Reaction mechanisms on liquid and solid acid catalysts. Correlation with acidity

The probe bases used for measurement of strength of acid catalysts need to be structurally similar to the catalytic substrates. Traditional acidity me asurements are inapplicable when the hydron transfer forms a tight ion pair , as on solid acids. Also, solid acids are much weaker than liquid acids of similar structure. Alkane activation by C-H and C-C cleavage, evidenced in strong superacids, does not occur in less strongly acidic media. The react ivity patterns and structure-reactivity relationships are the same for the cationoidic species (weakly coordinated carbocations) occurring in the less strong acids as for the full-hedged carbocations intervening in superacids . Even in trifluoromethanesulfonic acid, a weak superacid, the first step i n alkane activation is an oxidation, forming unsaturated carbocations. On s ulfated zirconia, the first step of alkane reactions is a one-electron oxid ation. Mechanistic features incompatible with standard (or "traditional") c arbocationic processes, are observed in the reaction of 3-methylpentane on the zeolite HZSM-5.