MO Description of Cycloadditions

We now turn our attention to the MOs involved in a Diels-Alder reaction. By studying the interactions between the relevant MOs we will gain insight into the reaction, and we will be able to explore the feasibility of reactions similar to Diels-Alder reactions. 

Now let s use the same approach (analyzing the frontier orbitals) to determine whether the following reaction is possible. 

In a [2+2] cycloaddition, the electron density must flow from the HOMO of one compound to the LUMO of the other compound. 

The ground state of ethylene absorbs a photon of light to achieve the excited state, in which the HOMO is i[)2- 

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