Problem 77 - A Complete Resource Book in Chemistry for JEE Main

Question

Polyacrylonitrile, characterized by the repeating unit is made from which of the following monomer? (a) \(\mathrm{CH}_{2}=\mathrm{CHCN}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CN}\) (c) \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (d) \(\mathrm{CH}_{2} \mathrm{CH}^{2}=\mathrm{CHCN}\)

Step-by-Step Solution

Answer

The monomer is (a) CH2=CHCN.

Step 1: Understand the structure of polyacrylonitrile

Polyacrylonitrile is a polymer characterized by repeating units of the formula -CH2-CH(CN)-. This repeating unit is derived from a vinyl group where one hydrogen is replaced by a nitrile group (CN).

Step 2: Identify the structural unit of the polymer

The repeating unit of polyacrylonitrile indicates that the structure is based on a vinyl monomer with a nitrile group attached directly to the vinyl carbon. The structure can be represented as -CH2-CH(CN)-.

Step 3: Match the repeating unit to the available monomers

Out of the given options, (a) CH2=CHCN matches the repeating unit -CH2-CH(CN)-, where the double bond opens up during polymerization, allowing the monomer to form the polymer chain.

Key Concepts

Polyacrylonitrile

Polyacrylonitrile, often abbreviated as PAN, is a versatile polymer used in various applications. It's most commonly known for its role in making synthetic fibers, like those used for fabric. This polymer is made up of repeating units of the molecule with the structure

• -CH₂-CH(CN)-

Each of these repeating units is derived from a specific type of monomer that has a vinyl group attached to a nitrile group. Polyacrylonitrile itself is noteworthy because of its high resistance to sunlight and oxidation, which makes it a durable material in textiles.

Vinyl Monomer

A vinyl monomer is a small molecule that can join together with other vinyl monomers to form a polymer. It is characterized by the presence of a vinyl group,

• -CH=CH₂,

which is a simple alkene group featuring a double bond between carbon atoms. This double bond is reactive, allowing the monomers to bond together in a process called polymerization. In the case of polyacrylonitrile, the relevant vinyl monomer is acrylonitrile,

• ext{CH}_{2}= ext{CHCN},

which includes a nitrile group attached to the vinyl group.

Nitrile Group

The nitrile group is a functional group in chemistry represented by the formula

• -C≡N,

consisting of a carbon atom triple-bonded to a nitrogen atom. This group adds unique properties to the molecule, such as polarity, which significantly influences the characteristics of the polymer it constitutes. In polymers like polyacrylonitrile, the presence of the nitrile group contributes to high tensile strength and resistance to heat. These features make the polymer ideal for applications where toughness and durability are required, such as in the production of carbon fibers.

Polymerization Process

Polymerization is the process by which small molecules, called monomers, link together to form a larger molecule, known as a polymer. For vinyl-type monomers like acrylonitrile, the polymerization typically involves a "chain growth" reaction. In this process, the double bond in the vinyl group opens up to form new single bonds with neighboring monomers. This results in a long chain of repeated units, building up the polymer structure:

• ...-{-CH₂-CH(CN)-}-...

The polymerization process can be initiated in several ways, including heat or the presence of a catalyst, to facilitate the reaction. The process results in transforming reactive monomers into stable, long-lasting material with applications ranging from textiles to high-performance materials.