Pulse Radiolysis Studies. I. Transient Spectra and Reaction-Rate Constants in Irradiated Aqueous Solutions of Benzene

Abstract

The radiation chemistry of aqueous benzene solutions has been studied by the electron‐pulse radiolysis technique. Ultraviolet absorption spectra of some of the transient species have been recorded by synchronized flash‐absorption spectroscopy. The elementary reactions occurring have been observed by fast photoelectric recording of the transient optical density. A transient spectrum having a broad absorption with a strong maximum at 313 mμ has been observed. On the basis of both spectrographic and kinetic evidence this spectrum is assigned to the hydroxycyclohexadienyl radical, (OH)C₆H₆·. The molar extinction coefficient is estimated to be ε₃₁₃₀=3500±800 M^—1cm^—1. A number of substituted cyclohexadienyl radicals have been observed in aqueous solution as well as in pure benzene and chloro‐benzene. A second transient observed in oxygenated aqueous benzene solution shows an absorption shifted to lower wavelengths. This is attributed to the hydroxycyclohexadienyl peroxy radical, (OH)C₆H₆O₂·. Absolute rate constants have been determined at 23°C for the following reactions: $$\begin{array}{cc}\mathrm{OH}+{\mathrm{C}}_6{\mathrm{H}}_6\mathrm{=(}\mathrm{OH}){\mathrm{C}}_6{\mathrm{H}}_6·& {\text{(4.3±0.9)×10}}^9{M}^{\text{−1}}{\sec }^{\text{−1}},\\ \mathrm{OH}+{\mathrm{C}}_6{\mathrm{D}}_6\mathrm{=(}\mathrm{OH}){\mathrm{C}}_6{\mathrm{D}}_6·& {\text{(4.7±0.9)×10}}^9{M}^{\text{−1}}{\sec }^{\text{−1}},\\ (\mathrm{OH}){\mathrm{C}}_6{\mathrm{H}}_6\mathrm{·+}{\mathrm{O}}_2\mathrm{=(}\mathrm{OH}){\mathrm{C}}_6{\mathrm{H}}_6{\mathrm{O}}_2·& {\text{(5.0±0.6)×10}}^8{M}^{\text{−1}}{\sec }^{\text{−1}}.\end{array}$$ The yield of phenol in oxygenated solution was found to decrease continuously with increasing pulse intensity. At the highest intensity used, G(C₆H₅OH) = 0.19 molecules/100 ev. At the lowest pulse intensity used, G(C₆H₅OH) = 1.9 molecules/100 ev, which approaches the values found in steady irradiations. Some additional phenol is formed in slow post‐irradiation reactions. Diphenyl was identified as a product in the deaerated system by gas chromatographic analysis. Its formation is largely the result of post‐irradiation reactions, the initial yield being substantially lower than previously reported. The mechanism of the radiation chemical reaction in both deaerated and oxygenated solutions is discussed on the basis of the conclusion that the hydroxyl radical enters the ring to form the hydroxycyclohexadienyl radical.