Electrophilic Bromination and Ipsosubstitution of Phenolsulfonic Acid

To determine the major product of the given reaction, we analyze the reaction step-by-step using the principles of electrophilic aromatic substitution (EAS) and ipso-substitution:

1. Activation and Directing Influence of the $-\text{OH}$ Group: In 4-hydroxybenzenesulfonic acid, the hydroxyl group ($-\text{OH}$) is a strong ortho/para-directing activating group. In an aqueous medium, hydrogen bonding and partial ionization enhance the electron density of the aromatic ring significantly, particularly at the positions ortho and para to the $-\text{OH}$ group (positions 2, 4, and 6).

2. Electrophilic Bromination at Ortho-Positions: The ortho-positions (positions 2 and 6 relative to $-\text{OH}$) are vacant and highly nucleophilic. The first two equivalents of aqueous bromine ($\text{Br}_2$) undergo electrophilic aromatic substitution at these positions: $\text{HO}-\text{C}_6\text{H}_4-\text{SO}_3\text{H} + 2\text{Br}_2 \longrightarrow \text{HO}-\text{C}_6\text{H}_2\text{Br}_2-\text{SO}_3\text{H} + 2\text{HBr}$ This yields 3,5-dibromo-4-hydroxybenzenesulfonic acid (or 2,6-dibromo-4-sulfophenol).

3. Ipso-Substitution (Desulfonation): The sulfonic acid group ($-\text{SO}_3\text{H}$) at position 4 (para to $-\text{OH}$) acts as a good leaving group during electrophilic substitution. In aqueous bromine, the third equivalent of $\text{Br}_2$ attacks the C-4 position occupied by the $-\text{SO}_3\text{H}$ group (ipso-attack).

   The loss of the $-\text{SO}_3\text{H}$ group as sulfuric acid ($\text{H}_2\text{SO}_4$) restores aromaticity, resulting in ipso-bromination: $\text{HO}-\text{C}_6\text{H}_2\text{Br}_2-\text{SO}_3\text{H} + \text{Br}_2 + \text{H}_2\text{O} \longrightarrow \text{HO}-\text{C}_6\text{H}_2\text{Br}_3 + \text{H}_2\text{SO}_4 + \text{HBr}$

4. Product Identification: The resulting molecule is 2,4,6-tribromophenol, which corresponds to compound $Q$.

Thus, the correct option is (B).