Question

In the following series of reactions identify the major products A and B respectively.

• A.
• B.
• C.
• D.

Hint:

In electrophilic aromatic substitution reactions with multiple substituents, consider the directing effects of each group. Deactivating groups are generally meta directing, while activating and weakly deactivating groups (like halogens) are ortho-para directing. If the directing effects conflict, the stronger activating group usually dictates the regioselectivity. Steric hindrance can also play a role in determining the major product, especially in ortho substitution.

Answer 1

The Correct Option is B

The reaction series starts with bromobenzene undergoing two electrophilic aromatic substitution reactions. 
Step 1: Sulfonation of bromobenzene
Bromine is an ortho-para directing group and is also deactivating towards electrophilic aromatic substitution due to its inductive electron-withdrawing effect being stronger than its resonance electron-donating effect. 
However, it still directs the incoming electrophile to the ortho and para positions. Sulfonation involves the electrophile SO\( _3 \). 
The major product will be the para isomer due to less steric hindrance. 
Therefore, major product A is 4-bromobenzenesulfonic acid. 
Step 2: Bromination of 4-bromobenzenesulfonic acid
Now, 4-bromobenzenesulfonic acid undergoes bromination with Br\( _2 \) in the presence of FeBr\( _3 \). 
We need to consider the directing effects of both the bromine and the sulfonic acid group (-SO\( _3 \)H). Bromine is ortho-para directing. The sulfonic acid group is meta directing because it is a strongly deactivating group due to the strong electron-withdrawing nature of the sulfur atom bonded to three electronegative oxygen atoms. The directing effects are conflicting. 
However, bromination is an electrophilic substitution reaction, and the sulfonic acid group strongly deactivates the benzene ring, making further substitution less favorable. If substitution occurs, it will be directed by the deactivating sulfonic acid group to the meta position relative to it. The positions ortho and para to the sulfonic acid group are positions 2 and 4 relative to the bromine atom. The meta positions to the sulfonic acid group are positions 3 and 5 relative to the bromine atom. Considering the initial bromobenzene, the sulfonic acid group went to the para position. Now, the incoming bromine will be directed meta to the -SO\( _3 \)H group. The positions meta to the -SO\( _3 \)H group are ortho to the bromine atom. Therefore, the incoming bromine will add at the ortho position relative to the bromine atom and meta to the sulfonic acid group. This is the 2-position relative to bromine and the 3-position relative to the sulfonic acid group. Therefore, major product B is 2,4-dibromobenzenesulfonic acid (the bromine adds ortho to the existing bromine and meta to the sulfonic acid group). 
Matching with the options, option (2) has A as 4-bromobenzenesulfonic acid and B as 2,4-dibromobenzenesulfonic acid.