Molecular Structure and Bonding

p Electrons in Conjugated Organic Molecules

Lecture 3 Synopsis:

Hückel Theory: p molecular orbitals for Butadiene and cyclic conjugated hydrocarbons; Aromaticity

Learning Objectives:

· To review the Hückel approximations

· To learn by example how to apply Hückel theory to the calculation of p molecular orbitals for any conjugated system. Example: Butadiene.

see: P.W. Atkins, Physical Chemistry, 6^th edn., pages 414-416.

· To appreciate the relationship between the nodal structure of the molecular orbitals and the energy.

· The concept of delocalization energy.

· To learn by example how to apply Hückel theory to cyclic conjugated polyenes and how this can rationalize the concept of aromaticity. Murrell, Kettle and Tedder “Valence Theory”, Page 268; Carroll ‘Persepectives…’pp.205-208-- The (4n+2) rule.

· The Geometric (or ‘Circle’) Method for finding the MOs of cyclic conjugated polyenes

The Hückel approximations (again!)