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Hayley Schiebel
University of North Carolina at Asheville
Subject Listing - Chemistry
Advisor: Dr. Bert Holmes
Friday, Oral Session 4, Presentation 1, Karpen Hall 038
UNIMOLECULAR RATE CONSTANTS FOR THE 1,2- AND 1,1-HF ELIMINATION FROM CHEMICALLY ACTIVATED 1,1,1,2,2-PENTAFLUOROETHANE
Hydrofluorocarbons (HFCs), the class to which CF3CF2H belongs, have been replacing chlorofluorocarbon compounds (CFCs) that have been implicated in the accelerated depletion of ozone in the earth's stratosphere. When chlorine is regenerated in CFCs, it is free to continue to break down other ozone molecules. This process continues for the atmospheric lifetime of the chlorine atom (one to two years), during which it destroys an average of 100,000 ozone molecules. Chlorine radicals are removed from the stratosphere after forming two compounds that are relatively resistant to dissociation by ultraviolet light: hydrogen chloride (HCl) and chlorine nitrate (ClONO2). Dissociation is slow enough so that these compounds can diffuse down to the troposphere, where they react with water vapor and are removed in rain. Because HFCs do not contain chlorine, they are better suitable for the atmosphere. CFCs were developed in the early 1930s and are used in a variety of industrial, commercial, and household applications. CFCs are non-toxic, non-flammable, and non-reactive with other chemical compounds. They are highly effective refrigerants that were developed in response to the pressing need to eliminate toxic and flammable substances, such as sulfur dioxide and ammonia, in refrigeration units and air conditioners. The most common commercial CFCs, marketed under the trade name Freon, are trichlorofluoromethane (CFC-11) and dichlorodifluoromethane (CFC-12). This work will report the reactions of a chemically activated HFC, CF3CF2H, which was prepared with approximately 95 kcal/mol of internal energy by the combination of CF3 and CHF2 radicals. The total rate constant was measured as 1.3 x 105 s-1 using CF2HCF2H as an internal standard and the rate constant for the 1,2-elimination of HF was measured from the pressure dependence of the [CF2=CF2/[CF3CF2H] ratio as 0.2 x 105 s-1. If the only decomposition channels are the 1,2- and 1,1-HF elimination, then the rate constant for the 1,1-channel is 1.1 x 105 s-1, a factor of 5 faster than the more common 1,2-HF elimination pathway. It also appears that the carbene formed by 1,1-HF elimination lacks sufficient energy to cause a 1,2-F migration to form CF2=CF2.
Advisor: Dr. Bert Holmes, Carson Distinguished Professor in the Sciences, Department of Chemistry, University of North Carolina at Asheville, Asheville, NC