Question

The maximum number of NO₂ group that can introduce by nitration of benzene is usually
A. 4
B. 2
C. 3
D. 6

Answer 1

Hint: Group NO₂ is electrophile and thus, during the nitration of benzene this group will attach at the position where there electron density is more. There are three positions in benzene, ortho, meta, and para. Ortho is 1,2 relation, meta is 1,3 relation and para is 1,4 relation as per numbering. Find no. of electron rich positions to get the maximum number of NO₂ groups to introduce.

Complete Step by Step Solution:
To know that how much NO2 group will attach on benzene it is required to draw resonating structures of nitrobenzene which firstly form after the first nitration such as


From all resonating structures of nitrobenzene, positive charge resides on ortho and para position with respect to carbon number 1 which is directly attached to the nitro group. Thus, electron density at ortho and para will be less and at meta will be relatively more. Thus, further nitration leads to attaching the NO₂ group at meta position.

So, NO₂ can attach to meta position with respect to carbon 1 (directly attached to NO₂ group). As there are two meta positions thus two NO₂ groups can join at this position. Altogether three NO₂ groups can be introduced by nitration of benzene.
Thus, the correct option is B.

Note: After nitration of benzene, HNO₂ group is attached directly to the ring. As nitrogen is attached to the benzene ring and also directly attached to the oxygen atom, this group will behave as an electron withdrawing group and attract the whole electron density of the ring towards itself.