BETA-FLUORIDE ELIMINATION-REACTIONS OF FLUORINATED ALKYL-GROUPS ON THE AG(111) SURFACE

The surface chemistry of CF3CH2(ad) and CF3CF2CH2(ad) on the Ag(111) s urface was studied by temperature-programmed-reaction (TPR) measuremen ts. These species, which are generated on the surface by thermal disso ciation of the C-I bond in the corresponding fluoroalkyl iodides, unde rgo beta-fluoride elimination to give the corresponding fluoroalkenes and adsorbed fluorine. Kinetic parameters obtained from variable heati ng rate TPR studies show that while the pre-exponential factors for th e beta-fluoride elimination reaction in CF3CH2(ad) and CF3CF2CH2(ad) a re the same, i.e., 10(14.7+/-0.8) s(-1), the activation energies are c onsiderably different, i.e., 18.1+/-0.9 and 13.9+/-0.7 kcal/mol, respe ctively. Utilizing the fact that F has a greater electron-withdrawing inductive effect than CF3, we conclude that the charge separation in t he transition state is of the form C-delta+...(F delta-). The adsorbed F generated by the beta-fluoride elimination reaction desorbs as AgF at similar to 850 K. Unlike adsorbed alkyl groups, which couple to pro duce alkanes, the hydrofluoroalkyl groups studied here react by the be ta-fluoride elimination pathway, probably due to the increase in the b arrier to coupling caused by fluorination at the beta-carbon.