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Title: The addition of halomethyl radicals to fluoroethylenes
Author: Sloan, John Purdie
Awarding Body: University of St Andrews
Current Institution: University of St Andrews
Date of Award: 1974
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The thesis records the competitive addition reactions of CFBr2, CF2I, CHF2. and CH2F radicals to each of vinyl fluoride, 1,1-difluoroethylene, trifluoroethylene and tetrafluoroethylene in the presence of ethylene, and a few additional reactions in the absence of ethylene. The results are discussed in conjunction with the corresponding results for addition of C3F7, (CF3)2CF, C2F5, CF3, CF2Br, CC13 and CH3 radicals. Part 1 records the photolytic reactions of CFBr3. At short wavelengths both CFBr2 and CFBr are formed but the contribution to product formation from the carbene was insignificant in the kinetic series conducted at 366 nm. Competitive addition of CFBr2 gave, from the Arrhenius plots, the A-factor and activation energy differences and the relative rates of the addition steps. The activation energy differences showed a good correlation with atom localisation energies for C3F7, CF3, CF2Br, CC13 and CFBr2 radicals. The photolytic reactions of CHF2I and CH2FI with the fluoroethylenes are recorded in parts 2 and 3 respectively. The primary photolysis step with CHF2I gives difluoromethyl radicals and excited iodine atoms. The excited I (3P[sub]1⁄2) atoms abstract hydrogen from CHF2I giving a system containing both CHF2 and CF2I radicals. The Arrhenius plots for addition of CF2I radicals follow the same pattern as for CFBr2 with much reduced selectivity. Addition of difluoromethyl and fluoromethyl radicals, apart from the normal Arrhenius behaviour observed for competitive addition between ethylene and tetrafluoro-ethylene, show anomalous Arrhenius behaviour with negative slopes corresponding to diverging adduct ratios with increasing temperatures, and A-factors varying by up to three orders of magnitude. Several potential factors causing the anomalies are considered and excluded; dimerisation and cross combination of radical species; telomerisation of and hydrogen abstraction by the adduct radicals; elimination of HI from the adducts; and reversibility of the addition steps. The evidence favours joint participation of ground state (CHF2, CH2F) and vibrationally excited (CHF2, CH2F) radicals in non-reversible addition reactions, ground state radicals predominating at high temperature and excited state at low temperature. Analysis of the complete set of results indicates that, for fully halogenated alkyl radicals, orientation of addition to unsymmetrical fluoroethylenes is a function of steric factors: the logarithms of orientation ratios at 150°C give a linear correlation with radical radii. With partially halogenated alkyl radicals the strong polar differences between hydrogen and halogen atoms requires the introduction of a polar term, which, combined with radical radii, gives a linear correlation including both partially and fully halogenated alkyl radicals with logarithms of the orientation ratios.
Supervisor: Tedder, John Michael Sponsor: Science Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD305.H15S6 ; Organofluorine compounds