Question

When nitrobenzene is treated with $Br_2$ in presence of $FeBr_3$, the major product formed is $m$-bromonitrobenzene. Statement which is related to obtain the $m$-isomer is

• A. the electron density on meta carbon is increased than that on ortho- and para-positions
• B. the intermediate carbonium ion formed after initial attack of $Br^+$ at the meta position is least destabilised
• C. loss of aromaticity when $Br^+$ attacks at the ortho and para positions and not at meta position
• D. easier loss of $H^+$ to regain aromaticity from the meta position than from ortho and para positions

Answer 1

Answer: (B)

Electron withdrawing groups decrease, electron density in the $o$- and $p$-positions without effecting $m$-positions, thus in such cases meta positions will has relatively higher electron density than the ortho and para positions it is not increased due to $-NO_2$ group. The intermediate carbocation formed by the attack of an electrophile on nitrobenzene will have relatively unstable resonating structure in case of ortho and para position.

Note that the above two structures are relatively unstable as $+ve$ charge is present on $C$ bearing electronegative group, while no such structure exists in case of meta substitution.