Alkenes

From WikidChem

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Alkenes

Alkenes are completely composed of C and H, so the only area of interest is the double bond which creates the pi bonding and antibonding orbitals. This is the region on which we will focus.

HOMO-The HOMO of an alkene is the pi bonding orbital (the Πp orbitals of the C atoms)

LUMO-The LUMO is the antibonding pi orbital (because the the Πp is the highest energy orbital, its antibonding will be the lowest energy of the unoccupied orbitals)

The splitting in this case will be bad because of the poor overlap of the p orbitals.

Alkenes are planar because of the pure Πp orbitals that are parallel to allow overlap. In ethene, for example, the high HOMO of another molecule will attack from the end, not the middle, because of the antibonding node between the carbons in the LUMO.

   * This is true unless the attacking HOMO itself has a node, so that half of it could overlap favorably with one end of the pi* LUMO of the alkene, and the other end could overlap favorably with the other. We'll see such examples. - JMM