Question

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

1. Chlorobenzene, 2,4-dinitrochlorobenzene, p-nitrochlorobenzene
2. Toluene, p-H3C-C6H4-NO2, p-O2N-C6H4-NO2.

Answer 1

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- 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 - nitrochlorobenzene $>$ 2, 4 - dinitrochlorobenzene

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(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:
Toluene $>$ p-CH3-C6H4-NO2, p -O2 N-C6H4-NO2