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-H_3C-C_6H_4 - NO_2, p-O_2N-C_6H_4-NO_2`.

Explanation

i)
Nitro group is electron withdrawing group. It deactivates the benzene ring towards reaction with electrophile. Thus, greater the number of nitro groups attached to the benzene ring, smaller is its reactivity with an electrophile, `E^(+)`.
ii)
Methyl group is an electron releasing group. It increases rectivity of benzene ring with an electrophile. On the other hand nitro is an electron with drawing group. It deactivates the benzene ring towards reacting with an electrophile. On the basis of this, we can conclude that greater the number of methyl groups attached to the ring greater is its reactivity with an electrophile, and greater the number of nitro groups on the thr ring, smaller is its reactivity with an electrophile.
The activating ability order of various groups is :
`NH_(2) gt -NHR gt -NR_(2) gt -OH gt - OR gt - X`