Ques. 1AIPMT 2000
2-Butene shows geometrical isomerism due to
- restricted rotation about double bond
- free rotation about double bond
- free rotation about single bond
- chiral carbon
Answer
(1) restricted rotation about double bond
Solution
∵ A carbon–carbon double bond consists of one σ-bond and one π-bond; rotation about it would break the sideways overlap of the p-orbitals forming the π-bond.
⇒ Rotation about C=C is restricted, so the groups on the doubly bonded carbons stay fixed in space.
In 2-butene (CH3-CH=CH-CH3) each doubly bonded carbon carries two different groups (H and CH3).
∴ As per NCERT, 2-butene exists as cis and trans isomers (geometrical isomerism) because of restricted rotation about the double bond.
Answer: (1)