Problem 143 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following substituted silanes the one which will give rise to cross linked silicone polymer on hydrolysis is \([2008]\) (a) \(\mathrm{R}_{4} \mathrm{Si}\) (b) \(\mathrm{RSiCl}_{3}\) (c) \(\mathrm{R}_{2} \mathrm{SiCl}_{2}\) (d) \(\mathrm{R}_{3} \mathrm{SiCl}\)

Step-by-Step Solution

Answer

(b) \(\mathrm{RSiCl}_3\) will give a cross-linked silicone polymer.

Step 1: Analyzing the Options

We need to identify which compound, upon hydrolysis, forms a cross-linked silicone polymer. Cross-linking generally occurs when a compound can form a network structure, typically with multiple bonding sites available.

Step 2: Examining Each Compound

1. **(a) **\(\mathrm{R}_4 \mathrm{Si}\)**: There are no chlorine atoms, so it cannot hydrolyze to form \(\mathrm{Si}-\mathrm{O}-\mathrm{Si}\) bonds.2. **(b) **\(\mathrm{RSiCl}_3\)**: Contains three chlorine atoms, allowing three cross-linking possibilities which can lead to a network structure.3. **(c) **\(\mathrm{R}_2 \mathrm{SiCl}_2\)**: Has two chlorine atoms, which limits cross-linking to a linear polymer.4. **(d) **\(\mathrm{R}_3 \mathrm{SiCl}\)**: Only has one chlorine atom, allowing for only simple linking without cross-links.

Step 3: Identifying the Cross-linkable Compound

Given the analysis:- **\(\mathrm{R}_4 \mathrm{Si}\)** cannot form any links.- **\(\mathrm{RSiCl}_3\)**, with three chlorines, can form three-dimensional networks leading to cross-linking.- **\(\mathrm{R}_2 \mathrm{SiCl}_2\)**, with two chlorines, is suitable for forming linear, not cross-linked polymers.- **\(\mathrm{R}_3 \mathrm{SiCl}\)**, with one chlorine, is incapable of forming cross-links.Thus, **\(\mathrm{RSiCl}_3\)** is the compound that will undergo hydrolysis to form a cross-linked silicone polymer.

Key Concepts

Substituted Silanes

Substituted silanes are chemical compounds made from silicon atoms bonded to other elements or groups, such as hydrogen, chlorine, or organic groups denoted as "R". These substitutions significantly affect the chemical behavior of silanes.
In this context, the key function of modified silanes is in their ability to participate in creating silicone polymers. The type of substituted silane is crucial. Specifically, the number of bonding sites available impacts their capacity to undergo reactions like hydrolysis, which is essential in polymer formation.
More reactive silanes have chlorine atoms, which can be replaced with hydroxyl groups during these reactions. The more chlorine atoms present, the more potential there is for creating complex molecular structures.

Hydrolysis

Hydrolysis is a chemical reaction involving water that breaks bonds in molecules. In substituted silanes, hydrolysis replaces chlorine atoms with hydroxyl groups (\( ext{OH}\)).
Here's a simplified view of the process: Chlorine atoms in silanes react with water, splitting to form hydrochloric acid (\( ext{HCl}\)) and silicon-hydroxyl units. This transformation is significant because the resultant \( ext{Si-OH}\) bonds serve as precursors to forming silicone networks.
The more chlorine atoms a silane has, the greater the occurrence of hydrolysis, leading to potential three-dimensional network structures in silicone polymers. As examined, options like \( ext{RSiCl}_3\) undergo such transformations, making them suitable for forming cross-linked silicone structures.

Silicone Polymer Formation

Silicone polymer formation is the procedure through which simple molecules link to form long chains or networks. These polymers are a result of reactions like hydrolysis and further molecular linkages.
The critical step is polymerization. In a silicon network, after hydrolysis, the reactive hydroxyl groups can condense, releasing water or other byproducts. This forms strong \( ext{Si-O-Si}\) linkages, transforming simple silane molecules into stable silicone networks.
For a silane like \( ext{RSiCl}_3\), more extensive networking occurs because it can form a tri-dimensional structure due to three available reactive sites. These extensive linkages are the foundation of cross-linked silicone networks typically seen in stronger, more robust silicone materials.

Network Structure

Network structures in silicone polymers provide the material with enhanced stability and integrity. This is achieved through a web of connections formed by \( ext{Si-O-Si}\) bonds.
The ability to form such networks relies on the number of available bonding sites. For instance, \( ext{RSiCl}_3\) has three points of attachment, making it ideal for creating complex, three-dimensional arrangements needed for cross-linking.
This cross-linking results in a more interconnected and rigid structure compared to simple linear polymers, which only consist of long chains. Consequently, network structures, which arise from compounds like \( ext{RSiCl}_3\), contribute to high durability and thermal stability, distinguishing these silicone polymers from others.