Question

Consider the following sequence of reactions. \( 2\text{CH}_3\text{Cl} + \text{Si} \xrightarrow[\text{573 K}]{\text{Cu}} X \xrightarrow{\text{H}_2\text{O}} Y \xrightarrow{\text{Polymerization}} Z \). The repeating structural unit in Z is

• A. \( -[\text{O}-\text{CH}_3\text{Si(O-)}]- \)
• B. \( -[(\text{CH}_3)_2\text{Si-O}]- \)
• C. \( -[\text{O}-\text{Si(CH}_3\text{)-O}]- \) with another CH3 attached to Si
• D. \( -[\text{CH}_2-\text{Si(CH}_3\text{)-O}]- \) with another CH3 attached to Si

Hint:

The synthesis of silicones follows this general path: 1. Rochow Process: \(R-Cl + Si \rightarrow R_nSiCl_{4-n}\) (mainly \(R_2SiCl_2\)). 2. Hydrolysis: \(R_2SiCl_2 + H_2O \rightarrow R_2Si(OH)_2\) (a silanediol). 3. Polymerization: The silanediol molecules condense, eliminating water to form \(-[R_2Si-O]-\) chains. The number of chlorine atoms in the initial silane determines the structure (2 for linear chains, 3 for cross-linking, 1 for chain termination).

Answer 1

The Correct Option is B

This sequence describes the Rochow process for the synthesis of silicones.
Step 1: Identify compound X.
Methyl chloride (\(\text{CH}_3\text{Cl}\)) reacts with silicon (Si) powder in the presence of a copper catalyst at high temperature (573 K). This direct process forms a mixture of methylchlorosilanes. The principal product is dimethyldichlorosilane.
\( 2\text{CH}_3\text{Cl} + \text{Si} \xrightarrow[\text{Cu catalyst}]{\Delta} (\text{CH}_3)_2\text{SiCl}_2 \).
So, X is dimethyldichlorosilane, \((\text{CH}_3)_2\text{SiCl}_2\).
Step 2: Identify compound Y.
Compound X undergoes hydrolysis with water. The chlorine atoms are replaced by hydroxyl (-OH) groups.
\( (\text{CH}_3)_2\text{SiCl}_2 + 2\text{H}_2\text{O} \rightarrow (\text{CH}_3)_2\text{Si(OH)}_2 + 2\text{HCl} \).
So, Y is dimethylsilanediol, \((\text{CH}_3)_2\text{Si(OH)}_2\).
Step 3: Identify the repeating unit in Z.
Compound Y, a silanediol, undergoes condensation polymerization. A molecule of water is eliminated between two hydroxyl groups of adjacent monomers.
\( ... -OH + (\text{CH}_3)_2\text{Si(OH)}-OH + (\text{CH}_3)_2\text{Si(OH)}-OH - ... \rightarrow ... -O-Si(\text{CH}_3)_2-O-Si(\text{CH}_3)_2-O- ... + n\text{H}_2\text{O} \).
This forms a long polymer chain with a backbone of alternating silicon and oxygen atoms (-Si-O-Si-O-).
The repeating structural unit in this linear polymer (Z) is \( -[(\text{CH}_3)_2\text{Si-O}]- \).
This is a polysiloxane, commonly known as silicone.