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

Abstract

Experimental investigations of -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 (∆E act ) to -hydrogen elimination in ethoxy groups (RCH 2 O (ad) f RCHdO (ad) + H (ad) , where R ) CH 3 , CFH 2 , CHF 2 , CF 3 ) and propyl groups (RCH 2 CH 2,(ad) f RCHdCH 2,(ad) + H (ad) , where R ) CH 3 , CF 3 ) 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 [RC δ+ ‚‚‚H δ-] ‡ . In this paper, deuterium kinetic isotope effects (DKIE) show that fluorination does not alter the mechanism for -hydrogen elimination from ethoxy groups. Furthermore, the DKIE measurements confirm that the effects of fluorine on the kinetics of -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 ∆E act for -hydrogen elimination in adsorbed alkoxy groups and the calculated reaction energetics (∆H rxn ) for gas-phase dehydrogenation of fluorinated alcohols in trans antiperiplanar conformations (e.g., RCH 2 OH (g) f RCHdO (g) + H 2,(g) , where the hydroxyl hydrogen is antiperiplanar to a carbon and the oxygen is antiperiplanar to a fluorine). Hammett plots for -hydrogen elimination give a reaction parameter of F ) -26. These correlations both suggest that the transition state for -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 -hydrogen elimination.

CH3CH2OH, CD3CD2OD, and CF3CH2OH Decomposition on ZnO $$ \left( {1\bar{1}00} \right) $$ 1 1 ¯ 00

“…Although b-hydride elimination is the expected reaction pathway for CH 3 CH=O (CD 3 CD=O) formation from ethoxy groups (step 2), this does not exhibit a deuterium isotope effect in the acetaldehyde desorption temperature. This is in contrast with results revealing a deuterium kinetic isotope effect during b-hydride elimination in ethoxy groups on Cu surfaces [24].…”

A dimensionless reaction coordinate for quantifying the lateness of transition states

“…Gellman et al performed experimental studies of b-H elimination from alkyl and alkoxy species on Cu(111) and Pt(111). [6][7][8][42][43][44][45] By extracting structure-reactivity coefficients from the experimentally measured activation . They also concluded that the transition state was late for b-H elimination of ethyl on different Cu surfaces (endothermic reaction), but the structure sensitivity of the forward and reverse reactions was found to be similar.…”

“…4,5 Gellman et al have suggested related ideas for classifying the sensitivity of barriers for catalytic reactions occurring on solid surfaces to the structure of these surfaces. [6][7][8] This proposal involves two separate hypotheses. The structure sensitivity postulate (SSP) asserts that the activation barriers for catalytic reactions occurring on solid surfaces are more sensitive to the structure of the surface if the transition state is late rather than early.…”

The chemistry of C2 perfluoroalkyl iodide on the Cu(111) single crystal surface

“…In the past two decades the chemistry [1][2][3][4][5] and the reaction kinetics [6][7][8] of alkyl moieties below C 3 prepared by the dissociative adsorption of alkyl halides on transition-metal have been widely studied. These results show that the alkyl moieties of C 2 or C 3 decompose on transition-metal surface by b-hydrogen elimination to form olefins (ethylene or propylene) and no C-C coupling reactions have been observed [1][2][3][4][5][6][7][8]. Fluorine-substituted alkyl surface moieties have also drawn increasing attention because the replacement of hydrogen with fluorine can result in a marked change in the physical and chemical properties.…”