The addition of halomethyl radicals to fluoroethylenes

Abstract

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]½) 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.