• In a 4n+2 cyclo addition n is a number, if it is 1 the total is 6. This is a Diels-Alder reaction.
• 4 electrons from 1,3-Butadiene and two from ethene are involved. The result is a six membered ring.

In the above MO picture of 1,3-Butadiene:

• π₁ and π₂ are bonding molecular orbitals. The four π electrons occupy these orbitals.
• Between them π₂ is higher in terms of energy hence HOMO (Highest occupied molecular orbital,)
• π₃*and π₄* are unoccupied,( ABMO)and π₃*is lower in terms of energy hence LUMO(Lowest unoccupied MO)
• Under photochemical conditions an electron undergoes transition to π₃* hence it will then be the HOMO
• When two molecules interact the HOMO of one interacts with the LUMO of another.( FMO theory)
• The two symmetry elements considered are C₂ axis and plane of symmetry.(In Woodwad Hoffmann treatment)
• S and A represent symmetric and anti-symmetric respectively.

In the above MO picture of ehtene:

• π₁ is the bonding molecular orbital. The two π₁ electrons occupy this orbital.
• There is only one occupied MO hence it will be the HOMO.
• π₁*is unoccupied, lower in terms of energy hence LUMO (Lowest unoccupied MO)
• Under photochemical conditions an electron undergoes transition to π₁* hence it will then be HOMO