REVERSIBILITY IN THE REACTION OF CYCLOHEXADIENYL RADICALS WITH OXYGENIN AQUEOUS-SOLUTION

Hydroxyl radicals were generated radiolytically and reacted with a num ber of benzene derivatives (PhH). In the presence of oxygen, the hydro xycyclo-hexadienyl radicals thus formed were converted into their corr esponding peroxyl radicals, Pulse radiolysis has shown the oxygen addi tion (forward reaction, f) to be reversible (reverse reaction, r) [Eq. (1)]. The peroxyl radicals can eliminate HO2. to yield phenols, along side some ring-fragmentation products (product-forming reaction, p). T he rate constants for the forward and reverse reactions (k(f) and k(r) ) and the corresponding stability constants K (= k(f)/k(r)) were deter mined for the hydroxy-cyclohexadienyl radicals derived from anisole, t oluene, fluorobenzene, benzene, chlorobenzene, benzyl chloride, benzoa te ion, phenylalanine,and terephthalate ion. The constants k(f) lie be tween 8x10(8) (anisole) and 1.6x10(7) dm(3) mol(-1) s(-1) (terephthala te ion), and k(r) between 7.5x10(4) (toluene) and 3.4x10(3) s(-1) (ter ephthalate ion). The stability constants lie between 2.6x10(4) (benzen e) and 3.3x10(3) dm(3) mol(-1) (phenylalanine). The rate constants for the product-forming reactions k(p) are between 5.5x10(3) (anisole) an d 3.4x10(2) s(-1) (benzoate). For the peroxyl radical derived from phe nylalanine, a bond dissociation energy of 5.5 kcal mol(-1) has been de rived. A number of hydroxy-cyclohexadienyl radicals (e.g., those deriv ed from benzoic acid, ethylbenzoate, benzonitrile, and nitrobenzene) r eact 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 5x10(6) dm(3) mol(-1) s(-1) except for nitrobenzene wher e the reaction is too slow For measurement. The rate constants k(r) ar e below 500 s(-1), and the product-forming reaction is too slow to be detected by pulse radiolysis. gamma-Radiolysis of N2O/O-2(4: 1)-satura ted aqueous solutions of benzonitrile gave dimeric compounds (e.g., di cyanobiphenyls) in low yield, alongside the three isomeric phenols; th is again proves the low reactivity of its hydroxycyclohexadienyl radic al toward oxygen.