Which of the following is a monomer of natural rubber?

Isoprene

The correct answer is option 1. Isoprene.

Let us dive into understand why isoprene is the correct monomer for natural rubber and how the other options relate to rubber-like materials.

1. Isoprene (2-methyl-1,3-butadiene) – Monomer of Natural Rubber

Natural rubber is made from polyisoprene, which is a polymer formed by the monomer isoprene. 

The chemical structure of isoprene is \(CH_2=C(CH_3)-CH=CH_2\), also known as 2-methyl-1,3-butadiene.

Polymerization of isoprene: In natural rubber, the isoprene units polymerize in a head-to-tail fashion, forming long, flexible chains. These chains have cis configurations around the double bonds, which give rubber its characteristic elasticity and flexibility.  Natural rubber is obtained from the latex of the rubber tree (Hevea brasiliensis) and is essentially poly-cis-isoprene.

\Other Options (Not Monomers of Natural Rubber)

2. Chloroprene (2-chloro-1,3-butadiene) – Monomer of Neoprene (Synthetic Rubber)

Chloroprene is a derivative of isoprene where a chlorine atom replaces one hydrogen atom. Its structure is \(CH_2=C(Cl)-CH=CH_2\).\ Neoprene is a synthetic rubber made by polymerizing chloroprene. It is known for its resistance to oil, heat, and weathering, making it useful in industrial applications and in making wetsuits, gaskets, and hoses. However, chloroprene** is not the monomer of natural rubber but a monomer used to produce synthetic rubber.

3. Ethylene (C₂H₄) – Not Related to Rubber

Ethylene is a simple alkene, with the formula \(CH_2=CH_2\). It is a key monomer used in producing polyethylene, which is a plastic rather than a rubber. Ethylene is not used in the formation of natural rubber or synthetic rubbers. It is mainly involved in the production of polymers like high-density polyethylene (HDPE) and low-density polyethylene (LDPE).

4. 1,3-Butadiene – Monomer of **Synthetic Rubbers (e.g., SBR, Buna-S)

1,3-Butadiene (\(CH_2=CH-CH=CH_2\)) is a diene, similar to isoprene but without the methyl group. It is used as a monomer in the production of several synthetic rubbers, such as:

Styrene-butadiene rubber (SBR), used in tires and other products. Nitrile rubber (NBR), used for making gloves, hoses, and seals.

However, 1,3-butadiene is not a monomer of natural rubber, which is based on isoprene.

Why Isoprene is the Correct Answer

Natural rubber is specifically made from polymerized isoprene. The long, flexible chains of polyisoprene (with cis-double bonds) give natural rubber its unique properties, such as elasticity, flexibility, and strength. While other dienes like chloroprene and 1,3-butadiene are used to make synthetic rubbers, they are chemically different from isoprene and form rubbers with different properties.

Thus, the correct answer is isoprene (option 1).