There is only one conserved symmetry element in this transformation – the mirror plane through the center of the reactants as shown to the left. From this we can assign the symmetry of the molecular orbitals of the reactants very simply. The molecular orbitals of the reactants are merely the set {Ψ1, Ψ2, Ψ3, Ψ4} of molecular orbitals of 1,3-butadiene shown above, along with π and π* of ethylene. Ψ1 is symmetric, Ψ2 is antisymmetric, Ψ3 is symmetric, and Ψ4 is symmetric with respect to the mirror plane. Similarly π is symmetric and π* is antisymmetric with respect to the mirror plane.