16.2b Pi Molecular Orbitals 1,3 Butadiene

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Pi Molecular Orbitals of 1,3-Butadiene

With a single sigma bond separating the pi bonds of 1,3-butadiene it is a conjugated system and some of the pi electron density will be delocalized between carbons 2 and 3 rather than just between 1 and 2 and 3 and 4.  There are four adjacent carbon atoms involved in the pi system and the combination of a p orbital from each of these atoms will result in four pi molecular orbitals: ψ1, ψ2, ψ3*, and ψ4* (also referred to as π1, π2, π3*, and π4*).

1,3 butadiene pi molecular orbitals

ψ1 and ψ2 are bonding molecular orbitals and are occupied in the ground state with ψ2 being the Highest Occupied Molecular Orbital (HOMO).  ψ3* and ψ4* are antibonding molecular orbitals and are unoccupied in the ground state with ψ3* being the Lowest Unoccupied Molecular Orbital (LUMO).  The orbitals are arranged in the following table in order of increasing energy.  It can also be seen that the number of vertical nodes in a molecular orbital increases as the energy increases. ψ1 has zero vertical nodes and they increase incrementally until ψ4* which has 3 vertical nodes.  It should also be pointed out that these vertical nodes are always arranged symmetrically in the molecular orbital diagrams.

ψ4 / π4* - 3
ψ3 / π3* LUMO 2
ψ2 / π2 HOMO 1
ψ1 / π1 - 0