Question

What is the product 'Z' in the following reaction sequence?

• A.

  

• B.

  

• C.

  

• D.

  

Hint:

Pay close attention to reaction conditions in aromatic substitution. For toluene: - Halogenation with Fe/dark (electrophilic substitution) occurs on the ring at ortho/para positions. - Halogenation with UV light/heat (free-radical substitution) occurs on the methyl group side-chain, forming benzyl halide.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
This is a multi-step organic synthesis starting from a diazonium salt. It involves a Sandmeyer reaction, a Wurtz-Fittig reaction, and an electrophilic aromatic substitution reaction.
Step 2: Tracing the Reaction Sequence:
- Step 1: Formation of X
The starting material is benzenediazonium chloride (C₆H₅N₂Cl). The reaction with Cu₂Cl₂/HCl is a Sandmeyer reaction. This reaction replaces the diazonium group (-N₂Cl) with a chlorine atom. \[ \text{C}_6\text{H}_5\text{N}_2\text{Cl} \xrightarrow{\text{Cu}_2\text{Cl}_2/\text{HCl}} \text{C}_6\text{H}_5\text{Cl} + \text{N}_2 \] So, X is chlorobenzene.
- Step 2: Formation of Y
Chlorobenzene (X) is reacted with methyl chloride (CH₃Cl) and sodium metal in dry ether. This is a Wurtz-Fittig reaction, which couples an aryl halide with an alkyl halide to form an alkylbenzene. \[ \text{C}_6\text{H}_5\text{Cl} + \text{CH}_3\text{Cl} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{C}_6\text{H}_5\text{CH}_3 + 2\text{NaCl} \] So, Y is toluene (methylbenzene).
- Step 3: Formation of Z
Toluene (Y) is reacted with chlorine (Cl₂) in the presence of iron (Fe) in the dark. This is a halogenation reaction, which is an electrophilic aromatic substitution. The methyl group (-CH₃) is an ortho-, para-directing and activating group. The reaction will produce a mixture of o-chlorotoluene and p-chlorotoluene.
\[ \text{C}_6\text{H}_5\text{CH}_3 + \text{Cl}_2 \xrightarrow{\text{Fe, dark}} o\text{-Cl-C}_6\text{H}_4\text{CH}_3 + p\text{-Cl-C}_6\text{H}_4\text{CH}_3 \] So, Z is a mixture of ortho- and para-chlorotoluene.
Step 3: Identifying Z from the Options:
We need to find the option that represents ortho- and/or para-chlorotoluene. - Option (A): Benzyl chloride (C₆H₅CH₂Cl). This would be formed by free-radical chlorination in the presence of UV light, not Fe/dark.
- Option (B): Toluene (Y). This is an intermediate.
- Option (C): This image shows p-chlorotoluene and o-chlorotoluene. This is the correct product mixture for Z. The image seems to show the para isomer.
- Option (D): Dichloromethane on a benzene ring, which is not a plausible product.
Looking at the options, option (C) correctly depicts one of the major products of the final step (p-chlorotoluene).
Step 4: Final Answer:
The product Z is a mixture of o-chlorotoluene and p-chlorotoluene. The structure shown in option (C) is p-chlorotoluene, a major product of the reaction. Therefore, (C) is the correct answer.