Question

Natural rubber is a polymer of isoprene. During polymerisation:

• A. 1,4 addition takes place
• B. 1,2 addition takes place
• C. 1,3 addition takes place
• D. both double bonds convert into single bonds

Hint:

Natural rubber = cis-1,4-polyisoprene (1,4-addition polymerization of isoprene).

Answer 1

The Correct Option is A

Natural rubber is a polymer of isoprene, which is scientifically known as 2-methyl-1,3-butadiene. The polymerization of isoprene involves the combination of multiple isoprene units through a series of chemical reactions to form polyisoprene, which constitutes the basis of natural rubber.

During the polymerization process, 1,4-addition takes place. This means that the polymerization occurs between the first carbon of one isoprene molecule and the fourth carbon of the next. The specific arrangement and type of addition ensure the formation of a long-chain polymer, which is necessary for the elasticity and properties of rubber.

Let's explore the possibilities:

1. 1,4 addition: This type of addition involves the terminal carbon of the diene (carbon 1) linking with the fourth carbon of the adjacent unit. This is the preferred polymerization route for isoprene in rubber, forming long polyisoprene chains that provide elasticity and flexibility. Hence, this is the correct answer.
2. 1,2 addition: This occurs when the polymerization bridges form between the first and second carbon atoms. This type of addition is not typical for natural rubber formation.
3. 1,3 addition: Involves a bond between the first and third carbons, also not typical or feasible for natural rubber formation.
4. Both double bonds convert into single bonds: This implies a complete saturation, which does not result in the formation of flexible polyisoprene chains required for rubber properties.

In summary, during the polymerization of isoprene to form natural rubber, the 1,4 addition takes place. This type of additional results in the necessary continuous chain structure characteristic of rubber.