THE BETA-HYDRIDE ELIMINATION MECHANISM IN ADSORBED ALKYL-GROUPS

We have examined the kinetics of the beta-hydride elimination reaction in propyl (CH3CH2CH2-Cu) and trifluoropropyl (CF3CH2CH2-Cu) groups on the Cu(111) surface. These species are formed at low temperatures (T < 120 K) by adsorption of propyl iodide (1-iodopropane, CH3CH2CH2I) and trifluoropropyl iodide (1,1,1-trifluoro-3-iodopropane, CF3CH2CH2I) and have been identified by their vibrational spectra. The adsorbed propyl groups decompose by beta-hydride elimination to give propylene (CH3CH=CH2) and trifluoropropylene (CF3CH=CH2). Fluorination of the terminal methyl group increases the barrier to beta-hydride elimination from 12.6 kcal/mol in adsorbed propyl to 21.0 kcal/mol in adsorbed trifluoropropyl. This dramatic influence of fluorine on the barrier indicates that charge separation in the transition state is of the form C(beta)delta+...H(delta-). Substitution of fluorine into the methyl group increases the reaction barrier by energetically destabilizing the cationic beta-carbon in the transition state.