Carbon-halogen bond dissociation on copper surfaces: Effect of alkyl chain length

Carbon-halogen bond dissociation in a series of straight-chain alkyl halides adsorbed on single crystal copper surfaces has been studied by high-resolution electron energy loss spectroscopy, temperature-programmed desorption, and work-function measurements. For two or more carbons in the alkyl chain, the rate of carbon-halogen (C-X) bond dissociation is independent of chain length, despite the fact that the heat of molecular adsorption (assuming a first-order preexponential factor of 10¹³ s^-1 for desorption) increases at the rate of 1.3 ± 0.2 (kcal/mol)/CH₂ group. The combination of these two effects produces a sharp transition in the branching between desorption and decomposition as a function of alkyl chain length. For alkyl chlorides the surface reaction path switches from desorption to decomposition between C₆ and C₇; for the alkyl bromides the transition is between C₂ and C₃. All of the alkyl iodides decompose. The C-X bond dissociation rates decrease in the order C-I > C-Br > C-Cl, and the activation energies for C-X bond scission are approximately 15% of the gas-phase bond energies in each case. The chain length dependence for the heat of adsorption suggests that the molecularly adsorbed alkyl halides bind with their carbon chains approximately parallel to the surface plane, while surface vibrational spectra indicate that the alkyl groups formed by carbon-halogen bond scission reorient to stand upright on the surface. The independence of the C-X bond dissociation rate on alkyl chain length suggests that reorientation of the carbon chain occurs after the transition state for C-X bond scission. The results are rationalized by using a modified Lennard-Jones picture of dissociative adsorption.