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 2,4,6-tribromophenol (c).

Step 1: Understand the Reaction Conditions

Phenol is being reacted with a mixture containing potassium bromide (KBr) and potassium bromate (KBrO₃). This mixture is often used to generate bromine in situ, which then reacts with phenol.

Step 2: Analyze Bromine Formation

In the aqueous acidic medium, the reaction between KBr and KBrO₃ generates bromine (Br₂) according to the reaction: \[ ext{5 Br}^- + ext{BrO}_3^- + ext{6 H}^+ ightarrow ext{3 Br}_2 + ext{3 H}_2 ext{O} \] This step is crucial as it creates bromine that will perform bromination of phenol.

Step 3: Phenol's Reaction with Bromine

Phenol undergoes electrophilic aromatic substitution with the bromine generated. Due to the activating effect of the hydroxyl group (-OH), phenol readily undergoes bromination. The ortho and para positions become activated.

Step 4: Determine the Major Product

Under the conditions where bromine is in excess, phenol gets brominated at all the activated positions (2, 4, and 6 positions of phenol). This is because the -OH group makes the ortho and para positions highly reactive towards electrophilic substitution, leading to the formation of 2,4,6-tribromophenol as the major product.

Key Concepts

Bromination of Phenol

In the world of organic chemistry, phenol is a fascinating compound. One interesting reaction phenol undergoes is with bromine, known as the bromination of phenol. This process involves the replacement of hydrogen atoms in the phenol ring with bromine atoms. When phenol reacts with a mixture of potassium bromide (KBr) and potassium bromate (KBrO₃), bromine (\(\mathrm{Br}_2\)) is generated in situ.

The reaction happens through a special mechanism called electrophilic aromatic substitution.

• Bromine acts as the electrophile, which is an atom or molecule with a strong affinity for electrons.
• The presence of the hydroxyl group (-OH) on phenol significantly influences this substitution process.

The hydroxyl group activates the aromatic ring, especially at the ortho and para positions. Hence, when bromine comes into contact with phenol under these conditions, 2,4,6-tribromophenol is formed as the major product.

This is because the ortho and para positions on the phenol ring are more susceptible to attack due to the electron-donating effect of the hydroxyl group.

Aromatic Compounds

Aromatic compounds are a unique class of hydrocarbons known for their stability and distinct chemical properties. The key characteristic of an aromatic compound is the presence of a conjugated ring system that follows Huckel's rule. This rule states that an aromatic compound must have \(4n + 2\) pi electrons, where \(n\) is an integer.

Phenol, characterized by its benzene ring and hydroxyl group, is a classic example of an aromatic compound.

• The benzene ring provides additional stability through its delocalized electrons.
• This stability is a result of the overlapping p orbitals, forming a continuous pi-electron cloud.

Aromatic compounds like phenol undergo reactions called electrophilic aromatic substitutions. In these reactions, an electrophile, a molecule attracted to electrons, attacks the electron-rich aromatic ring.

This feature is crucial for understanding many organic reactions, such as the bromination of phenol.

Activating Groups in Aromatic Chemistry

Activating groups play a vital role in aromatic chemistry, especially in the context of electrophilic aromatic substitutions. These groups increase the reactivity of the aromatic ring, making it more prone to attack by electrophiles.

An excellent example of such an activating group is the hydroxyl group (-OH) in phenol.

• The -OH group is known for its electron-donating properties.
• It increases the electron density on the aromatic ring, particularly at the ortho and para positions.

This electron-donating effect makes these positions more reactive towards incoming electrophiles, such as bromine.

When phenol undergoes bromination, we see how the activating effect of the hydroxyl group leads to the formation of 2,4,6-tribromophenol. This is due to the increased accessibility and reactivity of these positions, highlighting the significant influence activating groups have in aromatic chemical reactions.