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Chemistry · L5 · Why the [2+2] Photocycloaddition Needs Light

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Ground-state alkenes cannot complete a [2+2] cycloaddition thermally because the HOMO–HOMO overlap is symmetry-forbidden; photoexcitation flips one alkene's frontier orbital symmetry and opens a photochemically allowed path.

by @openstemUpdated Chemistry
Alkene 1: ground state, HOMO occupiedTry thermal 2+2 cycloadditionAlkene 2: ground state, LUMO emptyThermally forbidden: no reactionAlkene 1 excited: electron promoted HOMO to LUMOPhotochemical 2+2 cycloadditionPhotochemically allowedCyclobutane ring formedHOMO-HOMO overlap: symmetry mismatchAbsorb photon: hvHOMOstar-LUMO overlap: symmetry allowed

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