Todd:Chem3x11 ToddL12

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Chem3x11 Lecture 12

Incomplete Saturday Jun 2

This lecture is about pericyclic reactions.

(Back to the main teaching page)

Key concepts

  • Pericyclic reactions involve cyclic transition states

Some Opening Experimental Observations

Here are some reactions we've not seen before. The outcomes are, on the face of it, difficult to understand, particularly the stereochemical outcomes which are very specific. There are no apparent intermediates. Solvent polarity has little influence on the outcome. The reactions are equilibria. Imagine you were presented with these reactions and asked to explain the mechanism.

Scheme 1: Some Unexpected Reaction Outcomes we Ought to be Able to Explain
Scheme 1: Some Unexpected Reaction Outcomes we Ought to be Able to Explain

This was the situation in the 1960's when an explanation for these reactions, and several other types, finally emerged. The key was to look at the orbitals involved (an approach taken by Woodward and Hoffmann) and this was later simplified just to a consideration of the Frontier Orbitals (i.e., the HOMO and LUMO) (an approach pioneered by Fukui). To understand this, let's make sure we can draw the relevant frontier orbitals for molecules like this.

Frontier Orbitals of Polyenes

The σ framework of an organic molecule contains the strong (low energy) bonds, meaning the corresponding antibonding orbitals are high in energy. Our analysis of frontier orbitals in reactions of this kind typically focusses on the π system, where the higher energy bonding interactions are, as well as the lowest-lying antibonding molcular orbitals. If we therefore forget about the σ bonds and just focus on the π bonds, we can easily sketch the HOMO and LUMO for ethene.

Scheme 2: Frontier Molecular Orbitals for Ethene
Scheme 2: Frontier Molecular Orbitals for Ethene

(Remember that the MO's will actually look like clouds spread over the whole molecule, rather than just separate atomic AOs, but there will be nodes for every MO above the HOMO, as we'll see, where the constituent AO's are not in phase - as for the LUMO above)

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