The chemistry of alkyl iodides on copper surfaces. 2. Influence of surfacestructure on reactivity

The thermal chemistry of iodomethane, iodoethane, 1-iodopropane, 1-iodobuta ne, and 2-iodohexane on copper (100), (110), and (111) single-crystal surfa ces was characterized in this and previous studies by temperature-programme d desorption (TPD) spectroscopy. The main decomposition pathway available t o the methyl surface moiety that results from C-I bond activation in adsorb ed iodomethane is a-hydride elimination to methylene, a step that occurs ar ound 460-470 K on all three surfaces. Some methylene dimerization to ethane is also seen at higher coverages, at a rate that depends significantly on surface structure; ethane desorption peaks at 400 K on Cu(110), but only ab ove 440 K on Cu(100) and Cu(111). Ethyl groups produced by iodoethane decom position react at much lower temperatures and mostly undergo beta-hydride e limination to ethylene. The ethyl dehydrogenation reaction is structure sen sitive as well, a fact illustrated by the different ethylene desorption pea k maxima observed in the TPD experiments, at 225, 247, and 255 K on Cu(110) , Cu(111), and Cu(100), respectively (at saturation). Perhaps the more tell ing observations are the difference in feasibility of H-D scrambling in the ethylene resulting from conversion of a 1:1 mixture of normal and perdeute ro iodoethane, a reaction viable on Cu(100) but not on Cu(110), and the 10- fold difference in ethane yield between those two crystals. Additional stud ies with 1-iodopropane and 1-iodobutane provided some information on the ef fect of chain length on reactivity, and experiments with 2-iodohexane attes ted to the high selectivity for removal of internal hydrogen atoms during b eta-hydride elimination from alkyl species.