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

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.