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
(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