Return to Contents Chapter 10: Conjugation in Alkadienes and Allylic Systems Ch 10 contents

Molecular Orbital Analysis of Ethene Dimerisation

dimerisation of ethene to give cyclobutane
curly arrow mechanism for ethene dimerisation
Again compare the phases of the orbitals in the HOMO of one ethene molecule with the phases of the LUMO of the ethene molecule.

At one end there is a favourable bonding interaction (colours match) but there is an unfavourable anti-bonding interaction (colour mis-match) the other end.

Therefore the reaction is said to be a "symmetry forbidden" - interestingly, this reaction is rare and very slow !

molecular orbitals of ethene dimerisation reaction

A more complete analysis using molecular orbital methods (beyond our scope) can also explain:

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©  Dr. Ian Hunt, Department of Chemistry, University of Calgary