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Statement-I: The potential-energy barrier to rotation about the C=C double bond in 2-butene is much higher than the barrier in ethylene. Statement-II: The hyperconjugation effect decreases the double-bond character. Choose the correct option about these statements.

1. Statement-1 is true, statement-2 is true and statement-2 is the correct explanation for statement-1.
2. Statement-1 is true, statement-2 is true and statement-2 is NOT the correct explanation for statement-1.
3. Statement-1 is true, statement-2 is false.
4. Statement-1 is false, statement-2 is true.

Correct answer: Statement-1 is false, statement-2 is true.

Solution

The barrier to rotation about a C=C bond comes from breaking the pi overlap. In 2-butene the two methyl groups donate electron density by hyperconjugation, which decreases the C=C double-bond character relative to ethylene. Less double-bond character means a slightly lower rotation barrier, not a higher one. So Statement-I is false. Statement-II (hyperconjugation decreases double-bond character) is a true general statement.

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