JEE Main Chemistry question

Which of the following has the least hindered (lowest-barrier) rotation about its central carbon–carbon axis?

(A)Ethane
(B)Ethylene (rotation about the C=C is essentially forbidden, not merely hindered)
(C)AcetyleneCorrect
(D)Hexachloroethane

Answer: (C)

Solution

Acetylene's linear, cylindrically symmetric triple bond places no bulky substituents around the C–C axis to create eclipsing/steric interactions, so "rotation" about this axis has essentially zero barrier. Ethane has a real (if modest)

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Question ID VK-142628