Reversibility in the Reaction of Cyclohexadienyl Radicals with Oxygen in Aqueous Solution

Abstract

Hydroxyl radicals were generated radiolytically and reacted with a number of benzene derivatives (PhH). In the presence of oxygen, the hydroxycyclohexadienyl radicals thus formed were converted into their corresponding peroxyl radicals. Pulse radiolysis has shown the oxygen addition (forward reaction, f) to be reversible (reverse reaction, r) [Eq. (1)]. The peroxyl radicals can eliminate HO to yield phenols, alongside some ring‐fragmentation products (product‐forming reaction, p). The rate constants for the forward and reverse reactions (k_f and k_r) and the corresponding stability constants K (= k_f/k_r) were determined for the hydroxy‐cyclohexadienyl radicals derived from anisole, toluene, fluorobenzene, benzene, chlorobenzene, benzyl chloride, benzoate ion, phenylalanine, and terephthalate ion. The constants k_f lie between 8 × 10⁸ (anisole) and 1.6 × 10⁷ dm³ mol⁻¹ s⁻¹ (terephthalate ion), and k_r between 7.5 × 10⁴ (toluene) and 3.4 × 10³ s⁻¹ (terephthalate ion). The stability constants lie between 2.6 × 10⁴ (benzene) and 3.3 × 10³ dm³ mol⁻¹ (phenylalanine). The rate constants for the product‐forming reactions k_p are between 5.5 × 10³ (anisole) and 3.4 × 10² s⁻¹ (benzoate). For the peroxyl radical derived from phenylalanine, a bond dissociation energy of 5.5 kcal mol⁻¹ has been derived. A number of hydroxy‐cyclohexadienyl radicals (e.g., those derived from benzoic acid, ethylbenzoate, benzonitrile, and nitrobenzene) react too slowly to allow the equilibrium constant to be determined by means of pulse radiolysis. These reactions have rate constants k_f in the order of 5 × 10⁶ dm³ mol⁻¹ s⁻¹, except for nitrobenzene where the reaction is too slow for measurement. The rate constants k_r are below 500 s⁻¹, and the product‐forming reaction is too slow to be detected by pulse radiolysis. γ‐Radiolysis of N₂O/O₂(4:1)‐saturated aqueous solutions of benzonitrile gave dimeric compounds (e.g., dicyanobiphenyls) in low yield, alongside the three isomeric phenols; this again proves the low reactivity of its hydroxycyclohexadienyl radical toward oxygen.