Please explain the bonding in a 2+2 cyclodaddtion with only one excited electron.

Question

The photochemical 2 + 2 cycloaddition involves 1 electron in an excited pi* orbital of one alkene with an empty pi* orbital of the other ground state alkene. This gives the correct orbital overlap, but only involves one electron. How can this form a two electron bond?

Answer 1

In a photochemical 2+2 cycloaddition, even though only one electron is initially excited to a pi* orbital, the reaction can proceed because the excited electron in the pi* orbital of one alkene interacts with the empty pi* orbital of the ground state alkene, creating a favorable orbital overlap that allows for the formation of a new sigma bond, effectively "pulling" the other electron from the ground state alkene to participate in the bond formation; this process is facilitated by the symmetry of the orbitals and the overall energy lowering during the reaction.