Photochemistry of Dimethoxybenzenes in Aqueous Sulfuric Acid

Article abstract of DOI:10.1021/jo00061a042

The photochemistry of the three isomeric dimethoxybenzenes (1-3) has been studied in aqueous sulfuric acid.Two processes were found to take place: (i) photoprotonation of the ring resulting in exchange of the ring protons, which was observed for all three compounds and (ii) ipso substitution of the methoxy group by water, which was observed only for 1,2-dimethoxybenzene (3).Both of these photochemical reactions were catalyzed by acid, requiring acidities stronger than pH(D)2 for observable reaction.Quantum yields for proton exchange and ipso substitution are reported as a function of medium acidity.The fluorescence emissions of all three compounds were quenched by acid, to give sigmoid type quenching curves, that have a complementary relationship with the corresponding plots of quantum yield of exchange vs acidity, consistent with protonation of the benzene ring in S₁ as the primary photochemical step, to give cyclohexadienyl cation intermediates.Stern-Volmer analysis of fluorescence quenching by proton gave photoprotonation rates in the range 0.09-2.2*10⁹ M^-1 s^-1.The results of this work demonstrate unambiguously that methoxy-substituted benzenes are much stronger bases in S₁ (pK_BH+ ca. 1 to -2) than in the ground state.This enhanced basicity is manifested in regioselectivity of exchange of 1 and 3 and ipso substitution chemistry of 3, processes not observed in the ground states of these compounds.