Problem 138 - A Complete Resource Book in Chemistry for JEE Main

Question

Phenol reacts with bromine water in carbon disulphide at low temperature to give (a) o-bromophenol (b) 0 - and p-bromophenols (c) p-bromophenol (d) \(2,4,6\)-tribromophenol

Step-by-Step Solution

Answer

(b) o- and p-bromophenols

Step 1: Understanding the Reaction Environment

The reaction of phenol with bromine water in carbon disulfide under low temperatures is an example of electrophilic aromatic substitution. In this scenario, phenol acts as a phenoxide ion due to the presence of C6H5OH. Carbon disulfide acts as a solvent which affects the reaction pathway.

Step 2: Identify the Reactive Sites on Phenol

Phenol has an -OH group which is an activating group, directing the electrophilic substitution to ortho and para positions on the aromatic ring. This means that the bromine will likely attach to these positions because they are more electron-rich.

Step 3: Predict Possible Products

Given the conditions, the most likely products are ortho-bromophenol and para-bromophenol. In the presence of carbon disulfide and at low temperatures, reaction conditions limit excessive substitution that could lead to tri-brominated products.

Step 4: Analyze the Options Provided

Compare the predicted products (ortho-bromophenol and para-bromophenol) with the answer choices: (a) o-bromophenol, (b) o- and p-bromophenols, (c) p-bromophenol, and (d) 2,4,6-tribromophenol. The answer choice that includes both ortho and para substitution is (b).

Key Concepts

Bromination of Phenol

Bromination of phenol is a fascinating chemical reaction that falls under the category of electrophilic aromatic substitution. This reaction involves the addition of a bromine atom to the aromatic ring of phenol, transforming it into bromophenol. The conditions of the reaction are crucial. Typically, bromination occurs in the presence of bromine water and carbon disulfide as a solvent, under low-temperature conditions. This setup ensures controlled reaction rates and avoids extensive substitution that could complicate product formation.

The key player here is phenol, which has the chemical formula C₆H₅OH. The hydroxyl (-OH) group attached to the benzene ring makes phenol a highly reactive compound. Bromine, an electrophile, is attracted to the electron-rich aromatic ring of phenol. This attraction facilitates the substitution of hydrogen atoms on the ring by bromine atoms. Under the specified conditions, the reaction proceeds smoothly, selectively targeting specific positions on phenol's aromatic ring – primarily ortho and para positions.

Ortho and Para Substitution

The distinctive chemical structure of phenol influences the sites where bromination occurs. Specifically, it leads to ortho and para substitution. But what does this mean? The phenol molecule has a unique feature; it contains an activating group, the -OH group, which donates electrons to the benzene ring. This electron donation enhances certain positions on the ring, making them more susceptible to reaction with electrophiles like bromine.

Typically, electrophilic aromatic substitution in phenols favours positions adjacent to (ortho) and opposite to (para) the -OH group. Here's why these positions are favored:

• **Ortho Position:** The ortho positions are directly next to the hydroxyl group. This proximity allows for resonance stabilization, leading to enhanced reactivity at these sites.
• **Para Position:** The para position is directly opposite the hydroxyl group on the benzene ring. Like the ortho position, it also benefits from resonance stabilization, allowing electrophiles to target this site effectively.

Both these positions are significantly more reactive than the meta position, which is rarely targeted in such reactions. As a result, ortho-bromophenol and para-bromophenol are the primary products when phenol is brominated under controlled conditions.

Reaction Mechanisms

The mechanism for the bromination of phenol involves several well-defined steps, characteristic of electrophilic aromatic substitution reactions. Understanding this step-by-step process can clarify how phenol transforms into brominated derivatives.

Key steps in the mechanism:

• **Generation of the Electrophile:** It begins with the formation of bromine ions in the presence of bromine water and carbon disulfide. The bromine molecule itself acts as an electrophile, ready to attack the electron-rich aromatic ring of phenol.
• **Formation of the σ-complex (arenium ion):** Once the electrophile approaches the phenol, it forms a temporary complex known as an arenium ion. This complex is the intermediary where a bromine atom is effectively 'sticking' to the benzene ring, while the hydrogen atom at the point of attachment prepares to leave.
• **Deprotonation and Rearrangement:** The arenium ion undergoes deprotonation, releasing a hydrogen ion (H⁺) and restoring aromaticity to the benzene ring. This results in the brominated phenol product, either at the ortho or para position based on the mechanism pathway.

Each of these steps relies on both the electron-donating nature of the hydroxyl group and the specific conditions under which the reaction occurs. Together they ensure a directed, efficient substitution reaction ending in bromophenol formation.