Fluorine Substituent Effects on Alkoxide Chemistry and Orientation on the Cu(100) Surface

Article abstract of DOI:10.1021/j100143a043

The straight chain hydrocarbon alcohols (CH3(CH2)_nOH, n = 0-4) and fluorocarbon alcohols (CF3(CF2)_nCH2OH, n = 0-2) all adsorb reversibly on the clean Cu(100) surface.As the alkyl chain is lengthened, the heats of adsorption of the hydrocarbon alcohols are incremented by 1.2 kcal/mol per CH2 group while the heats of adsorption of the fluorinated alcohols are incremented by 0.7 kcal/mol per CF2 group.On the preoxidized Cu(100) surface the alcohols are all deprotonated to form alkoxides.The intensity of the ν_CO mode in the HREEL spectra suggests that the C-O bond lies along the surface normal in methoxide and the propoxides but is ted toward the surface in ethoxides, butoxides, and pentoxide.During heating the alkoxides decompose by β-hydride elimination to yield aldehydes.Fluorination of the alkyl chain in the Γ-position has a dramatic influence on the reaction kinetics, resulting in an increase in the β-hydride elimination barrier from 26 kcal/mol in hydrocarbon alkoxides to 42 kcal/mol in the fluorinated alkoxides.This is consistent with a description of the transition state in which charge separation is of the form C_β^δ+<*>H^δ- and the influence of the fluoroalkyl group is to energetically destabilize the cationic β-carbon in the transition state.