Write The Assumptions Of Huckel MO Theory in Brief (5 marks)
Answers
Explanation:
Demonstrate how Hückel's theory approximates teh full molecular orbital picture of molecules by treating the σ -bonding and π -bonding networks independently.
Molecular orbital theory has been very successfully applied to large conjugated systems, especially those containing chains of carbon atoms with alternating single and double bonds. An approximation introduced by Hückel in 1931 considers only the delocalized p electrons moving in a framework of π -bonds. This is, in fact, a more sophisticated version of a free-electron model.
The simplest hydrocarbon to consider that exhibits π bonding is ethylene (ethene), which is made up of four hydrogen atoms and two carbon atoms. Experimentally, we know that the H–C–H and H–C–C angles in ethylene are approximately 120°. This angle suggests that the carbon atoms are sp2 hybridized, which means that a singly occupied sp2 orbital on one carbon overlaps with a singly occupied s orbital on each H and a singly occupied sp2 lobe on the other C. Thus each carbon forms a set of three σ bonds: two C–H (sp2 + s) and one C–C (sp2 + sp2) (part (a) of Figure 4.13C.1 ).