(a) The typical reactions of benzene are electrophilic substitution reactions. Higher the electron density in the benzene ring, more reactive is the compound toward these reactions. Since \(\mathrm{NO}_2\) is more powerful electron-withdrawing group than \(\mathrm{Cl}\), therefore, more the number of nitro groups, less reactive is the compound. Thus, the overall reactivity decreases in order.
Chlorobenzene \(>\) p-nitrochlorobenzene \(>2\), 4-dinitrochlorobenzene.
(b) Here, \(\mathrm{CH}_3\) group is electron donating but \(\mathrm{NO}_2\) group is electron-withdrawing. Therefore, the maximum electrondensity will be in toluene, followed by \(p\)-nitro toluene followed by \(p\)-dinitrobenzene.
Thus, the overall reactivity decreases in the order :
Toluene \(>p-\mathrm{H}_3 \mathrm{C}-\mathrm{C}_6 \mathrm{H}_4-\mathrm{NO}_2>p-\mathrm{O}_2 \mathrm{~N}-\mathrm{C}_6 \mathrm{H}_4-\mathrm{NO}_2\)