Geometric or cis trans isomerism arises in an alkene because
- A. the double bond prevents free rotation, fixing the groups on either side
- B. the molecule contains a chiral carbon
- C. the double bond rotates freely
- D. alkenes contain a ring
Explanation
The pi bond locks the two carbons in a fixed relative orientation, so substituents are held either on the same side, cis, or on opposite sides, trans, and the two forms cannot interconvert without breaking the pi bond. Each doubly bonded carbon must also carry two different groups, otherwise the two arrangements are identical. Single bonds rotate freely, so alkanes show no such isomerism.
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