Cycloaddition of 2 ethenes to form cyclobutane (ethene dimerization)

We consider the reactants (2 ethene molecules) coming together so that their p orbitals overlap. This is the pathway for the reaction:

i.e.

· Two symmetry ELEMENTS (a two-fold rotation axis, C_2, and a mirror plane) are conserved throughout the reaction –

· They are present in the reactants (two ethenes) as they approach one another, and also in the products (and in the transition state of the reaction).

· These symmetry elements are shown above for the reactants and the products.

· There are other symmetry elements, but we will consider only the minimum number we need to analyse the reaction.

· We must now classify the symmetries of the orbitals of the reactants and the products with respect to these symmetry elements.

We must consider the combinations of the orbitals for their symmetry properties (e.g. the MOs of one ethene alone don’t have these symmetry elements).

The combined MOs must be either symmetric (S) or antisymmetric (A) for each symmetry element.

Classifying the MOs of the reactants (2 ethenes)