Kinetics of Gas Phase Halogenation Reactions

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Abstract
Making use of the independently measured rate constants for the reactions X+X+M⇌X2+M (where X=Br, I, etc.) it is possible to calculate the times needed to approach the stationary state for X atoms. On applying these results to the kinetic studies which have been made of halogenation reactions, it is found that for most thermal brominations, the times are in excess of 100 seconds. For thermal chlorinations, the times are in excess of 1000 seconds while for more strongly bonded diatomic molecules they are still higher. In photochemical systems the times are of the order of 0.1 to 1.0 second for absorbed intensities of 1012 quanta/cc sec which are quite usual. The implications of these values for many of the systems studied, are that heterogeneous initiation and termination of chains are contributing significantly to the kinetics of thermal halogenations at temperatures below 500°C. An examination of the brominations of isobutane, toluene, and neopentane shows that the mechanisms proposed in each case may be suspect and that heterogeneities are of likely importance and may account for the anomalous rate constants reported for these systems. Methods are employed for estimating to ±2 cal/mole °K the entropies of free radicals and molecules. The entropy of t‐butyl bromide is estimated to be 86.6 eu at 400°K while neopentyl bromide is 105.3 eu at 470°K. These entropy estimates make possible an examination of the self‐consistency of the rate data with the proposed rate laws. It is shown that for the systems mentioned above, the data lead to the conclusion that other termination processes must be important, in contradiction to the empirically observed rate laws. Finally, arguments are presented to show that bimolecular Arrhenius frequency factors should be less than collision frequencies.