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

The reaction of the 2-propyl cation with propene has been investigated by M P2/6-31G** and B3LYP/6-31G** calculations. An ion-molecule complex stabiliz ed by 10-11 kcal mol(-1) over the isolated reactants was identified. It con tinued over a barrier and gave 1,1,3-trimethyl-1-protonated cyclopropane as the reaction product. The process read in the opposite direction represent s 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 character istically different, both for the reaction product and for the ion-molecule complex, with the MP2 calculations showing a stronger stabilization of hyd rogen- and carbon-bridged species. The calculated volumes (packed-cell dime nsions) of the transition state for the isopropyl cation-propene alkylation and for the isopropyl cation-propane hydride exchange were compared. The f ormer had higher steric requirements in the transversal cross-section. Ther efore, in the cracking of alkanes on medium pore zeolites such as HZSM-5, t he beta cracking step should be more sensitive to the existence of methyl s ide-chains than the hydride transfer step. The cracking mechanism of alkane s and alkenes on medium-pore zeolites is discussed based on these findings.