The energy-level diagram in Figure 9.36 shows that the sideways overlap of a pair of p orbitals produces two molecular orbitals, one bonding and one anti-bonding. In ethylene there is a pair of electrons in the bonding π orbital between the two carbons. Absorption of a photon of the appropriate wavelength can result in promotion of one of the bonding electrons from the π2p to π*2p the molecular orbital.
(a) What would you expect this electronic transition to do to the carbon- carbon bond order in ethylene?
(b) How does this relate to the fact that absorption of a photon of appropriate wavelength can cause ready rotation about the carbon-carbon bond, as described in the "Chemistry and Life" box and shown in Figure 9.30?

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Transcribed Image Text:

с-с T bond 90 rotation around double bond - Antibonding MO AO AO Bonding MO End-on overlap 2p2 2p Sideways overlap 2px 2py