Introduction to
correlation diagrams
R.B. Woodward and R Hoffmann
“Conservation of Orbital Symmetry”, pages 1-64 -- They were awarded the Nobel
Prize for this!
We examine the following
aspects of a chemical reaction:
1) The symmetry of the reactants and products: can we identify any symmetry elements which are present throughout the reaction? These are things like axes of symmetry or planes of symmetry.
2) Identify the molecular orbitals which
participate in the reaction. Some
molecular orbitals, such as some C-H bonding MOs for bonds which are not
broken, are clearly not involved.
Include the antibonding orbitals corresponding to the highest few
bonding orbitals involved.
3) Assign symmetries to the MOs of A) Reactants B) Products.
4) Draw a correlation diagram. This is done by joining up the lowest orbital
with a particular set of symmetry attributes on the Reactant side with the
lowest MO with the same symmetry attributes on the Product side. This
is a correlation diagram.
5)
From this correlation diagram we will be able to see if there is
a barrier to the chemical reaction or not.
If the occupied MOs increase in energy there is a barrier.
We will see how this approach can be used
to examine the
cycloaddition reaction of two ethenes to
form cyclobutane