Animated Solution for Chemistry - Organic Chemistry: The major product of the following reaction is

Imagine you are looking at a standard phenol molecule, but with a twist. Attached directly opposite to the hydroxyl group, at the para position, is a bulky sulfonic acid group ($-S{O}_{3}H$). This is our starting material: $p$-hydroxybenzenesulfonic acid.

The hydroxyl group ($-OH$) is a powerhouse. It pumps electron density into the benzene ring through resonance, making the ring highly reactive towards electrophiles. It specifically directs incoming groups to the ortho and para positions. On the other hand, the $-S{O}_{3}H$ group is electron-withdrawing and deactivating, but its influence is completely overpowered by the mighty $-OH$ group.

When we introduce excess bromine ($B{r}_2$) into the system, we are essentially flooding the environment with hungry electrophiles, specifically bromonium ions ($B{r}^{+}$).

Because the ring is so strongly activated by the $-OH$ group, the $B{r}^{+}$ ions don't just politely wait; they aggressively attack the electron-rich ortho positions. Since both ortho positions are unsubstituted, they are quickly brominated, giving us bromine atoms at positions 2 and 6.

Now, here is where the plot thickens. You might look at the para position and think, "Well, it's already occupied by the $-S{O}_{3}H$ group, so the reaction must stop here." But there is a catch!

The $-S{O}_{3}H$ group is what chemists call a good leaving group in the context of electrophilic aromatic substitution. Because the $-OH$ group is pushing so much electron density into the para position, the $B{r}^{+}$ ion attacks that exact carbon anyway. This rare and fascinating event is called ipso-substitution.

As the $B{r}^{+}$ ion forms a bond with the para carbon, the ring temporarily loses its aromaticity. To regain stability, something has to give. The $-S{O}_{3}H$ group is expelled as sulfur trioxide ($S{O}_3$) and a proton ($H^{+}$). This specific elimination is known as desulfonation.

With the $-S{O}_{3}H$ group gone, the bromine atom takes its place. The dust settles, and we are left with bromine atoms at both ortho positions and the para position. Our final, major product is 2,4,6-tribromophenol, a compound famous for crashing out of solution as a brilliant white precipitate.