Rotation about a carbon to carbon double bond is restricted because
- A. the sigma bond is too strong
- B. the atoms are too heavy
- C. turning one carbon would break the sideways overlap of the pi bond
- D. double bonds are ionic
Explanation
The pi bond depends on two parallel p orbitals overlapping sideways, and rotating one carbon by ninety degrees would destroy that overlap, which costs far more energy than is available at room temperature. This rigidity is what makes cis and trans isomerism possible in alkenes. Single sigma bonds rotate freely because their overlap is symmetrical about the axis.
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