.beta.-Fluoride Elimination Reactions of Fluorinated Alkyl Groups on the Ag(111) Surface

Abstract

The surface chemistry of CF 3 CH 2 (ad) and CF 3 CF 2 CH 2 (ad) on the Ag(111) surface was studied by temperature-programmed-reaction (TPR) measurements. These species, which are generated on the surface by thermal dissociation of the C-I bond in the corresponding fluoroalkyl iodides, undergo β-fluoride elimination to give the corresponding fluoroalkenes and adsorbed fluorine. Kinetic parameters obtained from variable heating rate TPR studies show that while the pre-exponential factors for the β-fluoride elimination reaction in CF 3 CH 2 (ad) and CF 3 CF 2 CH 2 (ad) are the same, i.e., 10 14.7±0.8 s -1 , the activation energies are considerably different, i.e., 18.1 ± 0.9 and 13.9 ± 0.7 kcal/mol, respectively. Utilizing the fact that F has a greater electron-withdrawing inductive effect than CF 3 , we conclude that the charge separation in the transition state is of the form C δ+ ...F δ- . The adsorbed F generated by the β-fluoride elimination reaction desorbs as AgF at ∼850 K. Unlike adsorbed alkyl groups, which couple to produce alkanes, the hydrofluoroalkyl groups studied here react by the β-fluoride elimination pathway, probably due to the increase in the barrier to coupling caused by fluorination at the β-carbon.