The pictures that we used to explain the reaction between ethene and butadiene, can also serve to eplain why a reaction between two ethylene molecules will normally not take place:
| homo
| lumo
|
Overlap between HOMO and LUMO is not beneficial, as matching of lobes with
the right sign is cancelled by a mismatch at the other atom pair.
Neither is HOMO-HOMO overlap: the symmetry is allright, and two electrons
could occupy a newly formed, lower orbital, but the other two electrons
have to occupy the higher one, and the net effect is (close to) zero.
In LUMO-LUMO overlap there are no electrons at all to benefit from
the interaction.
Generally speaking, the 2+2 reaction is not favoured.
Neither is the 4+4 cycloaddition, for the same reason.
This picture changes however, when one electron is raised to the LUMO,
by photochemical excitation. This LUMO now becomes the new HOMO, and it will
favourably overlap with the empty LUMO of a ground state ethylene. The
newly formed, lower lying orbital is occupied by one electron, so the system
will be stabilized by the interaction.
Question: is the other combination, the 'formerHOMO'-HOMO interaction, beneficial?
These orbital symmetry considerations lead to the general (Woodward-Hoffmann) rule that
suprafacial cycloadditions of the type [i+j] are allowed in the ground state
if i+j equals 4n+2, and photochemically if it equals 4n.
The term suprafacial is illustrated on the
next page, where we discuss ketene 2+2 reactions.
