Bromination of Hydrocarbons. VI. Photochemical and Thermal Bromination of Toluene. Bond Dissociation Energies

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Abstract
With the aid of a high temperature, high vacuum, corrosion‐resistant valve and photometers operating with high sensitivity at low light intensities, it has been possible to study the homogeneous gas‐phase kinetics of the photochemical and thermal bromination of toluene. Infrared analyses indicated that the reaction is predominantly a side‐chain substitution, the products being benzyl bromide and hydrogen bromide. The photochemical reaction, studied in the temperature range 82° to 132°C, was strongly inhibited by hydrogen bromide. The thermal reaction was studied in the absence of added hydrogen bromide at 166°C. Except for the step involving the dissociation of bromine molecules, both the photochemical and thermal mechanisms involve the same type of atom and radical chain, as found previously for the bromination of simple alkanes. The corrected activation energy of the over‐all reaction, as determined from the temperature coefficient of the photochemical reaction, was found to be 7.2 kcal/mole. This value was assigned to the rate determining step Br+RH=R+HBr. From the temperature dependence of the hydrogen bromide inhibition, the activation energy for the reverse of this step was assigned the value 5.0 kcal/mole permitting the C–H bond dissociation energy in the alkyl side chain of toluene to be estimated as 89.5 kcal/mole at room temperature. The discrepancy between this value and Szwarc's value of 77.5 kcal/mole from pyrolysis studies has been pointed out. The efficiency of toluene as a third body in the homogeneous recombination of bromine atoms, and also problems relating to steric effects in atom and radical reactions have been discussed.