Question

The major products formed:

A and B respectively are:

• A.
• B.
• C.
• D.

Answer 1

The Correct Option is B

The reaction proceeds as follows: 1. The anisole (methoxybenzene) undergoes nitration with \( \text{HNO}_3/\text{H}_2\text{SO}_4 \) to form the product (A), which is 4-nitroanisole. 2. Subsequently, bromination with \( \text{Br}_2 \) (in the presence of excess iron) results in the brominated product (B), 4-bromo-2-nitroanisole.

Thus, the major products are (A) and (B) as represented in option (2).

Answer 2

The problem presents a two-step reaction sequence starting with anisole (methoxybenzene). We need to identify the major products 'A' and 'B' formed in each step.

Concept Used:

The solution involves the principles of electrophilic aromatic substitution (EAS) on a substituted benzene ring. The key factors are:

1. Directing Effects of Substituents:
   • Activating Groups: Electron-donating groups (like \(-OCH_3\)) activate the ring towards EAS and direct the incoming electrophile to the ortho and para positions. The methoxy group (\(-OCH_3\)) is a strong activating group due to its powerful positive resonance effect (\(+M\)).
   • Deactivating Groups: Electron-withdrawing groups (like \(-NO_2\)) deactivate the ring and direct the incoming electrophile to the meta position. The nitro group (\(-NO_2\)) is a strong deactivating group due to its strong negative resonance (\(-M\)) and inductive (\(-I\)) effects.
2. Steric Hindrance: When both ortho and para positions are available, the electrophile often preferentially attacks the less sterically hindered para position, making the para isomer the major product.
3. Reactions on Disubstituted Benzene: When two substituents are present, the position of the next substitution is determined by the combined directing effects. A strongly activating group will dominate over a deactivating group in controlling the position of substitution.

Step-by-Step Solution:

Step 1: Formation of Product 'A' (Nitration of Anisole)

The first step is the nitration of anisole using a mixture of concentrated nitric acid (\(HNO_3\)) and sulfuric acid (\(H_2SO_4\)). This mixture generates the electrophile, the nitronium ion (\(NO_2^+\)).

The methoxy group (\(-OCH_3\)) on the anisole ring is a strong activating group and is an ortho, para-director.

• Attack at the para position leads to the formation of 4-nitroanisole.
• Attack at the ortho position leads to the formation of 2-nitroanisole.

Due to the steric bulk of the \(-OCH_3\) group, the attack at the less hindered para position is favored. Therefore, 4-nitroanisole is the major product.

Product 'A' is 4-nitroanisole.

Step 2: Formation of Product 'B' (Bromination of 4-nitroanisole)

The second step is the bromination of product 'A' (4-nitroanisole) with excess bromine (\(Br_2\)) in the presence of a Lewis acid catalyst (Fe, which forms \(FeBr_3\) in situ). The electrophile is the bromonium ion (\(Br^+\)).

The reactant, 4-nitroanisole, has two substituents:

• The \(-OCH_3\) group at position 1 (a strong activator, ortho, para-directing).
• The \(-NO_2\) group at position 4 (a strong deactivator, meta-directing).

The powerful activating effect of the \(-OCH_3\) group governs the position of substitution. It directs the incoming \(Br^+\) to its ortho positions (positions 2 and 6), as its para position is already occupied by the \(-NO_2\) group. The \(-NO_2\) group also directs the incoming electrophile to its meta positions, which are also positions 2 and 6. Thus, both groups direct the substitution to the same positions.

Since the \(-OCH_3\) group is a very strong activator and excess bromine is used, bromination occurs at all available activated positions, which are both ortho positions (2 and 6).

Product 'B' is 2,6-dibromo-4-nitroanisole.

 

Conclusion:

Product 'A' is 4-nitroanisole, and product 'B' is 2,6-dibromo-4-nitroanisole. This corresponds to the structures shown in option (2).