Effects of Alkyl Chain Structure on Carbon−Halogen Bond Dissociation and β-Hydride Elimination by Alkyl Halides on a Cu(100) Surface

Abstract

The effects of alkyl chain structure on the rate of carbon−halogen bond scission in alkyl chlorides, bromides, and iodides on a Cu(100) surface and on the rates of β-hydride elimination by the alkyl products of these carbon−halogen bond scission reactions have been studied under ultra-high-vacuum conditions. It is found that the carbon−halogen bond dissociation rates increase in the order: C−Cl < C−Br < C−I and C(1°)−X < C(2°)−X < C(3°)−X, where X denotes the halogen and 1°, 2°, 3° refer to the number of alkyl substituents at the halogen-substituted carbon. β-Hydride elimination by the corresponding alkyl groups shows the following trends: (1) alkyl chain length (greater than three carbons) does not significantly affect the rate of β-hydride elimination; (2) the rate increases with alkyl substitution at the α-carbon in the order primary alkyls < secondary alkyls, (3) the rate of increase is substantially larger than expected on the basis of the increase in the number of β-hydrogens, and (4) for C5 and C6 alkyls the rate of this reaction is faster for 3-alkyls than for 2-alkyls. Differences in rate of up to 3 orders of magnitude are observed as a function of alkyl chain structure, and possible correlations between thermodynamic and kinetic effects are discussed.