Chapter 10: Conjugation in Alkadienes and Allylic Systems

Molecular Orbital analyzis of Ethene Dimerisation

• The dimerisation of ethene to cyclobutane appears to be a cycloaddition similar to the Diels-Alder reaction.
• However, the reaction does not occur readily, despite the seemingly simple mechanism.

• For further insight, we need to look at the interaction of the HOMO and LUMO of two ethene molecules:

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 !

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

• Why this reaction occurs more readily using photochemical techniques.