Question 13.22:-

Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, ${\mathrm{E}}^{+}$

(a)Chlorobenzene, 2,4-dinitrochlorobenzene, p-nitrochlorobenzene
(b)Toluene,

Electrophiles are reagents that participate in a reaction by accepting an electron pair in order to bond to nucleophiles.
The higher the electron density on a benzene ring, the more reactive is the compound towards an electrophile, ${\mathrm{E}}^{+}$ (Electrophilic reaction)
(a) The presence of an electron-withdrawing group (i.e., ${{\mathrm{NO}}_{2}}^{-}$and ${\mathrm{Cl}}^{-}$) deactivates the aromatic ring by decreasing the electron density.
Since ${{\mathrm{NO}}_{2}}^{-}$ the group is more electron-withdrawing (due to resonance effect) than the ${\mathrm{Cl}}^{-}$ group (due to inductive effect), the decreasing order of reactivity is as follows: Chlorobenzene > p – nitro chlorobenzene > 2, 4 – dinitrochlorobenzene

(b) While ${{\mathrm{CH}}_{3}}^{-}$ is an electron-donating group, ${{\mathrm{NO}}_{2}}^{-}$ group is electron-withdrawing. Hence, toluene will have the maximum electron density and is most easily attacked by ${\mathrm{E}}^{+}$. ${{\mathrm{NO}}_{2}}^{-}$ is an electron-withdrawing group. Hence, when the number of ${{\mathrm{NO}}_{2}}^{-}$ substituents is
greater, the order is as follows:
Toluene.$>p-C{H}_{3}-{C}_{6}{H}_{4}-N{O}_{2,}p-{O}_{2}N-{C}_{6}{H}_{4}-N{O}_{2}$