Control of oxidation selectivity by alkyl substitution: The reactions of t-butyl iodide and t-butanethiol on Rh(111)-p(2x1)-O

We report the formation of adsorbed t-butoxy by addition of transient t-butyl radicals to surface oxygen, in the reactions of t-butyl iodide and t-butanethiol on Rh(111)-p(2 x 1)-O. t-Butanethiol reacts by cleavage of the weak S-H bond, forming adsorbed t-butyl thiolate upon adsorption at 100 K. t-Butyl thiolate subsequently forms t-butanol isobutene and water in the range of 325-345 K via a transient t-butyl radical formed by cleavage of the C-S bond. Similarly, t-butyl iodide reacts by cleavage of the C-I bond to yield isobutene and t-butanol at 260 K. In both cases, isobutene is proposed to form via a combination of rapid P-hydrogen elimination of the t-butyl radical and decomposition of t-butoxide formed from addition of the radical to O-a. t-Butanol is produced via a competing hydrogenation of t-butoxide. The relative reaction temperatures correlate with the C-I and C-S bond strengths, indicating that homolytic C-I and C-S bond scission limit the rates of the reaction. Some t-butoxy remains on the surface up to 380 K, and is identified using high resolution electron energy-loss and temperature-programmed reaction spectroscopies. These studies demonstrate that the product distributions for reactions of alkyls on oxygen-covered Rh(lll) can be manipulated by altering the molecular structure. Specifically, we were able to induce partial oxidation of the alkyl radical to the corresponding alcohol by effectively eliminating the pathway for dehydrogenation at the carbon adjacent to the oxygen via alkyl substitution.