Ch 10: Introduction to MO theory

Chapter 10: Conjugation in Alkadienes and Allylic Systems

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 !

Introduction

The most important orbitals in molecules for reactivity are the two so called frontier orbitals.
These are called the HOMO and LUMO, see below:

	LUMO lowest unoccupied molecular orbital	LUMO receives electrons lowest energy orbital available characteristic for electrophilic component
	HOMO highest occupied molecular orbital	electrons from the HOMO are donated most available for bonding most weakly held electrons characteristic for nucleophilic component

Remember that the number of molecular orbitals = number of atomic orbitals combined to make them.
Basic process : combine two atomic orbitals (in-phase and out-of-phase) to generate two molecular orbitals.

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.