Microwave spectroscopic studies of the hydrogen transfer mechanism involved in isomerization

Abstract

The isomerization of cis-2-butene and 1-butene over alumina catalyst in the presence of perdeutero-propylene was studied in an attempt to elucidate the mechanism of n-butene isomerization. Microwave spectroscopy combined with gas chromatography and mass spectrometry was employed for the analyses of the reactants and products. Deuterium on the C₁ atom of perdeutero-propylene, e.g., CD₂, showed extensive exchange with the hydrogens in the coexisting butenes. Correspondingly, hydrogens at the C₁ position of 1-butene were replaced by deuterium. When this deuterium exchange was assumed to proceed independently of the isomerization of n-butene, the other features of the deuterium distribution resulting from the isomerization in the presence of perdeutero-propylene were similar to those obtained in the reaction of n-butene over deuterated p-toluene sulphonic acid. It was therefore concluded that the isomerization of n-butene over alumina, or at least the doublebond migration, occurs on some protonic sites formed by the adsorption of the olefin molecules; and that the deuterium scrambling between the terminal olefins, such as propylene and 1-butene, proceeds independently of the isomerization process.