Final answer:
To form cyclohexene, the HOMO of 1,3-butadiene and the LUMO of ethylene react in a Diels-Alder mechanism, involving a concerted movement of electrons and resulting in a six-membered ring.
Step-by-step explanation:
The molecular orbitals (MOs) that react to form cyclohexene through the Diels-Alder reaction are the Highest Occupied Molecular Orbital (HOMO) of 1,3-butadiene and the Lowest Unoccupied Molecular Orbital (LUMO) of ethylene (ethene). During this reaction, the filled π electron cloud of the butadiene's HOMO overlaps with the empty π* electron cloud of the ethylene's LUMO, facilitating the formation of new sigma bonds that result in the cyclic structure of cyclohexene. This reaction is characteristic of a concerted mechanism, where electrons move in a single, cyclic array, leading to the simultaneous formation of multiple bonds.