Problem 61 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following sequence of reactions. \(\mathrm{HC} \equiv \mathrm{CH} \stackrel{\text { (i) } \mathrm{CuCl}, \mathrm{NH}_{4} \mathrm{Cl}, \text { (ii) } \mathrm{HCl}}{\longrightarrow}(\mathrm{X})\) \(\underline{\text { Polymerization }}{\longrightarrow}(\mathrm{Y})\) The polymer (Y) is (a) PVC (b) Saran (c) Neoprene (d) Chloroprene

Step-by-Step Solution

Answer

The polymer (Y) is polyvinyl chloride (PVC).

Step 1: Analyze Initial Reactants

The sequence starts with acetylene, represented as \( \mathrm{HC} \equiv \mathrm{CH} \). This indicates the presence of a terminal alkyne which will undergo chemical transformations.

Step 2: Determine the Transformation Process

The first part of the process involves adding copper(I) chloride \( \text{(CuCl)} \) and ammonium chloride \( \text{(NH}_4\text{Cl)} \), followed by hydrochloric acid \( \text{(HCl)} \). This converts acetylene to vinyl chloride \( \text{(CH}_2\text{=CHCl)} \) through a series of reactions.

Step 3: Understand the Polymerization

Polymerizing vinyl chloride \( \text{CH}_2\text{=CHCl} \) results in the formation of polyvinyl chloride (PVC), a type of plastic. This is due to the addition polymerization process, where vinyl chloride monomers link together to form long polymer chains.

Step 4: Identify the Polymer

The compound "Y" that results from the polymerization of vinyl chloride is polyvinyl chloride (PVC). This is typically used in construction and various applications due to its durability and versatility.

Key Concepts

Alkyne Transformations

In organic chemistry, alkyne transformations are fascinating due to their versatility in forming various products. Alkynes are hydrocarbons characterized by a carbon-carbon triple bond, depicted as \( \mathrm{R-C} \equiv \mathrm{C-R} \). This triple bond is highly reactive, making alkynes suitable for numerous chemical reactions.
One common starting alkyne is acetylene \((\text{HC} \equiv \text{CH})\), a simple and useful molecule which can be transformed into other valuable chemicals. In the provided exercise, acetylene undergoes a sequence of reactions. First, it interacts with copper(I) chloride \((\text{CuCl})\) and ammonium chloride \((\text{NH}_4\text{Cl})\) forming acetylide intermediates. These intermediates are highly reactive, paving the way for further reactions.
Following this, hydrochloric acid \((\text{HCl})\) is added to the reaction mixture. This step ultimately transforms the acetylide intermediates into vinyl chloride \((\text{CH}_2=\text{CHCl})\), a compound with a carbon-carbon double bond. This transformation is possible due to alkyne addition reactions, where new atoms or groups are added across the triple bond, helping convert it into a double-bond compound like vinyl chloride.

Polymerization Processes

Polymerization is a chemical reaction where small molecules called monomers join together to form long chains or three-dimensional networks, known as polymers. The beauty of polymerization lies in its ability to create materials with unique properties, suitable for countless applications.
In the context of the exercise, vinyl chloride is the monomer that undergoes polymerization. The type of polymerization it undergoes is known as addition polymerization, a process where monomers add sequentially to the growing polymer chain without the loss of any atom or small molecule.

• During the polymerization of vinyl chloride, the double bond \((\text{C=C})\) in the vinyl chloride monomer opens up, allowing it to form new single bonds with neighboring monomers.
• This sequential addition creates long chains of polyvinyl chloride \((\text{PVC})\), characterized by repeating units of \((\text{CH}_2-\text{CHCl})\).
• Addition polymerization is typically initiated by heat, pressure, or catalysts, which helps in breaking the double bond and facilitating the chain growth.

In essence, the nature of polymerization allows for the transformation of simple monomers into functional, complex polymeric structures with diverse properties.

Polyvinyl Chloride (PVC)

Polyvinyl Chloride, commonly known as PVC, is one of the most widely used synthetic plastic polymers globally. PVC is made from the polymerization of vinyl chloride, like shown in the exercise.
PVC is renowned for its versatility, durability, and flame resistance, making it suitable for many applications. It can be flexible or rigid depending on how it is processed, with each form serving different purposes.

• Rigid PVC is often used in construction, like for pipes, doors, and windows, due to its strength and resistance to environmental degradation.
• Flexible PVC is used in products like wiring and cable insulation, clothing, and plastic films because it is softer and more pliable.

PVC's adaptability stems from its ability to be mixed with various additives, such as plasticizers, which can modify its flexibility and heat stability. This ability makes PVC crucial in manufacturing because it can be tailored to meet specific needs. Overall, the creation of PVC from vinyl chloride accommodates a spectrum of industry demands and applications.