H/D isotope exchange between methane and magic acid (HSO₃F–SbF₅): an in situ NMR study

Abstract

The kinetics of hydron exchange between methane and a series of DSO 3 F-SbF 5 superacids were measured by in situ 2 H decoupled 1 H NMR spectroscopy. The rates of exchange showed a strong dependence on antimony pentafluoride concentration, with the free energy of activation DG # (30 C) decreasing from 97 to 84 kJ mol À1 over the range of concentration 19 to 49 mol % SbF 5 . The constant free enthalpy of activation DH # (ca. 65 kJ mol À1 ) and the decreasing entropy of activation DS # seem to indicate that an increase in acidity of the superacid system does not substantially change the nature of the transition state but rather acts on its solvation.

Neopentane and Solid Acids: Direct Hydron Exchange before Cracking

“…Recently we have studied the H/D exchange between methane and DSO 3 F/SbF 5 varying the SbF 5 concentration and we have demonstrated not only a unique transition state for all systems but also a strong participation of the anionic part in the transition state. 13 An analogy between these superacidic media and zeolites can then be proposed on the basis of kinetical analysis, and the rate of exchange at a given temperature should correlate directly with the acidity of the system.…”

“…Nonclassical cations were earlier suggested as a transition state or strongly solvated intermediates in the H/D exchange reaction between methane and superacids DF/SbF 5 12 or DSO 3 F/SbF 5 . 13 Whereas the reactivity of hydrocarbons in liquid superacids is clearly explained by the σ-basicity of their C-C and C-H bonds, 14,15 the mechanistic pathway should obviously involve the surface of solid acids and the participation of nucleophilic oxygen lone pairs compensating the lack of superacidity and stabilizing charges. [16][17][18] 2,2-Dimethylpropane (neopentane) is a "limiting" case on the σ-basicity scale for alkanes 19 because the protolysis can only take place either on a primary C-H bond or on a C-C bond.…”

Superacid‐Catalyzed Reactions

“…Only the unsolvated superacid H 2 F þ was found to be strong enough to protonate methane to yield the methonium ion solvated by HF as a potential energy minimum. 51 The rates of exchange, much lower than in HF-SbF 5 , showed a strong dependence on antimony pentafluoride concentration, with the free energy of activation DG z 30 C decreasing from 23 to 20 kcal mol À1 over the range of concentration 19 to 49 mol% SbF 5 . As methane is also slightly soluble in Magic Acid, the hydrogen exchange rate between methane and a series of DSO 3 F-SbF 5 superacids could be measured by in situ 2 H-decoupled 1 H NMR spectroscopy ( Figure 5.3).…”

Reactions in Zeolites

“…For instance in the case of isobutane 29,82,83 a regiospecifi c exchange on the methyl groups was observed in the H/D exchange reaction with solid acids being in agreement with a general mechanism based on intermediate carbenium ions (classical trivalent carbocation). 91 While the reactivity of hydrocarbons in liquid superacids is clearly due to the σ -basicity of their C − H bonds, 66,92 the mechanistic pathway involves the surface of solid acids and the participation of nucleophilic oxygen lone pairs compensating the lack of superacidity and stabilizing charges. Similarly, we 84,85 and others 86,87 have studied more recently both the H/D exchange and the skeletal rearrangement in the reaction between propane and sulfated zirconia as well as zeolite using deuterium and carbon -13 as labels.…”