Vapour-phase bromination of fluoroform and methane

Abstract

The thermal bromination of fluoroform has been studied in the range 361-431°C and the photobromination in the range 275360°C. The Arrhenius parameters for the reaction 9 Br+CF3H+HBr+CF3, 1 1 have the values : E9 = 23.5 f l . 0 kcal mole-1 and A9 = 2.9 x 1013 cm3 mole-1 sec-1. These results have been confirmed by competitive and comparison experiments on the photobromination of methane. The photolysis of hexafluoroacetone in the presence of H B r gives El 1 < 2.4 kcal mole-1 and from this and other evidence it is concluded that El1 = 1 f l kcal mole-1. For the equilibrium Br2+ CF3H +HBr+ CF3Br (1) AH1 = -4-2i-0.6 kcal mole-1 and AS1 = -0.77f0.75 cal/mole deg. at 477°C. The above data lead to D(CF3-HJ = 109-5&1.5, AHf(CF3) = -105f2and AHf(CF3Br) = -150f1, kcalmole-1 at 25°C. It is suggested that D (CH3-H) = 103 kcal mole-1 at 25°C. The accepted values of the bond dissociation energy in methane D(CH3-H) and the heat of formation of the methyl radical are largely based on studies of the bromination of methane by Kistiakowsky and Van Artsdalen.1 Their work has been reinterpreted by Fettis and Trotman-Dickenson 2 who obtain D(CH3-H) = 103.9kcal mole-1. We have made an analogous study of the bromination of fluoroform in order to find D(CF3-'€3) and AHf(CF3). The relative rates of bromination of fluoroform and methane have also been investigated. EXPERIMENTAL MATERIALS A.R. bromine was left in contact with A.R. KBr for a long period to remove chlorine. HBr was made from bromine and tetralin. CF3H and CF3Br were given by the National Smelting Corporation and no impurities could be detected by infra-red or gas chromatographic analysis. All these compounds were subjected to bulb-to-bulb distillations on a vacuum line and a middle cut of each was retained. Methane was made by treating methyl magnesium iodide with ethanol under vacuum. The product was passed through a liquid-air trap into a trap at -206°C and a middle cut from the latter was retained. A P P A R A T U S A N D P R O C E D U R ETHERMAL ExPERIMEmS.-The Pyrex glass reaction vessel was about 17 cm long, 4 cm d i m . and had a volume of 190.1 ml. The oven containing the reaction vessel was thermostatted to fl"C and the temperature variation along the reaction vessel was less than 2°C. Temperatures were measured with a calibrated chromel/alumel thermocouple. The vessel was connected through a Springham greaseless tap to a calibrated volume plus glass spiral gauge then through a second greaseless tap to a high-vacuum system.Mixtures of bromine and fluoroform were left in the reaction vessel for suitable times and then analysed as follows. The pressure of the mixture in a known volume was measured 541609

Simultaneous Adjustment of Experimentally Based Enthalpies of Formation of CF3X, X = nil, H, Cl, Br, I, CF3, CN, and a Probe of G3 Theory

“…The 20 original links used by Syverud 16 are listed in Table . These reflect a substantial body of various calorimetric, equilibrium, and kinetic measurements. − The individual links were already very thoroughly analyzed and discussed by Syverud…”

Activation energy requirements in hydrogen abstractions. Quantitative description of the causes in terms of bond energies and infrared frequencies

“…Another notable example is provided by a comparison of the reactions of methyl radicals with ethane and chloroform. The molecular parameters for reactant and product C-H bonds are quite similar in the two reactions (Table II); however, Ea = 11 for abstraction from ethane and only about 6.5 for abstraction from chloroform, evidently a demonstration of the "polar effect" creating a difference of 104 on the rate at room temperature. The most substantial difference in the molecular data for the two reactions is rXY = 960 and 1075 cm-1 for CH3-CHí-CHs and for CC13-CH3, respectively; the higher frequency is partially due to the presence of a dipole in the C-C bond of the latter molecule.…”

Conversion of CHF3 to CH2CF2 via reaction with CH4 in the presence of CBrF3: An experimental and kinetic modelling study

“…Br can then react with CHF 3 [30][31][32] and CH 4 [32,33], which subsequently initiates a series of chain reactions. No notable enhancement or inhibition of the conversion level of CHF 3 was observed following CBrF 3 (Fig.…”