Question

In the above reaction electrophile is substituted at meta position only, due to

I. Electron density is more at ortho & para position
II. Electron density is relatively less at ortho & para position
III. Electron density is less at meta position
IV. Electron density is relatively more at meta position Correct answer is

• A. I, III only
• B. II, IV only
• C. I only
• D. III only

Hint:

Electron-withdrawing groups (like \( -NO_2 \)) are meta-directing because they decrease the electron density of the benzene ring, especially at the ortho and para positions, making the meta positions relatively electron-rich and thus more favorable for electrophilic attack. Draw the resonance structures of the intermediate carbocations formed by electrophilic attack at ortho, meta, and para positions to understand their relative stabilities.

Answer 1

The Correct Option is B

The given reaction shows the nitration of nitrobenzene.
The nitro group (\( -NO_2 \)) is a meta-directing group in electrophilic aromatic substitution reactions.
This means that the incoming electrophile (\( E^+ \)) preferentially substitutes at the meta position relative to the nitro group already present on the benzene ring.
The reason for this meta-direction can be understood by examining the effect of the nitro group on the electron density of the benzene ring through resonance.
The nitro group is an electron-withdrawing group, and it withdraws electron density from the benzene ring, especially at the ortho and para positions.
Let's consider the resonance structures of nitrobenzene: The nitro group has the following resonance structures that contribute to its electron-withdrawing nature: When an electrophile attacks the ortho or para positions, one of the resulting resonance structures of the intermediate carbocation has a positive charge directly adjacent to the electron-withdrawing nitro group.
This makes these intermediates less stable.
When an electrophile attacks the meta position, none of the resonance structures of the resulting intermediate carbocation have a positive charge directly adjacent to the electron-withdrawing nitro group.
This makes the intermediate formed via meta attack relatively more stable compared to those formed via ortho or para attack.
Therefore, the nitro group deactivates the benzene ring towards electrophilic substitution, and it deactivates the ortho and para positions more strongly than the meta positions.
This results in lower electron density at the ortho and para positions and relatively higher electron density at the meta positions.
The electrophile, being electron-loving, will attack the positions with relatively higher electron density, which are the meta positions in this case.
Thus, the electrophilic substitution occurs at the meta position because: - Electron density is relatively less at ortho and para positions (Statement II is correct).
- Electron density is relatively more at meta position (Statement IV is correct).
Statements I and III are incorrect because they describe the opposite situation.
The correct answer is the combination of statements II and IV.