Solvation as a main factor that determines the strength of liquid superacids and the selectivity of the acid-catalyzed reactions of olefins

Abstract

Analysis of literature and ab initio quantum chemical calculations indicates that the solvation of protons in anhydrous H 2SO 4 or HF is considerably weaker than in aqueous solutions. This results in very low constants of autoprotolysis of anhydrous superacids. In contrast, hydration of protons in moderately concentrated acids is much stronger, resulting in higher dissociation constants. Therefore, the strength of superacids is connected to the high chemical activity of the weakly solvated protons, rather than to the amount of protons. In a similar way, solvation of protonated active intermediates of the acid-catalyzed transformations of olefins in anhydrous superacids is also weaker than in aqueous solutions. This results in the real carbenium ion mechanisms of the isoparaffin–olefin alkylation or “conjunct oligomerization” of olefins. In contrast, solvation with water in moderately concentrated or dilute sulfuric acid transforms alkyl carbenium ions into oxonium ions. Therefore, cationic polymerization of olefins in moderately concentrated sulfuric acid is not a real carbenium ion reaction. Similar to the acid-catalyzed transformations of olefins on zeolites, it involves oxonium ions as the stable active intermediates and alkyl carbenium ions as transition states.