Hückel’s Molecular Orbital Theory By Sean Hanley Hückel Theory Hückel’s Molecular Orbital Theory By Sean Hanley

Overview Explain Hückel’s Rule Aromatic Compounds Explain Molecular Orbital (MO) Theory Hückel Theory in Quantum Sense So What??

Hückel’s Rule Any conjugated monocyclic polyene that is planar and has (4n+2)π and/or nonbonding electrons, with n = 0,1,2, etc., will exhibit the special stability associated with aromaticity. (1930)

Hückel’s Rule n 4n + 2 Pi electrons 0 4(0) + 2 = 2 1 4(1) + 2 = 6 2 4(2) + 2 = 10 3 4(3) + 2 = 14 4 4(4) + 2 = 18

Which are Aromatic??? 3 2 1 5 2 3 4 8 6 7

Some Common Aromatic Compounds

Comparison of Molecules Benzene is Stabilized by 36 kcal/mol called the Empirical Resonance Energy

Molecular Orbital Theory Simplified Hybrid Orbitals The Bonding of Chemistry Atomic Orbitals

Shape of P and D orbitals

Atomic Orbitals to Molecular Orbitals

Molecular Orbital Theory Dioxygen O2

Molecular Orbital Theory 2 Bonds

Molecular Orbital Theory Dicarbon + ion

Molecular Orbital Theory 1.5 Bonds

Overlap of Orbitals 2pz orbitals overlap causing stabilization a semi bonding Ethene Example sp^2 hybridized 1 Double Bond Ψpi= c12pzA + c22pzB

Ethene Example Determinant associated with Wave function H11=H22 Sij set to zero unless i=j Sii=1 Diagonal elements are assumed to be the same for all carbon atoms denoted α (Alpha) Nearest neighbor carbon atoms assumed the same denoted β (Beta)

Benzene Structure

Benzene Pi System

Benzene Hückel Theory

Benzene Pz Wave Functions

MO Diagram of Benzene

Benzene MO

Aromatic Stabilization

Aromatic Stabilization

Recent Research in Aromaticity In 2000 chemists in Germany formulated a rule to determine when a fullerene would be aromatic. In particular, they found that if there were 2(n+1)^2 π electrons Buckminsterfullerene (Buckyball), with 60 π electrons, is non-aromatic, since 60/2=30

Questions????