Does the energy barrier cause the existence of two distinct isomers?

1 Answer
Jun 26, 2015

Yes, the energy barrier for rotation about the double bonds in alkenes causes the existence of two distinct isomers.


The π bond of an alkene is stable because the electrons are attracted to both carbon nuclei.

Rotation about the C=C axis breaks the π bond and puts one #p# electron on each carbon atom.

This takes energy of about 200 kJ/mol.

The average kinetic energy of a molecule at room temperature is about 4 kJ/mol.

This is not enough energy to break the pi bond, so alkenes like but-2-ene can exist as distinct cis and trans isomers.