3B, Frontier Molecular Orbital theory
Apart from electrostatic interactions, the overlap between orbitals may
favour the reaction between an electron donor and an electron acceptor.
A high (in energy) lying occupied orbital in the donor may overlap with a low
lying empty one in the acceptor, leading to a net stabilization.
The strength of the interaction is determined by:
These effects can account for the subtle differences that are found in
facial selectivity, caused by
- the energy difference between the two orbitals involved (the smaller the better).
- the amount of overlap between the orbitals (the larger the better).
Example: Diels-Alder reaction of fluorocyclopentadiene, which takes place
on the side where the F resides. Sterically the other side would be preferred,
so an electronic effect must play a role.
We can investigate the orbital
concerned, the HOMO of fluorocyclopentadiene, and other cyclopentadienes.
- steric effects which hinder overlap, or by
- asymmetry of a pi orbital, which favours overlap on that side where the
orbital is larger.
See also the VRML
representation of frontier orbitals (HOMO, LUMO) in a series of Diels-Alder
reactions, at Imperial College,
and a related paper.
This is paragraph B of chapter 3: More theory
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