Problem 4 - A Complete Resource Book in Chemistry for JEE Main

Question

The repeating units of PTFE are (a) \(\mathrm{F}_{2} \mathrm{C}=\mathrm{CF}_{2}\) (b) \(\mathrm{FCl} \mathrm{C}=\mathrm{CF}_{2}\) (c) \(\mathrm{F}_{3} \mathrm{C}-\mathrm{CF}_{3}\) (d) \(\mathrm{Cl}_{2} \mathrm{CH}-\mathrm{CH}_{3}\)

Step-by-Step Solution

Answer

The repeating units of PTFE are (a) \( ext{F}_2 ext{C}= ext{CF}_2\).

Step 1: Understand PTFE

PTFE stands for Polytetrafluoroethylene, which is a type of polymer. It is commonly known as Teflon. It consists of repeating units that form the polymer chain.

Step 2: Identify PTFE's Monomer

The monomer used to create PTFE is tetrafluoroethylene, which has the chemical structure \( ext{F}_2 ext{C}= ext{CF}_2\). This means in PTFE, this unit repeats to form the long polymer chain.

Step 3: Analyze the Options

Let's examine each option:- (a) \( ext{F}_2 ext{C}= ext{CF}_2\) matches the monomer structure of PTFE.- (b) \( ext{FClC}= ext{CF}_2\) includes chlorine, which is not part of PTFE.- (c) \( ext{F}_3 ext{C}- ext{CF}_3\) is a saturated compound and not the monomer for PTFE.- (d) \( ext{Cl}_2 ext{CH}- ext{CH}_3\) does not relate to PTFE as it neither contains fluorine nor has a double bond necessary for polymerization.

Step 4: Conclude the Correct Answer

Option (a) \( ext{F}_2 ext{C}= ext{CF}_2\) is the correct structure of the repeating unit of PTFE because it matches the monomer tetrafluoroethylene.

Key Concepts

PTFE (Polytetrafluoroethylene)

PTFE, or Polytetrafluoroethylene, is a synthetic polymer that many people commonly know as Teflon, which is a brand name. It is renowned for its non-stick properties, making it very popular in cookware, but it has many other applications as well.

What makes PTFE special is its chemical structure, consisting of carbon and fluorine atoms. The strong bonds between these atoms give PTFE its unique characteristics. Here are a few noteworthy properties:

• **High Melting Point**: PTFE has a very high melting point, meaning it can withstand significant heat without breaking down.
• **Chemical Resistance**: It is highly resistant to chemicals, which is why it's often used in coatings and sealants that need to withstand harsh environments.
• **Low Friction**: Due to its slick surface, this polymer has a low coefficient of friction, which minimizes wear and tear, enhancing durability and efficiency.

Such properties make PTFE valuable in various industries, from cookware and automotive to pharmaceutical and electronics. It serves both practical and specialized purposes, thanks to its robust and versatile features.

Monomer Identification

In the world of polymers, a monomer is a small molecule that can bind chemically to other molecules to form a polymer. For PTFE, the crucial monomer is tetrafluoroethylene. This molecule is denoted chemically as \(\mathrm{F}_2\mathrm{C}=\mathrm{CF}_2\).

Understanding the monomer is essential because it defines the repeating unit of the polymer. Tetrafluoroethylene contains carbon atoms double-bonded to each other, with each carbon atom also bonded to two fluorine atoms.

When these monomers undergo a process called polymerization, they link together, removing the double bond and forming the long chains that make up PTFE. This monomer's structure is critical because it imparts PTFE with its distinguishing features, like high temperature tolerance and chemical inertness.

Polymerization Process

The process of polymerization is key to transforming monomers into polymers. For PTFE, this involves the monomer tetrafluoroethylene. The polymerization of tetrafluoroethylene typically occurs through a free-radical mechanism.

Free radicals are molecules with unpaired electrons, which makes them highly reactive and able to initiate the polymerization process. Here's a simplified breakdown of the process:

• **Initiation**: A free radical initiator, often a peroxide, introduces radicals to start the reaction.
• **Propagation**: The active sites created on the monomers allow them to connect, forming a long chain as the tetrafluoroethylene units link.
• **Termination**: Eventually, the process concludes when radicals react with each other, ceasing further bonding.

This series of steps ensures the monomers open their double bonds and establish a continuous chain, forming the solid structure of PTFE. The mechanism is highly effective, allowing the production of stable and enduring polymer chains necessary for the various applications of PTFE.