Browsing by Author "Whittle, E."

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    Bond dissociation energies from equilibrium studies: Part 4.—The equilibrium Br2+ CH4⇌ HBr + CH3Br. Determination of D(CH3—Br) and ΔH °f(CH3Br, g)
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1973, 69 (2), 1973) Ferguson, K. C.; Okafo, E. N.; Whittle, E.
    The equilibrium Br2+ CH4⇌ HBr + CH3Br (2) has been studied in the range 347–477°C, with equilibrium being approached from both sides. Products additional to those required by eqn (2) were formed but it is believed that reliable values of the equilibrium constant K2 have been measured. Third-law calculations lead to ΔH°2=–26.4 ± 0.7 kJ mol–1 at 298 K from which ΔH°f(CH3Br, g)=–34.3 ± 0.8 kJ mol–1.
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    Competitive study of the reactions Br + RBr → Br2+ R (R = CF3, C2F5)
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1972, 68 (4), 1972) Ferguson, K. C.; Whittle, E.
    The overall reactions CF3Br + HBr → CF3H + Br2, C2F5Br + HBr → C2F5H + Br2 have been studied competitively over the range 195–476°C. Reaction was initiated by bromine atoms generated both thermally and photochemically. Initiation reactions are Br + CF3Br → Br2+ CF3(3), Br + C2F5Br → Br2+ C2F5(4) for which log k3/k4=(0.264 ± 0.075)–(2330 ± 220)/θ where θ= 2.303 RT/cal mol–1. This confirms previous work on reactions (3) and (4) studied separately. The relevnce of the results to the determination of the bond dissociation energies D(CF3—Br) and D(C2F5—Br) is discussed.
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    Kinetics of the Reaction Between HBr and C2F5Br Determination of the Bond Dissociation Energy Z>(C2F5—Br)
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1972, 68 (2), 1972) Ferguson, K. C.; Whittle, E.
    The kinetics of the overall thermal reaction between HBr and C2F5Br, HBr + C2F5Br ⇌ Br2+ C2F5H (3, –3) were studied in the range 365–516°C. Reactions involved are Br + Br + M ⇌ Br2+ M kc, Br + C2F5Br ⇌ Br2+ C2F5(4, –4), C2F5+ HBr → C2F5H + Br. (5) The rate law expected from this mechanism was confirmed and the experimental rate constant for the bromine-atom transfer reaction (4) is given by log k4/(cm3 mol–1 s–1)=(14.30 ± 0.16)–(24070 ± 530)/θ(10) where θ= 2.303 RT/cal mol–1. These studies also yield values of k–4/k5, which when combined with previous results, lead to log(k–4/k5)=(0.65 ± 0.20)+(2360 ± 500)/θ. We propose that the value of E4 in eqn (10) should be modified to 22.8 kcal mol–1 which leads to a value of the bond dissociation energy D(C2F5—Br)= 68.6 kcal mol–1. The relation of this result to D(C2F5—H) and to ΔH°3, obtained from previous studies of equilibrium (3), is discussed. New data on equilibrium (3) are presented.
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    Photochemistry of Anhydrides: Part 2.—Photolysis of Perfluoropropionic Anhydride : A New Source of C2F5 Radicals
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1972, 68 (1), 1972) Chamberlain, G. A.; Whittle, E.
    The photolysis of the vapour of (C2F5CO)2O in the temperature range 20–213°C is described quantitatively by the overall reaction (C2F5CO)2O +hν→ CO + CO2+ n-C4F10, with the effective primary process being (C2F5CO)2O +hν→ CO + CO2+ 2C2F5. The anhydride was photolyzed in the presence of both CH4 and cyclohexane and the following reactions occur: C2F5+ CH4→ C2F5H + CH3(7), C2F5+ c-C6H12→ C2F5H + c-C6H11(8), C2F5+ C2F5→ C4F10. (4) For these reactions, log(k7/k½c)/(cm[fraction three-over-two] mol–½ s–½)=(5.01 ± 0.28)–(11490 ± 560)/θ, log(k8/k½c)/(cm[fraction three-over-two] mol–½ s–½)=(5.68 ± 0.25)–(6440 ± 380)/θ, where θ= 2.303 RT/cal mol–1 and kc is the rate constant for reaction (4). It is concluded that Arrhenius parameters for abstraction of H from a given RH by CF3, C2F5 and n-C3F7 radicals are almost identical. When (C2F5CO)2O is photolyzed with cyclohexene both H-abstraction (kH) and addition to the double bond (kadd) occur with log(kH/kadd)=(0.67 ± 0.07)–(2530 ± 120)/θ.
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    Photochemistry of Anhydrides: Part 1.—Photolysis of Perfluoroacetic Anhydride Vapour: A New Source of CF3 Radicals
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1972, 68 (1), 1972) Chamberlain, G. A.; Whittle, E.
    The photolysis of perfluoroacetic anhydride was studied in the temperature range 19–177°C using wavelengths in the range 254–265 nm. The photolysis is exceptionally simple and is quantitatively described by the reactions (CF3CO)2O +hν→ 2CF3+ CO + CO2, CF3+ CF3 [graphic omitted] C2F6, The anhydride was tested as a source of CF3 radicals by photolyzing it in the presence of CH4 and of cyclohexane. For the reactions CF3+ CH4→ CF3H + CH3(7), CF3+ c-C6H12→ CF3H + c-C6H11, (12), we obtain log(k7/k1c)/(cm3/2 mol–1/2 s–1/2)=(5.93 ± 0.32)–(12500 ± 700)/θ, (logk12/k1/2c)/(cm3/2 mol–1/2 s–1/2)=(5.18 ± 0.15)–(5560 ± 220)/θ, where θ= 2.303 RT/cal mol–1. These results agree with previous ones obtained using conventional sources of CF3 radicals. We conclude that perfluoroacetic anhydride is a convenient new source of CF3 radicals.
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    Reaction of C2F5 Radicals with HC1: Determination of the Bond Dissociation Energy D(C2F5-H)
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1972, 68 (3), 1972) Bassett, J. E.; Whittle, E.
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    Reactions of trifluoromethyl radicals with organic halides: Part 7.—Chloro- and fluorochloro-ethanes
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1972, 68 (5), 1972) Quick, L. M.; Whittle, E.
    The reactions of CF3 radicals with the compounds C2H5Cl, CH2ClCH2Cl, CH3CHCl2, CH3CCl3, CHCl2CHCl2, CF3CH2Cl, CF3CHCl2, CF3CCl3, CF2ClCFCl2 and CH3CF2Cl were studied. Arrhenius parameters were measured for abstraction of H by CF3, where the molecule contained abstractable H, and for abstraction of Cl by CF3 from most of the multi-chlorinated molecules. Reactions of some of the chloroethanes were complicated by αβ-elimination of HCl, photosensitized by the hexafluoroacetone which was used as a source of CF3 radicals. However, it is believed that reliable rate constants for abstraction were measured. The variations in activation energies for H and halogen abstraction are discussed in terms of polar effects and enthalpy changes. There are substantial variations in A factors for both H and halogen abstraction by CF3 and there is a correlation between variations in A and E both for hydrogen abstraction and halogen abstraction.