Problem 118 - A Complete Resource Book in Chemistry for JEE Main

Question

Buna \(-\mathrm{N}\) synthetic rubber is copolymer of: [2009] (a) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}_{2}\) and \(\mathrm{H}_{5} \mathrm{C}_{6}-\mathrm{CH}=\mathrm{CH}_{2}\) (b) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CN}\) and \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}_{2}\) (c) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CN}\) and C=CC(=C)C C=CC(=C)Cl (d) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{C}=\mathrm{CH}_{2}\) and \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}_{2}\)

Step-by-Step Solution

Answer

Buna-N is a copolymer of acrylonitrile and butadiene as in option (b).

Step 1: Understand Buna-N rubber composition

Buna-N rubber, also known as nitrile rubber, is a copolymer composed of acrylonitrile and butadiene.

Step 2: Identify acrylonitrile

Acrylonitrile has the chemical formula \(\mathrm{CH_2=CHC\equiv N}\). It contains a vinyl group \(\mathrm{CH_2=CH-}\) and a cyano group \(-\mathrm{CN}\).

Step 3: Identify butadiene

Butadiene is a simple conjugated diene with the formula \(\mathrm{CH_2=CH-CH=CH_2}\).

Step 4: Match options to copolymers

Examine each option and check if they match the acrylonitrile and butadiene components of Buna-N. Option (b) \(\mathrm{H}_2 \mathrm{C}=\mathrm{CH}-\mathrm{CN}\) and \(\mathrm{H}_2 \mathrm{C}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}_2\) matches. The first component is acrylonitrile, and the second is butadiene.

Key Concepts

Acrylonitrile

Acrylonitrile is an organic compound with the molecular formula \( \mathrm{CH_2=CHC\equiv N} \). It is known for its pungent odor and is often used in the production of synthetic fibers, such as acrylics. While working with acrylonitrile, it's important to note the presence of its two functional groups:

• Vinyl group \( \mathrm{CH_2=CH-} \): This group contributes to the molecule's ability to polymerize, joining with other molecules to form long chains.
• Cyano group \( -\mathrm{CN} \): The cyano group introduces polarity to the molecule, which heavily influences its reactivity and the properties of the polymers formed.

Acrylonitrile is crucial in creating Buna-N rubber due to its ability to combine with butadiene. In Buna-N, it provides chemical resistance, particularly against oils and fuels, thanks to its polar nature.

Butadiene

Butadiene is a simple conjugated diene with the molecular formula \( \mathrm{CH_2=CH-CH=CH_2} \). As a conjugated diene, it has alternating double bonds, which makes it highly reactive and ideal for polymerization. Butadiene serves as a building block in various rubber manufacturing processes. Here are some key points about butadiene:

• Conjugated Double Bonds: These allow for strong covalent bonds with other molecular structures, increasing the polymer's elasticity and resilience.
• Flexibility in Structure: Its ability to polymerize easily with other monomers makes butadiene a versatile component in creating synthetic rubbers like Buna-N.

In Buna-N, butadiene contributes to the rubber's flexibility and durability. This makes products made from Buna-N, such as seals and gaskets, highly reliable in various industrial applications.

Copolymer

A copolymer is a polymer derived from two (or more) different monomers. In the case of Buna-N rubber, these monomers are acrylonitrile and butadiene. The cooperation of these elements creates a material with unique properties:

• Combination of Features: By combining acrylonitrile and butadiene, the copolymer acquires superior resistance to oils and fuels from acrylonitrile, paired with elasticity and wear resistance from butadiene.
• Enhanced Performance: This synergy is why Buna-N is often used in environments where the material needs to withstand harsh conditions, such as automotive and aviation applications.

The term "copolymer" is widely used in chemistry to differentiate materials where two or more distinct monomer types have been united. This classification helps in predicting the material's properties based on its monomeric components.