Steric requirements of the alkene–carbocation alkylation in relation to the hydride transfer and proton cleavage of carbon–carbon bonds. Significance for the reactions of alkanes in zeolites

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Abstract
The reaction of the 2-propyl cation with propene has been investigated by MP2/6-31G** and B3LYP/6-31G** calculations. An ion–molecule complex stabilized by 10–11 kcal mol−1 over the isolated reactants was identified. It continued over a barrier and gave 1,1,3-trimethyl-1-protonated cyclopropane as the reaction product. The process read in the opposite direction represents the cracking of 2-methylpentane, occurring nominally from the 2-methyl-4-pentyl cation (open structure of 1,1,3-trimethyl-1-protonated cyclopropane). The geometries predicted by the MP2 and B3LYP calculations were characteristically different, both for the reaction product and for the ion–molecule complex, with the MP2 calculations showing a stronger stabilization of hydrogen- and carbon-bridged species. The calculated volumes (packed-cell dimensions) of the transition state for the isopropyl cation–propene alkylation and for the isopropyl cation–propane hydride exchange were compared. The former had higher steric requirements in the transversal cross-section. Therefore, in the cracking of alkanes on medium pore zeolites such as HZSM-5, the β cracking step should be more sensitive to the existence of methyl side-chains than the hydride transfer step. The cracking mechanism of alkanes and alkenes on medium-pore zeolites is discussed based on these findings.

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