Nitrobenzene reacts with Br₂ in the presence of FeBr₃ to give m-bromonitrobenzene as major product. Which of the following provides the best reason for the formation of m-bromonitrobenzene as the major product?

• (A) The electrons density at the meta position is greater than those at the ortho and para positions.
• (B) Aromaticity is lost in the σ-complexes formed by the attack of Br⁺ at the ortho and para positions but not at the meta position.
• (C) The σ-complex formed by the attack of Br⁺ at the meta position is the least destabilized and the most stable among the three σ-complexes.
• (D) In the final step of regeneration of benzene ring by the loss of H⁺ from the σ-complexes, the meta-oriented σ-complex loses H⁺ most readily.

Nitrobenzene reacts with bromine in the presence of iron(III) bromide to predominantly yield m-bromonitrobenzene. The best explanation for this outcome is option (C): the σ-complex formed by the attack of Br⁺ at the meta position is the least destabilized and the most stable among the three σ-complexes. This stability allows the reaction to favor the formation of the meta product over the ortho and para products.

Understanding the Reaction Mechanism

In electrophilic aromatic substitution reactions, the stability of the intermediates plays a crucial role in determining the major product. Here’s a breakdown of why m-bromonitrobenzene is favored:

• Stability of σ-complexes: The meta position allows for a more stable σ-complex compared to the ortho and para positions.
• Electron Density: The electron-withdrawing nitro group decreases electron density at the ortho and para positions, making them less favorable for electrophilic attack.

Why Other Options Are Less Suitable

Let’s briefly examine why the other options do not provide the best explanation:

• Option A: While electron density is a factor, it does not solely determine the major product.
• Option B: Aromaticity is indeed important, but the stability of the σ-complexes is a more direct reason for the product distribution.
• Option D: The ease of losing H⁺ is relevant, but it is the stability of the σ-complex that primarily influences the reaction pathway.

In summary, the formation of m-bromonitrobenzene as the major product is best explained by the stability of the σ-complex formed at the meta position, making option (C) the correct choice.