Carbon-halogen bond dissociation at copper surfaces: Alkyl radicals versus metal alkyls

Thesis/Dissertation · Fri Jan 01 04:00:00 EST 1993

OSTI ID:7283831

Lin, Jong-Liang

In this thesis, ultra-high vacuum techniques, electron spectroscopies, and mass spectrometry are applied to study the formation of alkyl radicals and surface-bound alkyl groups during the dissociation of carbon-halogen bonds on atomically-clean, single crystal copper surfaces. Straight-chain alkyl halides of one to five carbons in length (C[sub 1]-C[sub 5]) have been investigated on Cu(111) and Cu(100) surfaces. The initiating step for alkyl halide reactions on copper surfaces is carbon-halogen bond dissociation as a result of the relatively weak carbon-halogen bond as compared with the carbon-hydrogen bond. The alkyl fragments derived from carbon-halogen bond dissociation show three different reaction pathways: ejection from the surface, adsorption onto the surface, and diproportionation/coupling reactions. For CH[sub 3]I, methyl ejection and adsorption occur at 140 K during C-I bond dissociation, while for longer chain alkyl iodides, adsorption, disproportionlation and coupling occur at high coverages. For the alkyl bromides and chlorides, the carbon chain length has a profound effect on the carbon-halogen bond dissociation yield through its effect on the adsorption energy and geometry. These effects are rationlized via atom and electron transfer mechanisms for carbon-halogen bond dissociation, and analogies are drawn to the oxidative addition of alkyl halides to organometallic compounds in solution. The chemistry of the adsorbed alkyl groups formed by carbon-halogen bond dissociation has also been investigated. The reaction pathways depend on the carbon chain length. Adsorbed methyl groups are stable on copper surfaces up to 400 K; above this temperature C-H bonds dissociate and C-C bonds are formed to produce methane, ethane, and propylene. Some adsorbed methyls also desorb from the surface into the gas phase. Longer chain adsorbed alkyls undergo [beta]-hydride elimination above 200 K to produce the corresponding alkene, alkane, and hydrogen.

• Research Organization: Columbia Univ., New York, NY (United States)
• OSTI ID: 7283831
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400200* -- Inorganic
Organic
& Physical Chemistry
ALKYL RADICALS
CHEMICAL BONDS
CHEMICAL REACTION KINETICS
COPPER
DISSOCIATION
ELEMENTS
HALOGENS
KINETICS
METALS
NONMETALS
RADICALS
REACTION KINETICS
SORPTIVE PROPERTIES
SURFACE PROPERTIES
TRANSITION ELEMENTS