CHEM 2211 1st Edition Lecture 25 Outline of Last Lecture I II III IV V VI Delocalized Electrons Bonding in benzene Drawing Resonance structures Factors that decrease stability Delocalization adds stability Criteria for Aromaticity Outline of Current Lecture I II III IV V Delocalization Energy Criteria for Aromatic compounds Aromaticity of Heterocyclic compounds Antiaromaticity Drawing molecular orbitals and Frost Diagrams Current Lecture I II III Delocalization Energy A Aromatic compounds like benzene have very large delocalization energies Criteria for Aromatic compounds Aromaticity of Heterocyclic compounds A Heterocyclic compounds are compounds with one or more atoms that isn t carbon B Heterocyclic atoms must have a pi orbital in order to fulfill the aromaticity rules C Most common hetero atoms are N O and S D Examples Example of Heterocyclic compounds IV Antiaromaticity A Characterized by instability B Is considered to be antiaromatic if the compound fulfills the first rule but not the second rule for aromaticity C Molecuels that do not adhere to either rule are non aromatic D Example Non aromatic Anti aromatic V Aromatic Drawing Molecular Orbitals MO s and frost diagrams A Molecular orbitals demonstrate pi bonding and illustrate the pi system in a compound B The number of MO s is dependent upon the number of pi bonds in a compound i Each MO has a node The number of nodes is the orbital number minus one ii Nodes separate out of sync orbitals iii Orbitals that are in sync with each other are shaded on the same side of the balloons and vice versa for out of sync orbitals C Example 1 3 butene i Red line nonbonding line ii Blue dashed line nodes iii Purple numbers number of carbon iv Blue numbers orbital level D When drawing Frost diagrams make sure that the molecule has a point facing downward i Each carbon is represented by a line level ii Each set of levels is bisected by the non bonding line iii The bottom half contains the bonding orbitals and the top half contains the antibonding orbitals E Example Aromatic Anti aromatic
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