Problem 15 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Benzyl chloride \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{Cl}\right)\) can be prepared from toluene by chlorination with (a) \(\mathrm{SO}_{2} \mathrm{Cl}_{2}\) (b) \(\mathrm{SOCl}_{2}\) (c) \(\mathrm{Cl}_{2}\) (d) \(\mathrm{NaOCl}\)

Step-by-Step Solution

Answer

The correct reagent is (a) \( \text{SO}_2\text{Cl}_2 \).

Step 1: Identify Reactants and Product

We need to identify the starting material (toluene) and the desired product (benzyl chloride, \( \text{C}_6\text{H}_5\text{CH}_2\text{Cl} \)). The question is asking which reagent can be used to chlorinate toluene to give benzyl chloride.

Step 2: Understand Chlorination Process

Chlorination involves the substitution of a hydrogen atom with a chlorine atom. Specifically, we want to substitute the hydrogen of the \( -\text{CH}_3 \) group attached to the benzene ring in toluene.

Step 3: Evaluate Each Option

(a) \( \text{SO}_2\text{Cl}_2 \) is used for side-chain chlorination.(b) \( \text{SOCl}_2 \) is typically used for converting alcohols to chlorides, not for side-chain chlorination.(c) \( \text{Cl}_2 \) in the presence of UV light can substitute on the side chain but is less selective.(d) \( \text{NaOCl} \) is an oxidizing agent, not used for direct chlorination of hydrocarbons.

Step 4: Choose the Appropriate Chlorinating Agent

Option (a), \( \text{SO}_2\text{Cl}_2 \), is known to be an efficient reagent for the chlorination at the side chain of toluene to form benzyl chloride, making it the correct option.

Key Concepts

Chlorination Process

Chlorination is a chemical reaction that involves the substitution of hydrogen atoms in a molecule with chlorine atoms. This process is crucial for the production of many chemicals, including benzyl chloride. In the context of toluene, which is a hydrocarbon, the chlorination specifically targets the methyl group (\(-\text{CH}_3\)) attached to the aromatic benzene ring. The selection of the chlorinating agent is key. Different agents can impact the outcome based on their selectivity and reactivity.
For instance, using \(\text{SO}_2\text{Cl}_2\) or sulfuryl chloride, facilitates an efficient substitution of hydrogen atoms in hydrocarbons, favoring the side-chain position rather than the more stable aromatic ring. This selectivity ensures the desired product, benzyl chloride, is generated. Understanding the chlorination process is essential for chemical manufacturing and synthesis, especially when precise control over the substitution is required.

Toluene Reactivity

Toluene, an aromatic hydrocarbon, is known for its unique reactivity due to the benzene ring and its \(-\text{CH}_3\) side chain. The aromatic ring in toluene provides a stable, delocalized electron cloud that typically resists substitution unless specific conditions are applied. The methyl group attached to the benzene ring is more reactive, particularly in reactions like halogenation under appropriate conditions.
In chemistry, we often aim to modify the side chain of toluene without affecting the stable benzene ring. This is because reactions occurring at the side chain can create more valuable products such as benzyl chloride, a precursor in many chemical syntheses. Leveraging the intrinsic reactivity of toluene through controlled conditions is a fundamental technique in organic chemistry.

Side-Chain Chlorination

Side-chain chlorination is a specific type of chlorination that focuses on substituting hydrogen atoms in the side chain of a molecule, rather than the main aromatic ring. For toluene, side-chain chlorination aims to replace a hydrogen atom in the \(-\text{CH}_3\) group with a chlorine atom, leading to the formation of benzyl chloride. This reaction is favorable when using reagents like \(\text{SO}_2\text{Cl}_2\), known for its effectiveness in targeting side-chain hydrogens.
The conditions under which side-chain chlorination is conducted are crucial. For example, the use of \(\text{Cl}_2\) along with UV light can achieve side-chain chlorination, though it might not be as selective as sulfuryl chloride. The choice of reagents and conditions greatly influences both the yield and the selectivity of the chlorination process, guiding chemists in producing desired compounds efficiently.