NMR study of the role of isopropylsulfates in the two-step "conjunct oligomerization" of propylene and isopentane-propylene alkylation catalyzed by 95% sulfuric acid

"Conjunct oligomerization" of propylene or the isopentane-propylene alkylat ion catalyzed by an excess of 95% sulfuric acid was performed in two consec utive steps. First di-isopropylsulfate was prepared by interaction of sulfu ric acid with propylene. The ester was then either decomposed at room tempe rature in the presence of the 5-10 molar excess of 95% acid or was used in the acid-catalyzed alkylation of isopentane. In situ H-1 and C-13 NMR study of the reaction mixture of "conjunct oligomerization" indicated that the d iester participates in two equilibria with sulfuric acid. The first one tra nsforms the diester into a monoester. The second equilibrium corresponds to protonation of the monoester with an excess of sulfuric acid. This convert s a minor fraction of the mono-alkylsulfate into isopropyl carbenium ions t hat are only weakly solvated with sulfuric acid: C3H7HSO4 + H2SO4 reversibl e arrow C3H7+ H2SO4 + HSO4-. The subsequent reactions of alkyl carbenium io ns with the non-protonated alkylsulfate result in final products of "conjun ct oligomerization" while in the presence in the reaction mixture of isopen tane, alkylation with the predominant formation of C-8 branched paraffins t akes place. A very low yield of propane indicates a minor role of hydride t ransfer in alkylation. Another unexpected result is the absence in both rea ction mixtures of propylene. These findings are in contradiction with the c lassical mechanism of isoparaffin-olefin alkylation by Schmerling. Therefor e, an alternative mechanism of this reaction is suggested via a direct alky lation of isopentane with the mono-alkylsulfate.