Microwave study of the mechanism of n-butene isomerization over p-toluene sulphonic acid

Abstract

The isomerization of n-butene over deuterated p-toluene sulphonic acid was studied by means of microwave spectroscopy combined with gas chromatography and mass spectrometry. The deuterium content at each hydrogen position in the butene molecule was thereby determined. In double-bond migration of cis (and trans)-2-butene, the 3-d₁(CH₂CHCHDCH₃) species of 1-butene was the only product in the initial stages of the reaction. When 1-butene was isomerized to cis-2-butene, the 1-d₁(CH₂DCHCHCH₃) species was the only cis-2-butene produced in the early stages of the reaction. When trans-2-butene was converted to cis-2-butene by cis-trans isomerization, the d₀ and the 2-d₁(CH₃CDCHCH₃) species were formed in approximately equal amounts initially. These results lead to the conclusion that cis-trans isomerization of n-butene over p-toluene sulphonic acid proceeds via a carbonium ion. The kinetics of the reaction were analyzed on the basis of the reaction scheme which took both the carbonium ion and the concerted mechanisms into consideration. The results showed that the carbonium ion mechanism makes the major contribution to the isomerization of n-butene over p-toluene sulphonic acid, while the reaction by a concerted mechanism is almost negligible.