Summary
The
MO diagram for the product ( cyclobutane) is
We can now draw a CORRELATION DIAGRAM which shows
how the orbitals of the reactants pass over or change into those of the
products.
The
key to remember in drawing the correlation diagram is that the symmetry of the
orbitals is conserved throughout!
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CORRELATION DIAGRAM
·
As the reaction proceeds the two electrons in the HOMO of the
reactants rise sharply in energy.
·
This creates a large energy barrier to the reaction and we say
that it is symmetry forbidden.
·
If we excite one of the electrons in the reactants, using UV
light, from the HOMO to the LUMO then the overall energy as we go from excited
reactants to excited product decreases.
·
Thus we say that the photon-assisted reaction is symmetry
allowed.