Effects of Alkyl Chain Structure on Carbon−Halogen Bond Dissociation and β-Hydride Elimination by Alkyl Halides on a Cu(100) Surface

Abstract

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.

Thermal reactions of 2-iodoethanol on Cu(100)

“…Furthermore, ICH 2 CH 2 OH decomposes, via C-I scission, on Ni(1 0 0) at 150 K [6]. On Cu(1 1 1) and Cu(1 0 0) surfaces, the C-I bonds of C 2 H 5 I and the iodides with longer carbon chains dissociate below 120 K [15,16].…”

Reactivity of Ethyl Groups on a Sn/Pt(111) Surface Alloy

“…Most commonly, this problem has been surmounted by using thermal, photochemical or electron induced dissociation (EID) of ethyl halides (C 2 H 5 X, X = Cl, Br, I) adsorbed at low temperatures on metal single crystals [3,4]. In this manner, studies of ethyl chemistry have been reported on copper [5][6][7][8][9], silver [10,11], nickel [12,13], platinum [14][15][16][17], and rhodium [18]. A disadvantage of this approach is the introduction of coadsorbed halogen adatoms on the surface.…”

Mechanisms of hydrocarbon conversion reactions on heterogeneous catalysts: analogies with organometallic chemistry

“…Additional work using alkyl groups with constrained structures, cyclic compounds in particular, have been employed to point to the planar nature of the transition state [44]. Selectivity towards the abstraction of hydrogens from inner carbons has also been reported [34,41]. All these characteristics parallel the behavior reported on discrete organometallic compounds.…”

“…For instance, the CH 3 symmetric stretching (around 2965-2963 cm )1 ) and deformation (1366 and 1390-1395 cm )1 ) modes in normal and neopentyl-a-d 2 become narrower and split upon scission of the CAI bond, and the CD 2 deformation in both a-d 2 and perdeuterio isotopomers shifts down (from 927-917 to 918-902 cm )1 ). Typically, most CAI or CABr bonds in halohydrocarbons on late transition metals cleave below 200 K, and display activation barriers on the order of 5 kcal mol )1 [33,34]. The preparation of surface species, alkyls in particular, via oxidative addition of appropriate halohydrocarbon precursors has allowed us to characterize the surface chemistry of hydrocarbon conversion catalysis in great detail [19,21,23].…”

“…It has long been established that the loss of hydrogen atoms in organometallic compounds occurs preferentially at the beta position, that is, at the second carbon away from the metal center [36]. Since our initial report of selective hydrogen abstraction from the methyl moiety in ethyl groups on Pt(1 1 1) surfaces [37], b-H elimination has also been widely recognized in adsorbed systems [34,[38][39][40][41]. In fact, a number of clever experiments have provided some insight on the microscopic details of this elementary step.…”