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-H3C – C6H4 – NO2, p-O2N – C6H4 – NO2.
We know that electrophiles are the reagents or species that participate in a reaction by accepting an electron pair in order to bond with nucleophiles.
Also, the higher the electron density of a benzene ring, the more reactive is the compound towards an electrophile, E+ (Electrophilic reaction).
(a) The presence of an electron withdrawing group like 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 the reactivity is as follows:-
Chlorobenzene > p – nitrochlorobenzene > 2, 4 – dinitrochlorobenzene
(b) We know that CH3- is an electron donating group and NO2- the 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 decreasing order of the reactivity order is as follows:
Toluene > p-H3C – C6H4 – NO2> p-O2N – C6H4 – NO2