Reactions of n-butenes on nickel films

Abstract

The reactions on nickel films of all three butenes (exchange, double-bond migration and cis–trans isomerization) were investigated at 0–20°C in the presence of C₃D₆ and at –84°C in the presence of D₂. Analyses of the reaction products were made by mass spectrometry and microwave spectroscopy. The hyperfine distributions of the [²H₁] and [²H₂]but-1-enes obtained by exchange suggest a vinylic-type dissociative mechanism (Farkas mechanism). Cis–trans isomerization takes place either by an associative Horiuti–Polyani mechanism or by a direct process without deuterium incorporation. Three reaction mechanisms account for double-bond migration: intramolecular hydrogen shift, associative mechanism and allylic mechanism. The latter involves a number of interconversions between π-olefinic and π-allylic adsorbed species and yields multideuterated molecules.

The above reaction mechanisms may be classified into three groups, according to their activation energies, and are believed to occur on three different types of sites with different coordination numbers. This view is supported by the changes in reaction mechanisms that result from film modification.