1,3-Butadiene’s lowest-energy MO is particularly stable because it has three
bonding interactions and its two electrons are delocalized over all four nuclei—they
encompass all the carbons in the system. The MO next in energy ( ) is also a bond-
ing MO because it has one more bonding interaction than antibonding interaction; it is
not as strongly bonding or as as low in energy as These two bonding MOs show
that the greatest electron density in a compound with two double bonds joined by
one single bond is between C-1 and C-2 and between C-3 and C-4, but there is some
electron density between C-2 and C-3—just as the resonance contributors show. They
also show why 1,3-butadiene is most stable in a planar confomation: If 1,3-butadiene
weren’t planar, there would be little or no overlap between C-2 and C-3. Overall, is
an antibonding MO: It has one more antibonding interaction than bonding interaction,
but it is not as strongly antibonding as which has no bonding interactions and three
antibonding interactions.
Both and are symmetric molecular orbitals; they have a plane of symmetry,
so one half is the mirror image of the other half. Both and are fully asymmetric;
they do not have a plane of symmetry, but would have one if one half of the MO were
turned upside down. Notice that as the MOs increase in energy, they alternate from
being symmetric to asymmetric.c 2 c 4c 1 c 3c 4 ,c 3ppc 1.p c 21 c 12288 CHAPTER 7 Electron Delocalization and Resonance• More About Molecular Orbital Theoryenergy of the
p atomic orbitalsmolecular orbitals
1,3 butadieneetheneenergy levelsnodeEnergy432121Figure 7.9N
Four patomic orbitals overlap to
produce four molecular orbitals in
1,3-butadiene, and two patomic
orbitals overlap to produce two
molecular orbitals in ethene. In
both compounds, the bonding MOs
are filled and the antibonding MOs
are empty.symmetric molecular orbitals fully asymmetric molecular orbitalsBRUI07-263_297r4 21-03-2003 11:32 AM Page 288