The correct option is B IV
has the least acidic strength
Conjugate base formed after loss of H+
for each of the following compounds is :
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, IV is the least stable followed by I.
In compounds II and III :
III 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 III
Correct order of stability of conjugate bases is :
More the stablility of conjugate base more is the acidic strength, so the correct acidic strength order is :