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

Reaction of 3-methylpentane (3MP) was conducted in a two-phase liquid phase system, with the weak superacid trifluoromethanesulfonic acid (TFMSA) as c atalyst, below 40 degrees C. The reaction pattern depended upon the way in which the process was conducted. If the acid layer was homogenized periodic ally, isomerization to 2-methylpentane occurred, with very little cracking. If the acid layer was left undisturbed, a yellow zone (indicating unsatura ted organic species) at the interface with the hydrocarbon layer was formed , the reaction showed an induction period after which the overall conversio n was greater than for the other reaction mode, and the extent of cracking (direct or following alkylation) increased significantly. For the same quan tities of TFMSA and 3MP, increasing the diameter of the reaction tube incre ased the reaction rate, indicating that the reaction takes place at the int erface. The reaction kinetics for the clean isomerization mode (dispersion of the unsaturated organic species formed in the acid layer from the interf ace) were analyzed by the rate equations reported previously, allowing for catalyst deactivation. Rate measurements at five temperatures between 14 an d 32 degrees C gave the activation parameters Delta H double dagger 19 kcal mol(-1) and Delta S double dagger approximate to -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 contr olled reaction as found for the HF-SbF5 catalyst. Instead, the ionization o f an alkyl trifluoromethanesulfonate intermediate or a methyl group shift i n the cation could be rate-determining. No intermediate could be evidenced by NMR in either layer, but the C-13 NMR spectrum of the acid layer at the end of the reaction showed the presence of several alkenyl cations (polyalk ylcyclopentenyl and possibly even some polyalkylcyclohexenyl cations).