Conservation of orbital symmetry: cycloaddition of two ethenes to form cyclobutane
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.