How does a Diels-Alder reaction form a ring?
3D model · turn it, step through it, tap the parts
An interactive 3D scene of s-cis butadiene and ethylene approaching face to face, the diene's HOMO lining up with the dienophile's LUMO, then electrons flowing around a six-atom loop as three π bonds break and two σ bonds and one π bond form together, leaving cyclohexene.