Transition State for β-Elimination of Hydrogen from Alkoxy Groups on Metal Surfaces

Abstract

Experimental investigations of β-hydrogen elimination from alkoxy and alkyl groups bound to a Cu(111) surface have been coupled with computational studies of gas-phase analogues to provide insight into the transition state for catalytic hydrogenation and dehydrogenation on metal surfaces. Previous studies have shown that fluorination increases the activation barrier (ΔEact) to β-hydrogen elimination in ethoxy groups (RCH2O(ad) → RCHO(ad) + H(ad), where R = CH3, CFH2, CHF2, CF3) and propyl groups (RCH2CH2,(ad) → RCHCH2,(ad) + H(ad), where R = CH3, CF3) on the Cu(111) surface. The increase in barrier height with increasing fluorination was attributed to the inductive influence of fluorine, which energetically destabilizes a hydride-like transition state of the form [RCδ+···Hδ-]‡. In this paper, deuterium kinetic isotope effects (DKIE) show that fluorination does not alter the mechanism for β-hydrogen elimination from ethoxy groups. Furthermore, the DKIE measurements confirm that the effects of fluorine on the kinetics of β-hydrogen elimination do not result from the change in mass when hydrogen is substituted by fluorine. A systematic study of fluorine substitution of surface-bound isopropoxy groups reveals combined steric and electronic effects. An excellent correlation is found between the ΔEact for β-hydrogen elimination in adsorbed alkoxy groups and the calculated reaction energetics (ΔHrxn) for gas-phase dehydrogenation of fluorinated alcohols in trans antiperiplanar conformations (e.g., RCH2OH(g) → RCHO(g) + H2,(g), where the hydroxyl hydrogen is antiperiplanar to a carbon and the oxygen is antiperiplanar to a fluorine). Hammett plots for β-hydrogen elimination give a reaction parameter of ρ = −26. These correlations both suggest that the transition state for β-hydrogen elimination develops a greater partial positive charge on the carbinol carbon than is found in the adsorbed reactant. Furthermore, the transition state is energetically late in the reaction coordinate for β-hydrogen elimination.