Problem 214 - A Complete Resource Book in Chemistry for JEE Main

Question

Phenol is heated with a solution of mixture of \(\mathrm{KBr}\) and \(\mathrm{KBrO}_{3} .\) The major product obtained in the above reaction is (a) 3-Bromophenol (b) 4-Bromophenol (c) \(2,4,6\)-Tribromophenol (d) 2-Bromophenol

Step-by-Step Solution

Answer

The major product is (c) \(2,4,6\)-Tribromophenol.

Step 1: Understand the Reagents

Phenol, when treated with a combination of \(\text{KBr}\) and \(\text{KBrO}_3\), undergoes bromination. The presence of \(\text{KBrO}_3\) acts as an oxidizing agent, which facilitates the production of bromine from \(\text{KBr}\). This setup leads to the formation of bromine molecules in situ, which can then react with phenol.

Step 2: Analyze the Reaction Pathway

Phenol is known for its high reactivity towards electrophilic aromatic substitution reactions due to the -OH group's activating effect. When bromine is introduced, it preferentially substitutes at ortho and para positions relative to the -OH group. However, the reaction with such a powerful brominating system will result in multiple substitutions on the aromatic ring.

Step 3: Determine the Bromination Outcome

Given that the \(\text{KBr}/\text{KBrO}_3\) mixture is a strong brominating agent, phenol undergoes bromination at all available ortho and para positions. Therefore, the major product of this reaction will be \( 2,4,6\)-tribromophenol, as the significant activation by the hydroxyl group leads to full bromination at all positions that are ortho and para to the hydroxyl group.

Key Concepts

Bromination

Bromination is a chemical reaction where bromine atoms are introduced into a compound. In the context of aromatic compounds like phenol, bromination typically involves an electrophilic aromatic substitution. In this type of reaction, the compound reacts with an electrophile, in this case, bromine. The bromine molecule initiates the reaction by interacting with the electron-rich aryl ring. An essential aspect to consider is the brominating agents. A mixture of KBr and KBrO 3 serves this purpose effectively. KBrO 3 acts as an oxidizing agent converting KBr into bromine molecules. This creates bromine in situ, which is highly reactive and enhances the bromination process. The reactivity of this mixture allows for the multiple bromination of phenol, resulting in tribromophenol under the right conditions.

Phenol

Phenol is a type of aromatic compound which features a hydroxyl group (-OH) directly attached to a benzene ring. This structural characteristic makes phenol considerably more reactive than benzene in electrophilic aromatic substitution reactions. The oxygen in the hydroxyl group donates electron density into the benzene ring through resonance and inductive effects. This electron-donating effect activates the benzene ring, making it more susceptible to bromination. When bromine is present, the increased electron density makes the ortho and para positions relative to the hydroxyl group more prone to substitution. In strong bromination reactions, phenol can undergo bromination at all available ortho and para positions. In the reaction with KBr and KBrO 3, phenol's reactivity leads to the formation of 2,4,6-tribromophenol as multiple substitutions occur.

Aromatic Compounds

Aromatic compounds are a class of organic compounds characterized by the presence of one or more planar ring structures with delocalized electrons. These compounds form stable arrangements due to resonance. In electrophilic aromatic substitution reactions, such as bromination, the aromatic ring serves as a nucleophile. Its electron-rich pi system interacts with electrophiles, like bromine, allowing substitutions to occur. The degree of substitution on an aromatic ring can be influenced by substituents already attached to the ring. For example, in phenol, the hydroxyl group increases the ring's nucleophilicity, influencing the reaction to favor ortho and para substitutions. Through appropriate activation, aromatic compounds can undergo multiple substitutions, which is evident in the case of phenol being transformed into 2,4,6-tribromophenol when reacting with a potent brominating agent.