The Diels–Alder reaction is an example of a thermal pericyclic reaction between a conjugated diene and an alkene or alkyne, commonly referred to as a dienophile. The reaction involves a concerted movement of six π electrons, four from the diene and two from the dienophile, forming an unsaturated six-membered ring. As a result, these reactions are classified as [4+2] cycloadditions.

From a molecular orbital perspective, the interacting lobes of the two π systems must be in phase to permit the formation of new σ bonds in a synchronous manner. For molecules in the ground state, the interaction between HOMO (diene) and LUMO (dienophile) or HOMO (dienophile) and LUMO (diene) satisfies the orbital symmetry requirements. The two π components interact suprafacially in each case, making Diels–Alder a thermally allowed [4+2] cycloaddition reaction.