Question

Which of the following compounds is the most acidic?

• A.
• B.
• C.
• D.

Answer 1

The correct option is B

Conjugate base formed after loss of $H^{+}$ for each of the following compounds is :
$\text{Acidic strength}\propto -I,-H,-M\propto \frac{1}{+I,+H,+M}$


$-C{H}_3$ and $-OC{H}_3$ when attached to benzene ring show +H and +R effect respectively, which destabilizes the conjugate base .
Since, +R effect is more predominant than +H effect so, III is the least stable followed by I.
In compounds II and IV :
IV is stabilized by delocalization of negative charge on benzene ring. The resonating structures so formed are not equivalent resonating structures.
II is stabilized by -R effect of $-CN$ group. So, it is more stable than IV
Correct order of stability of conjugate bases is :
$III<I<IV<II$
More the stablility of conjugate base more is the acidic strength, so the correct acidic strength order is :
$III<I<IV<II$