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

Molecular Orbital Analysis of Diels-Alder reaction

simplest Diels-Alder reaction: 1,3-butadiene and ethene to cyclohexene

Notice how the phases of the orbitals at C1 and C4 of the HOMO of butadiene match the phases of the orbitals of C1 and C2 in the LUMO of ethene (i.e. the colours match).

This means there is a favourable bonding interaction at each of these positions, which corresponds to the positions where the two new s bonds are formed.

Therefore the reaction is said to be a "symmetry allowed" - this is good news, since we know the reaction works !

molecular orbitals of a Diels-Alder reaction

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

  1. The preference for the endo product
  2. Substituent effects on the rate of reaction
  3. Substituent effects on the regiochemistry of the reaction
  4. Catalysis by Lewis acids

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