Coupling and disproportionation reactions of alkyl iodides on a single-crystal copper surface: Alkyl radicals versus metal alkyls

Previous studies of straight-chain alkyl iodides (C₂-C₅) adsorbed on a Cu(111) surface at submonolayer coverages have shown that these molecules dissociate at ∼ 120 K to generate adsorbed alkyl groups and iodine atoms. The surface-bound alkyls (with an alkyl-metal bond energy of ∼30 kcal/mol) undergo β-hydride elimination above ∼200 K. Here, the thermal chemistry of these alkyl iodides at high surface coverage (up to saturation of the monolayer) is investigated by using temperature-programmed react ion/desorption to identify the surface reaction products and reaction kinetics. The intriguing aspect, which is different from the reaction at low coverage, is the formation of alkyl coupling and disproportionation products at the temperature where the C-I bond dissociates. The relative yields of these products suggest a radical mechanism. The parallel reaction channels for forming alkyl radicals and adsorbed alkyls during C-I bond dissociation on the Cu(111) surface are discussed in the context of analogous reactions of alkyl halides with metal surfaces in solution.