JEE Main 2022 · 28 June · evening · Single correct · +4 / −1
Given below are two statements, one is Assertion (A) and other is Reason (R).
Assertion (A) : Natural rubber is a linear polymer of isoprene called cis-polyisoprene with elastic properties.
Reason (R) : The cis-polyisoprene molecules consist of various chains held together by strong polar interactions with coiled structure.
In the light of the above statements, choose the correct one from the options given below :
- (a) Both (A) and (R) are true and (R) is the correct explanation of (A).
- (b) Both (A) and (R) are true but (R) is not the correct explanation of (A).
- (c) (A) is true but (R) is false.
- (d) (A) is false but (R) is true.
Answer and solution
Answer (c) (A) is true but (R) is false.
Solution
Chemistry ma’am · 4 steps
Rubber stretches because its chains are held only weakly. Strong polar forces would make it rigid.
- Check the Assertion. Natural rubber is a linear polymer of isoprene (2-methyl-1,3-butadiene, CH₂=C(CH₃)-CH=CH₂). Every double bond in the chain has the cis arrangement, so it is cis-1,4-polyisoprene, and it is elastic. (A) is true.
- Why it is coiled. The cis double bonds put a bend in the chain at regular points. So the chains cannot lie straight and pack closely, and they stay coiled. The coiled part of (R) is correct.
- Check the forces. Polyisoprene is a pure hydrocarbon, made only of C and H, so it has no polar groups. Its chains are held together by weak van der Waals forces, not strong polar interactions. These weak forces let the coils stretch out under a pull and spring back when the pull is released. So (R) is false.
- So. (A) is true but (R) is false, which is option (c).
The idea to keep. Elastomers have the weakest intermolecular forces among polymers, so their coiled chains can stretch and recoil. Strong forces between chains give fibres instead.
Answer from the NTA final answer key. Solution by PrepWiser.