Question

Arrange the following compounds in order of decreasing reactivity towards electrophilic substitution reaction.
(I) Benzene
(II) Nitrobenzene
(III) 1, 3-Dinitrobenzene
(IV) 1, 3, 5-Trinitrobenzene

• A. I > II > III > IV
• B. I > III > II > IV
• C. III > I > IV > II
• D. IV > III > II > I

Answer 1

The correct option is A I > II > III > IV

Rate of electrophilic substitution reaction is proportional to the stability of the carbocation intermediate (Arenium ion) formed in the reaction.

$\text{Stability of Arenium ion}\propto \text{+R, +H, +I groups}$

$\text{Stability of Arenium ion}\propto \frac{1}{\text{-R, -H, -I groups}}$
Here, $N{O}_2$ is a electron withdrawing group it will attract the electron density toward itself and reduce the stability of carbocation intermediate formed.
Increase in number of $N{O}_2$ group will decrease the stability of intermediate more.
Thus, compound (IV) will have least stability followed by compound (II) and (III).
Compound (I), benzene has more stable intermediate because it is not destabilised by $N{O}_2$ group.
Thus, the correct order of decreasing reactivity toward electrophilic substitution is
(I) > (II) > (III) > (IV)