Effects of alkyl chain structure on carbon-halogen bond dissociation and beta-hydride elimination by alkyl halides on a Cu(100) surface

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 beta-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 degrees)-X < C(2 degrees)-X < C(3 degrees)-X, where X denotes the halogen and 1 degrees, 2 degrees, 3 degrees refer to the number of alkyl substituents at the halogen-substituted carbon. beta-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 beta-hydride elimination; (2) the rate increases with alkyl substitution at the alpha-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 beta-hydrogens, and (4) for C-5 and C-6 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.