The kinetics of isomerization of 3-methylpentane catalyzed by trifluoromethanesulfonic acid1

Abstract

Reaction of 3-methylpentane (3MP) was conducted in a two-phase liquid phase system, with the weak superacid trifluoromethanesulfonic acid (TFMSA) as catalyst, below 40 °C. The reaction pattern depended upon the way in which the process was conducted. If the acid layer was homogenized periodically, isomerization to 2-methylpentane occurred, with very little cracking. If the acid layer was left undisturbed, a yellow zone (indicating unsaturated organic species) at the interface with the hydrocarbon layer was formed, the reaction showed an induction period after which the overall conversion was greater than for the other reaction mode, and the extent of cracking (direct or following alkylation) increased significantly. For the same quantities of TFMSA and 3MP, increasing the diameter of the reaction tube increased the reaction rate, indicating that the reaction takes place at the interface. The reaction kinetics for the clean isomerization mode (dispersion of the unsaturated organic species formed in the acid layer from the interface) were analyzed by the rate equations reported previously, allowing for catalyst deactivation. Rate measurements at five temperatures between 14 and 32 °C gave the activation parameters ΔH ^‡ = 19 kcal mol^–1 and ΔS ^‡ ≈ –16 cal mol^–1 deg^–1, which did not change when the ratio of catalyst to substrate was varied by a factor of two. These values are not compatible with a mass-transfer controlled reaction as found for the HF–SbF₅ catalyst. Instead, the ionization of an alkyl trifluoromethanesulfonate intermediate or a methyl group shift in the cation could be rate-determining. No intermediate could be evidenced by NMR in either layer, but the ¹³C NMR spectrum of the acid layer at the end of the reaction showed the presence of several alkenyl cations (polyalkylcyclopentenyl and possibly even some polyalkylcyclohexenyl cations).