The molecular orbital description of bonding in dienes is an extension of the jump from alkene to allyl--one more sp2 center with one more p orbital is involved. There is an increase in the net stability of the system versus isolated double bonds, though the HOMO is now higher in energy than the alkene HOMO.


Show pz orbitals or Orbitals off
Show the highest energy MOHighest lying pi MO of butadiene: all p orbitals out of phase.
Show the LUMOLUMO of butadiene (Lowest Unoccupied Molecular Orbital): phase changes between C1 and C2, and between C3 and C4.
Show the HOMOHOMO of butadiene (Highest Occupied Molecular Orbital). A phase change between C2 and C3.
Show the fully bonding pi MOLowest pi MO of butadiene. All p orbitals have the same phase.

 The links are listed in descending order of energy; the lowest is the most stable. The bottom 2 are occupied (4 π electrons total).

We can draw a number of resonance forms but we start to get to a point where resonance does not really help us any more. There are a couple observations that come from the MO analysis:
  1. Net stabilization of the pi system versus isolated C=C bonds. Evidence: Hydrogenation of 2 eq. propene gives ΔH° = -60.6 kcal/mol; for 1,3-butadiene ΔH° = -57.1 kcal/mol
  2. Somewhat restricted rotation about C2-C3. Experimentally, the barrier is appr. 6 kcal/mol.
  3. Despite the net stabilization, the HOMO is higher and the LUMO lower than for an alkene. Experimentally observed by UV spectroscopy.


Black background
White background
Spacefilling model
Wireframe
Ball & Stick


Last updated: 12/15/2019