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
The accepted values of the important quantities D(CH3 -H) and AH(: (CH3) are largely bascd on studies of the bromination of methane. We now report on a corresponding study of the bromination of fluoroform.
Thermal bromination of CHsHThe kinetics have been studied in d:tail over the range 360-43OOC and the mechanism is found to be similar to that for the bromination of methane. Important steps are :The overall activation energy Eobs = E2 + %El = 45.6 f 0.7 kcal so that E2 = 23.0 kcal. Now AH2 = E2 -E-2 and we estimate from the inhibiting effect of HBr in the above system and from the photolysis of CF&OCFs/HBr mixtures that E-2 = 1 f 1 kcal. Thus AH2 = 22 kcal and D(CF3-H) = 109 kcal at 2980.This suprising result has been checked as follows :
Photobromination of CF3HThis has been studied in the range 275-36OOC giving an activation energy of 24.7 f 0.5 kcal which gives D(CF3-H = 110 kcal at 2980K, and confirms the above value.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.