15-19P Show that the [4+2] Diels-Alder ... [FREE SOLUTION]

Chapter 15: 15-19P (page 782)

Show that the [4+2] Diels-Alder reaction is photochemically forbidden.

Short Answer

Expert verified

In the Diels-Alder cycloaddition, the LUMO of the dienophile and the excited state of the HOMO of the diene (HOMO^*) generate one bonding and one antibonding interaction. Hence, it is a photochemically forbidden process.

Step by step solution

Diels-Alder reaction

Diels-Alder reactions are a kind of cycloaddition reaction. Two molecules link in a single step to generate a new ring in this reaction.

Forbidden cycloaddition

If a cycloaddition creates an overlap of positive-phase p orbitals with the negative-phase orbitals, antibonding interactions are formed. These interactions increase the activation energy and are symmetry forbidden.

Showing that the [4+2] Diels-Alder reaction is photochemically forbidden

When ultraviolet light is used instead of heat for [4+2] Diels-Alder reaction, it is photochemically not allowed. A photon with the appropriate energy strikes 1, 3-butadiene, one of the pi electrons gets excited to the next higher molecular orbital. This higher molecular orbital is occupied and is the new (HOMO^*) of the excited state.

Bonding and the antibonding interaction of the Diels-Alder cycloaddition

In the Diels-Alder cycloaddition, the LUMO of the dienophile and the excited state of the HOMO of the diene(HOMO^*) generate one bonding and one antibonding interaction. Hence, it is a photochemically forbidden process.