Neopentyl iodide on Pt(1 1 1): II. Hydrogenation and H–D exchange

The reactivity of neopentyl iodide with coadsorbed hydrogen or deuterium on a Pt(1 1 1) single-crystal surface was studied by temperature programmed desorption (TPD). It was determined that the neopentyl surface groups resulting from an early C–I bond scission are easily hydrogenated to neopentane, the yield of which amounts to more than 99% of all the products from the surface reactions. TPD yields and desorption maxima were used to estimate relative rates for the different hydrogenation and dehydrogenation steps, and isotope labeling experiments allowed for the identification of additional H–D reactions. Hydride elimination from the α carbon of neopentyl moieties is approximately six times faster than removal from the γ position. A significant normal kinetic isotope effect, of a factor of approximately eight, was also measured, and determined to be associated with differences in the preexponential factors of the associated reaction rate constants. H–D exchange appears to be favored at the α position, a consequence of a complex interplay between the relative rates of α- vs. γ-H elimination in neopentyl groups and the relative stability of the resulting neopentylidene and 2,2-dimethyl propane-1,3-diyl intermediates.