Bromination of Hydrocarbons. VII. Bromination of Isobutane. Bond Dissociation Energies from Bromination Kinetics

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Abstract
The kinetics of the gas phase photochemical bromination of isobutane to yield 2‐bromo 2‐methylpropane was investigated in the temperature range 40—95°C and the reaction found to proceed through an atom and radical chain mechanism analogous to that observed with a number of other hydrocarbons. The thermal reaction was studied at 112 and 130°C and shown to have the same mechanism except for the mode of production of bromine atoms. Bromination was inhibited by hydrogen bromide, a product of the reaction, and also by oxygen. The dissociation energy of the tertiary C–H bond in isobutane was found to be 89.6 kcal/mole at 0°K, in agreement with the idea that tertiary bonds are considerably weaker than primary bonds. A general review of activation energies of bimolecular reactions involved in bromination is given as well as a recapitulation of values of C–H and C–C bond dissociation energies derived from bromination studies.