The Diels–Alder reaction is a thermally allowed [4+2] cycloaddition between a 4π conjugated diene and a 2π dienophile to form a six-membered cyclic product.

In this process, two weaker π bonds are converted to two stronger σ bonds.

The reaction involves a concerted movement of six π electrons via a cyclic transition state.

For the two new σ bonds to form simultaneously, the frontier molecular orbitals of the two π systems must align such that the interacting lobes are in phase.

Under thermal conditions, a constructive overlap is possible when the ground state HOMO of the diene and the ground state LUMO of the dienophile interact in a suprafacial manner.

The same is true for the alternative HOMO-LUMO pairing. However, the former pairing is preferred since a smaller HOMO-LUMO energy gap leads to a better overlap.

So, the diene is generally the electron-rich component and the dienophile electron-deficient.

Lastly, the Diels–Alder reaction is syn stereospecific, meaning that the stereochemistry of the dienophile is retained in the product.