Chemistry / Unit 6 Chem 1120 short lesson, about 20 seconds
Pi Bonding Restricts Rotation
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The idea
Rotating one atom of a double bond relative to the other would break the pi bond's side-by-side overlap, so it strongly resists rotation.
In practice
The two carbons in ethylene's double bond stay fixed in one plane, unlike the freely rotating single bond in ethane.
Where people slip
This restriction comes specifically from the pi bond's overlap geometry; a molecule's single sigma bonds rotate freely.
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