The two modes of reaction of hexane catalyzed by trifluoromethanesulfonic acid

The initial reaction observed on reacting hexane (H) with trifluoromethanes ulfonic acid (TFMSA) under mild conditions was a dehydrogenation with the f ormation of alkenyl cations, identified by UV-visible spectroscopy. When th ese ions were dispersed from the liquid/liquid interface, isomerization to methylpentanes (2MP and 3MP) occurred. The reaction rates were measured at low conversions and gave Delta H double dagger = 15 kcal mol(-1) and Delta S double dagger approximate to -40 cal mol(-1) deg(-1). When the acid layer was not homogenized, a much faster reaction, mostly cracking and dispropor tionation, was observed, after an induction period needed to achieve a crit ical concentration of initiators at the interface. The homogenized acid ini tiated the cracking mode after a much longer time, when the alkenyl ions re ached the critical concentration throughout the acid phase. The induction p eriod was reduced by the addition of small amounts of one-electron oxidizer s, such as ferric ions. The relative reactivity 3MP/H, which in the isomeri zation mode was about the same as for HF-based catalysts (1000), was reduce d to about 10 in the cracking mode. Some key reaction features of the crack ing mode are reminiscent of zeolite catalysis. These are: the dramatic acce leration of the reaction of n-hexane relative to the reaction of 3-methylpe ntane, a large excess of the branched isomers in the C-4 and C-5 fractions above the equilibrium ratio, the absence of unsaturated cracking products ( the unsaturated products are retained by the catalyst in both cases), and f ormation of dibranched C6H14 isomers, particularly 2,2-dimethylbutane (2,2D MB) as primary products. Neither steric control in cages or channels, nor i ntermediacy of pentacoordinated carbocations, invoked as explanations for t he reactions in zeolites, can apply to the reaction with TFMSA as catalyst.