Question

Question:Using orbital symmetry, explain why a Diels–Alder reaction does not take place under photochemical reaction conditions.

Short answer

Answer

The pathway of the reaction is concerted. The p-orbitals overlap in a suprafacial manner. Upon photochemical excitation, the concerted pathway is no longer followed.

Step-by-step solution

Diels-Alder reaction

Diels-Alder reaction is a type of [4+2] cycloaddition between a diene and a dienophile. Like any other electrocyclic reaction, Diels-Alder also proceeds via a concerted pathway and occurs under thermal conditions.

Orbital symmetry in Diels-Alder reaction

The participating reactants of a Diels-Alder reaction are:

Reactants of Diels-Alder reaction

The bonding of the diene and dienophile occurs as shown below:

Bonding of diene and dienophile

The orbital overlap is given below

Orbital overlap of HOMO(diene)-LUMO(dienophile)

The like phases face the same side; thus orbital overlap can occur in a suprafacial manner.

It is necessary for [4+2] cycloaddition to occur under thermal conditions, since under photochemical conditions, the electron at the HOMO of diene may get excited, as shown below:

When the excited HOMO overlaps with the LUMO of the dienophile, the like phases are not facing the same direction for one of the terminal carbons. Thus the molecule has to be twisted, following the antarafacial pathway.

In this condition, a concerted pathway can no longer be followed.

Hence, photochemical conditions are forbidden for the Diels-Alder reaction.