The two modes of reaction of hexane catalyzed by trifluoromethanesulfonic acid

Abstract

The initial reaction observed on reacting hexane (H) with trifluoromethanesulfonic acid (TFMSA) under mild conditions was a dehydrogenation with the formation of alkenyl cations, identified by UV-visible spectroscopy. When these 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 ΔH ^‡ = 15 kcal mol^–1 and ΔS ^‡ ≈ –40 cal mol^–1 deg^–1. When the acid layer was not homogenized, a much faster reaction, mostly cracking and disproportionation, was observed, after an induction period needed to achieve a critical concentration of initiators at the interface. The homogenized acid initiated the cracking mode after a much longer time, when the alkenyl ions reached the critical concentration throughout the acid phase. The induction period was reduced by the addition of small amounts of one-electron oxidizers, such as ferric ions. The relative reactivity 3MP/H, which in the isomerization mode was about the same as for HF-based catalysts (1000), was reduced to about 10 in the cracking mode. Some key reaction features of the cracking mode are reminiscent of zeolite catalysis. These are: the dramatic acceleration of the reaction of n-hexane relative to the reaction of 3-methylpentane, a large excess of the branched isomers in the C₄ and C₅ fractions above the equilibrium ratio, the absence of unsaturated cracking products (the unsaturated products are retained by the catalyst in both cases), and formation of dibranched C₆H₁₄ isomers, particularly 2,2-dimethylbutane (2,2DMB) as primary products. Neither steric control in cages or channels, nor intermediacy of pentacoordinated carbocations, invoked as explanations for the reactions in zeolites, can apply to the reaction with TFMSA as catalyst.