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SOLVATION EFFECTS IN CATALYTIC TRANSFORMATIONS OF OLEFINS IN SULFURIC ACID

Analysis of literature and ab initio quantum chemical calculations indicate that the properties of anhydrous H2SO4 as a superacid are determined not by the extent of dissociation of this acid but by the unusually high chemical activity of the weakly solvated protons. In a similar way, the selectivity of the acid-catalyzed transformations of olefins is also determined by solvation of the active intermediates. Depending on the concentration of sulfuric acid, the properties of the active species are changing from almost free carbenium ions in the anhydrous acid to the hydrated oxonium ions in the moderately dilute sulfuric acid. In the later case, similar to zeolites, the carbenium-ion-like alkyl species are formed only as transition states. The subsequent hydrolysis of oxonium ions upon stronger dilution of the acid results in the formation of alcohols. These conclusions are supported by ab initio quantum chemical calculations and by in situ 13C-NMR of protonated active intermediates.