Ortho Photocycloaddition of Alkenes and Alkynes to the Benzene Ring
Photoexcitation of benzene into its first excited singlet state ( ^ u ) turns that stable molecule into a very reactive species. The amount of electronic energy in the excited molecule, relative to the ground electronic state, is 112.5 kcal/mol and the six electrons are no longer in pairs filling the three bonding TT-orbitals in a closed-shell configuration. The molecule has lost its aromatic stability, is rich in energy, and therefore capable of undergoing chemical reactions not possible in the ground state. For reactions in the ground state of benzene, rather agressive reagents are necessary. Among these reactive species are molecules in their elec tronically excited states.