Problem 66 - A Complete Resource Book in Chemistry for JEE Main

Question

Benzyl chloride \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{Cl}\right)\) can be prepared from toluene by chlorination with (1) \(\mathrm{SO}_{2} \mathrm{Cl}_{2}\) (2) \(\mathrm{SOCl}_{2}\) (3) \(\mathrm{Cl}_{2}\) (4) \(\mathrm{NaOCl}\) (a) 1 and 2 (b) 1 and 4 (c) 2 and 3 (d) 4

Step-by-Step Solution

Answer

The correct choices are (c) 2 and 3.

Step 1: Understand the Reaction

Benzyl chloride is prepared by chlorination of toluene, which involves substituting a hydrogen atom in the methyl group of toluene (C_{6}H_{5}CH2 ight)6Cl ight)9). Identify the reagents that facilitate this reaction by replacing a hydrogen atom with chlorine (6Cl ight)9).

Step 2: Analyze the Given Options

Analyze each option to determine which reagents are suitable for this chlorination process. - SOCl2 ight)9) is generally used for converting alcohols to alkyl chlorides, not for direct substitution on aliphatic hydrogen. - SO2 ight)6Cl2 ight)9) is a reagent for chlorinating aliphatic hydrogens like in toluene. - Cl2 ight)9) is suitable for free radical chlorination, a common method for hydrogen substitution in hydrocarbons. - NaOCl2 is more of an oxidizing agent than a direct chlorinating agent.

Step 3: Match Reagents to Processes

Identify the appropriate reagents for chlorination based on the analysis: - SO2 ight)6Cl2 ight)9) and Cl2 ight)9) can facilitate substitution chlorination to produce benzyl chloride from toluene.

Step 4: Select the Correct Options

Based on the matched reagents (Step 3), recognize that (1) SO2 ight)6Cl2 ight) ) and (3) Cl2 ight) ) are both capable of preparing benzyl chloride from toluene. Therefore, the correct combination of reactions for this conversion is choice (c), 2 and 3.

Key Concepts

Chlorination Reaction

Chlorination is a chemical reaction where a chlorine atom bonds with another atom or molecule. This process is critical in organic synthesis and is used to transform hydrocarbons into more reactive chlorinated compounds. It's an important method in industrial chemistry, particularly for making intermediates that serve as building blocks for complex molecules.
In the specific case of benzyl chloride preparation from toluene, chlorination involves introducing chlorine to replace hydrogen in the methyl group. This can be done using different reagents, each facilitating the chlorination in unique ways.

• Free Radical Chlorination: A mechanism where chlorine radicals are used to replace hydrogen atoms. This is usually initiated by heat or light and commonly employs \( \mathrm{Cl}_2 \) as the chlorinating agent.
• Catalytic Methods: Some reagents, such as \(\mathrm{SO}_2\mathrm{Cl}_2\) (sulfuryl chloride), provide both the chlorine source and conditions suitable for chlorination.

These processes are widely used and understood through the lens of organic chemistry principles.

Toluene Conversion

Toluene, with the formula \(\mathrm{C}_6\mathrm{H}_5\mathrm{CH}_3\), is a simple aromatic hydrocarbon known for its versatility in chemical reactions. Converting toluene into benzyl chloride involves a key transformation—substitution of the hydrogen in its aliphatic methyl group with a chlorine atom.

• This conversion is essential in chemical manufacturing because it creates benzyl chloride, which is a useful intermediate in synthetic chemistry.
• For successful conversion, it's crucial to use appropriate chlorinating agents that specifically target the hydrogen atoms in the methyl group.

Successful chlorination of toluene to yield benzyl chloride relies on using \(\mathrm{Cl}_2\) or \(\mathrm{SO}_2\mathrm{Cl}_2\), as these reagents effectively facilitate the substitution process without affecting the aromatic ring.

Aliphatic Hydrogen Substitution

Aliphatic hydrogen substitution is a fundamental reaction in organic chemistry where hydrogen atoms in aliphatic compounds are replaced by other atoms, such as chlorine. In toluene, the hydrogen atoms in the methyl group are prime targets for substitution.

• Mechanism: The substitution can occur via radical reactions, where chlorine radicals replace hydrogen atoms, or through other mechanisms involving specific reagents.
• Reactivity: The reactivity of aliphatic hydrogens varies based on their position and the presence of other functional groups. Toluene, being an aromatic compound with an aliphatic side chain, displays unique reactivity.

Understanding the principles of aliphatic hydrogen substitution helps in selecting the appropriate reagents and reaction conditions to achieve desired chemical transformations. This makes the process of converting toluene into benzyl chloride possible through careful reagent choice and reaction management.