Question
Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile E+:
(a) Chlorobenzene; 2, 4-dinitrochlorobenzene, p- nitro chlorobenzene
(b) Toluene, p-H3C – C6H4 – NO2, p-O2N – C6H4 – NO2
Solution
(a) 
Nitro (NO2) group is a deactivating group. Its presence on the benzene ring will deactivate it towards electrophile attack since electrophile seeks a centre of high electron density. Thus, more the number of nitro groups present, lesser will be the reactivity of the compounds towards electrophilic substitution.
(b) Here, CH3 group is electron donating but NO2 group is electron withdrawing. Therefore, the maximum electron density will be in toluene, followed by p-nitrotoluene followed by p-dinitrobenzene. Thus, the overall reactivity decrease in the order:
