Question 13.22:-

Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, E+

(a)Chlorobenzene, 2,4-dinitrochlorobenzene, p-nitrochlorobenzene
(b)Toluene, p-H3CC6H4NO2, p-O2NC6H4NO2.

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, E+ (Electrophilic reaction)
(a) The presence of an electron-withdrawing group (i.e., NO2-and Cl-) deactivates the aromatic ring by decreasing the electron density.
Since NO2- the group is more electron-withdrawing (due to resonance effect) than the Cl- group (due to inductive effect), the decreasing order of reactivity is as follows: Chlorobenzene > p – nitro chlorobenzene > 2, 4 – dinitrochlorobenzene

(b) While CH3- is an electron-donating group, NO2- group is electron-withdrawing. Hence, toluene will have the maximum electron density and is most easily attacked by E+. NO2- is an electron-withdrawing group. Hence, when the number of NO2- substituents is
greater, the order is as follows: