Molecular Structure and Bonding

p Electrons in Conjugated Organic Molecules

Lecture 2 Synopsis:

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

Summary of topics

· Review of 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, 7^th edn., pages 435-436.

· 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. Atkins, 7^th edn, Page 437; Carroll ‘Perspectives on Structure and Mechanism in Organic Chemistry’pp.205-208-- The (4n+2) rule.

· The Geometric (or ‘Circle’) Method for finding the MOs of cyclic conjugated polyenes (Carroll, pg. 205)

The Hückel approximations (again!)