The correct option is
B III<I<IV<IIConjugate base formed after loss of
H+ for each of the following compounds is :
Acidic strength∝−I,−H,−M∝1+I,+H,+M
−CH3 and
−OCH3 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