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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.

Even though dehydrocyclization is widely practiced in heterogeneous catalysis for the conversion of straight chain hydrocarbons into aromatic compounds, knowledge of the mechanism of this process remains limited, largely because it has not previously been possible to carry out the reaction under conditions amenable to detailed mechanistic studies. We report here ultrahigh vacuum studies of the dehydrocyclization of submonolayer coverages of 1-hexene to benzene on a Cu3Pt(111) single-crystal surface. On the basis of temperature-programmed reaction/desorption (TPR/D) studies of dehydrocyclization of 1-hexene as compared to the reactions of cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, benzene, 1,3-hexadiene, and 1,3,5-hexatriene with a Cu3Pt(111) surface, it is found that a rate-determining step in the overall reaction is cyclization. The obtained results show that at low coverages of mono- and bi-unsaturated cyclic compounds, benzene is the only gas-phase hydrocarbon product of reaction of these compounds with a Cu3Pt(111) surface, while after a certain threshold in coverage, molecular desorption of these compounds commences. The temperature of benzene evolution for all the cyclic compounds studied is between 200 and 300 K, whereas for linear chain hydrocarbons this temperature is ∼400 K. TPR/D studies of product hydrogen evolution show that all cyclic compounds evolve hydrogen upon reaction with the surface at the temperatures close to that of hydrogen recombination−desorption reaction, 220−290 K. On the other hand, 1-hexene evolves hydrogen upon reaction with this surface at two different temperatures:  ∼270 and ∼405 K. Combining these results with the studies of hydrogen evolution from 1,3,5-hexatriene, which occurs at 400 K, we suggest that cyclization is a rate-determining step and that mono- and bi-unsaturated C6 cyclic compounds are not the reaction intermediates for the dehydrocyclization of 1-hexene to benzene.

The addition reaction of gas-phase D atoms to olefin monolayers adsorbed on a Cu(100) surface is studied, with a focus on the regioselectivity of deuterium addition onto monolayer 1-butene and 1-pentene molecules. Both 1- and 2-alkyl groups are generated from D atom additions to 2° and 1° carbons, respectively. The alkyl groups are separated based on a difference in their β-hydride elimination kinetics, with the rate of 2-alkyl groups losing β-hydrogens about 2 orders of magnitude faster than 1-alkyl groups on the copper surface. The results suggest that D addition to terminal (1°) carbon is favored by a factor of ∼3 for 1-butene and ∼4 for 1-pentene molecules adsorbed on the surface.