Phenol is heated with a solution of mixture of KBr and KBrO3. The major product obtainedin the above reaction is

When phenol is heated with a mixture of potassium bromide (KBr) and potassium bromate (KBrO3), the primary reaction that occurs is bromination. This process leads to the formation of brominated phenol, specifically 2-bromophenol, as the major product. Let's break down how this reaction takes place and why this particular product is favored.

The Reaction Mechanism

The bromination of phenol involves the electrophilic aromatic substitution mechanism. Here’s how it unfolds:

• Formation of Bromine Species: In the presence of KBrO3, bromate ions (BrO3^-) can generate bromine (Br₂) in situ. This occurs through an acid-base reaction where bromate is reduced, typically in acidic conditions, producing bromine.
• Electrophilic Attack: The generated bromine acts as an electrophile. Phenol, with its hydroxyl group (-OH), is highly activated towards electrophilic substitution due to the electron-donating nature of the -OH group. This makes the aromatic ring more nucleophilic.
• Substitution Position: The hydroxyl group directs the incoming bromine to the ortho and para positions relative to itself. However, due to steric hindrance and the stability of the intermediates, the ortho position is often favored, leading to the formation of 2-bromophenol as the major product.

Why 2-Bromophenol?

The preference for 2-bromophenol over other possible products can be attributed to several factors:

• Sterics: The ortho position is slightly hindered compared to the para position, but the stability of the transition state and the resonance stabilization of the resulting carbocation intermediate favors the ortho substitution.
• Resonance Effects: The -OH group can stabilize the positive charge that develops during the formation of the sigma complex (the intermediate formed during electrophilic substitution). This stabilization is more effective at the ortho position due to the proximity of the -OH group.

Summary of the Reaction

In summary, when phenol reacts with a mixture of KBr and KBrO3, the major product is 2-bromophenol. This outcome is a result of the electrophilic aromatic substitution mechanism, where bromine acts as the electrophile and the hydroxyl group on phenol directs the bromination to the ortho position. Understanding this reaction not only illustrates the principles of electrophilic substitution but also highlights the influence of functional groups on reaction pathways.