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

the electron density on meta-carbon is more than that on ortho- and para-positions

The correct answer is option 1. the electron density on meta-carbon is more than that on ortho- and para-positions.

When nitrobenzene is treated with \(Br_2\) in the presence of \(FeBr_3\), the major product formed is m-bromonitrobenzene. The nitro group \((-NO_2)\) is a strongly electron-withdrawing group that deactivates the benzene ring towards electrophilic substitution, particularly at the ortho and para positions. Here’s a detailed explanation of why the meta position is favored:

Nitro Group as a Meta-Directing Deactivator

The nitro group has both inductive \((-I)\) and resonance \((-M)\) effects that withdraw electron density from the benzene ring. Let’s break down these effects:

Inductive Effect (-I Effect):

The nitro group is highly electronegative and pulls electron density through the sigma bond network. This inductive withdrawal of electron density affects all positions on the benzene ring, but the effect is strongest at the ortho and para positions relative to the nitro group. As a result, these positions are less electron-rich and less favorable for electrophilic attack.

Resonance Effect (-M Effect):

The nitro group can participate in resonance by withdrawing electron density through the pi system. This effect further reduces electron density at the ortho and para positions. The resonance structures below show the delocalization of the nitro group:

These resonance structures indicate that the positive charge is more stabilized at the ortho and para positions, making these positions less favorable for electrophilic attack because they are electron-deficient.

Relative Electron Density

Due to the combined inductive and resonance effects of the nitro group, the electron density at the meta position is relatively higher compared to the ortho and para positions. This makes the meta position more favorable for electrophilic substitution.

Formation of the Sigma Complex

When bromine (\(Br^+\)) attacks the benzene ring, an intermediate sigma complex (arenium ion) is formed.

Ortho and Para Attack: If bromine attacks at the ortho or para positions, the resulting sigma complex would have a positive charge on the carbon adjacent to the nitro group. Because the nitro group is electron-withdrawing, it destabilizes the positive charge, making these intermediates less stable.

Meta Attack: If bromine attacks at the meta position, the resulting sigma complex does not have the positive charge directly adjacent to the nitro group, making this intermediate more stable compared to the ortho and para intermediates.

Conclusion

The meta position is favored because:

The electron density on the meta-carbon is relatively higher than on the ortho and para positions. The intermediate sigma complex formed when bromine attacks the meta position is more stable compared to those formed from ortho and para attacks due to the lack of direct destabilizing interaction with the electron-withdrawing nitro group.

Therefore, the correct statement explaining why the m-isomer is obtained is: (1) the electron density on meta-carbon is more than that on ortho- and para-positions