of ${\text{CH}}_3\text{CHO}$ is ${\text{CH}}_2^{-}\text{CHO}$, where the charge is delocalized over one oxygen.
5. The resonance stabilization provided by the two oxygens in the nitronate ion makes ${\text{CH}}_3{\text{NO}}_2$ the most acidic.
6. Hence the answer is (B).