1. MO’s for Cyclobutadiene
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2. Energy Diagram for Cyclobutadiene
Following Hund’s rule, two electrons are in separate orbitals.
This diradical would be very reactive =>

3. Polygon Rule
The energy diagram for an annulene has the same shape as the cyclic compound with one vertex at the bottom.
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4. Aromatic Requirements
Structure must be cyclic with conjugated pi bonds.
Each atom in the ring must have an unhybridized p orbital.
The p orbitals must overlap continuously around the ring. (Usually planar structure)
Compound is more stable than its open-chain counterpart =>

5. Anti- and Nonaromatic
Antiaromatic compounds are cyclic, conjugated, with overlapping p orbitals around the ring, but the energy of the compound is greater than its open-chain counterpart.
Nonaromatic compounds do not have a continuous ring of overlapping p orbitals and may be nonplanar =>

6. Hückel’s Rule
If the compound has a continuous ring of overlapping p orbitals and has 4N + 2 electrons, it is aromatic.
If the compound has a continuous ring of overlapping p orbitals and has 4N electrons, it is antiaromatic =>

7. [N]Annulenes [4]Annulene is antiaromatic (4N e-’s)
[8]Annulene would be antiaromatic, but it’s not planar, so it’s nonaromatic.
[10]Annulene is aromatic except for the isomers that are not planar.
Larger 4N annulenes are not antiaromatic because they are flexible enough to become nonplanar =>

8. MO Derivation of Hückel’s Rule
Lowest energy MO has 2 electrons.
Each filled shell has 4 electrons.
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9. Cyclopentadienyl Ions
The cation has an empty p orbital, 4 electrons, so antiaromatic.
The anion has a nonbonding pair of electrons in a p orbital, 6 e-’s, aromatic.
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10. Acidity of Cyclopentadiene
pKa of cyclopentadiene is 16, much more acidic than other hydrocarbons.
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