Thermal cycloadditions are reactions where the source of activation energy needed to initiate the reaction is provided in the form of heat. A typical example of a thermally-allowed cycloaddition is the Diels–Alder reaction, which is a [4 + 2] cycloaddition. In contrast, a [2 + 2] cycloaddition is thermally forbidden.

The reaction occurs between the highest occupied molecular orbital (HOMO) of one π component and the lowest unoccupied molecular orbital (LUMO) of the other. These are known as the frontier molecular orbitals. Under thermal conditions, the reaction proceeds via the ground state HOMO and LUMO. For cycloadditions to take place in a concerted manner, the terminal lobes of the interacting systems must have the same symmetry.

In a [4 + 2] cycloaddition, the terminal lobes of the 4 π and the 2 π components are in phase and interact suprafacially. Consequently, the reaction is thermally allowed. However, in a [2 + 2] cycloaddition, the symmetry mismatch leads to a suprafacial interaction on one end and antarafacial on the other. The geometric constraint makes it thermally forbidden.