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

π Molecular Orbitals

Qualitative Molecular Orbital theory is a fascinating aspect of organic chemistry that can provide a remarkable insight into the workings of organic reactions based on how orbitals interact to control the outcome of reactions. The ideas were developed by Fukui, Hoffmann and Woodward, leading to a Nobel Prize for Fukui and Hoffmann in 1981.

An introduction to the basic components is presented here.  A more thorough analysis probably requires another semester !

frontier molecular orbitals LUMO lowest
  • LUMO receives electrons
  • lowest energy orbital available
  • characteristic for electrophilic component
HOMO highest
  • electrons from the HOMO are donated
  • most available for bonding 
  • most weakly held electrons
  • characteristic for nucleophilic component
Over the next couple of pages we will look at the molecular orbitals of the p-systems in ethene and 1,3-butadiene then look at how molecular orbital theory provides some insight  into the Diels-Alder reaction and another cycloaddition reaction.

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