JEE Challenger
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ChemEasyAdv 2016 • Paper 1

Product Formed on Complete Hydrogenation of Natural Rubber

Upon full hydrogenation, natural rubber produces which of the following polymers?

Options

A

ethylene-propylene copolymer

Correct
B

vulcanised rubber

C

polypropylene

D

polybutylene

Topics & Concepts

PolymersPolymers

Step-by-Step Solution

Natural rubber is a polymer of isoprene (22-methyl-1,31,3-butadiene), specifically cis-1,41,4-polyisoprene.

The repeating unit of natural rubber is: −[CH2−C(CH3)=CH−CH2]n−-\left[ \text{CH}_2 - \text{C}(\text{CH}_3) = \text{CH} - \text{CH}_2 \right]_n-

Upon complete hydrogenation, the double bond (>C=CH−>\text{C}=\text{CH}-) in each repeating unit undergoes addition of hydrogen (H2\text{H}_2) to form a saturated polymer chain: −[CH2−C(CH3)=CH−CH2]n−→H2/catalyst−[CH2−CH(CH3)−CH2−CH2]n−-\left[ \text{CH}_2 - \text{C}(\text{CH}_3) = \text{CH} - \text{CH}_2 \right]_n- \xrightarrow{\text{H}_2/\text{catalyst}} -\left[ \text{CH}_2 - \text{CH}(\text{CH}_3) - \text{CH}_2 - \text{CH}_2 \right]_n-

Rearranging the saturated repeating unit shows that it consists of alternating ethylene (−CH2−CH2−-\text{CH}_2-\text{CH}_2-) and propylene (−CH2−CH(CH3)−-\text{CH}_2-\text{CH}(\text{CH}_3)-) units: −[−CH2−CH(CH3)−⏟Propylene unit    −CH2−CH2−⏟Ethylene unit]n−-\left[ \underbrace{-\text{CH}_2 - \text{CH}(\text{CH}_3)-}_{\text{Propylene unit}} \;\; \underbrace{-\text{CH}_2 - \text{CH}_2-}_{\text{Ethylene unit}} \right]_n-

Thus, the product formed on complete hydrogenation of natural rubber is structurally identical to an ethylene-propylene copolymer.

Correct Answer: (A) ethylene-propylene copolymer

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