Practice question · Multiple choice
Why can an alkene not rotate freely about its carbon-carbon double bond?
Hints
- Consider which of the two component bonds depends on orbital alignment.
- A pi bond needs two parallel p orbitals; rotating destroys that alignment.
Show the answer
B. Twisting would break the sideways overlap of the pi bond
Why
The sigma bond is cylindrically symmetric and would permit rotation on its own. The pi bond requires the two orbitals to stay parallel, so rotating by destroys the overlap entirely, a real energy barrier of roughly 260 kJ/mol.
Practise Hybridisation and Molecular Orbitals
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