0. I. MiCiC and M. T. Nenadovic
940
Pulse Radiolytic Investigations of Some Peroxyhydroxycyclohexadienyl Radicals 0.1. Micic" and M. T. Nenadovic Boric Kid& Institute of Nuclear Sciences. VinEa, Yugoslavia (Received September 29, 1975) Publication costs assisted by the Petroleum Research Fund
The absorption spectra and decay kinetics of the peroxy OH and H adduct of phenol have been studied using the pulse radiolysis technique. Both peroxy radicals decompose by a first-order reaction. OzC6H50H(OH) decomposes to HO2 and a product which partly forms hydroquinone after some intramolecular rearrangement. In alkaline deaerated solutions of phenol disproportionation of phenoxy1 radicals produces benzosemiquinone radical anions. Formation of this transient is suppressed by the presence of 0 2 . The reaction rate constants of C~HSOH(OH), CGH~OH(H), C6H50, C&f4(0H)2(H),and C&I4(0H);2(0H) radicals with oxygen have been determined.
Introduction The formation of hydroxycyclohexadienyl radicals by addition of hydroxyl radicals to aromatic rings or of H atoms to hydroxybenzenes has been studied in a number of cases.l Reaction rates of some hydroxycyclohexadienyl radicals with oxygen have been determined by pulse radiolys k 2 No experimental evidence is, however, available about the peroxy radicals formed in these reactions, except for the study by Dorfman et al.,3 when the absorption spectrum of the peroxy radical was obtained in oxygenated solutions of benzene. Many organic peroxy radicals decompose, directly or indirectly, to yield the perhydroxyl radical, thus creating a new experimental problem of identification and discrimination between their ~pectra.~-lO In the present work, we investigated oxygen reactions with mono- and dihydroxycyclohexadienyl radicals, formed in aqueous phenol solution. An attempt was made to obtain information about the formation of peroxy radicals and their subsequent reactions. Hydroquinone was used for comparison because of its capability to form isomeric radicals by reacting with H atoms. In this work, pulse radiolysis was applied in the study of the spectral properties and kinetic behavior of the intermediates formed. In some cases, for complementary information steady state irradiations with a y source were also performed. Experimental Section Solutions were prepared from analytical grade chemicals (BDH or Merck). Triply distilled water was used. The solution pH was adjusted with HC104, NaOH, or phosphate buffers. For neutral solutions the buffer concentration was M and the pH of the solution changed from only 6 X 7.2 to 6.8 during the pulse. Higher buffer concentration could not be used since phosphate ions catalyze decomposition of the OH adduct of phenol.ll Oxygen was removed by bubbling with argon or NzO. Different oxygen concentrations were obtained by mixing appropriate volumes of NzO and 0 2 saturated solutions from two syringes. For pulse radiolysis a Feberton 707 (Field Emission Corp.) electron accelerator was used, and the operation conditions were similar to those in previous work.12 The essential features of the pulse radiolysis set-up were a pulsed 450-W Xe lamp, all Suprasil silica glass optics, a double The Journal of Physical Chemistry, Vol. 80, No. 9, 1976
monochromator OPTON MM 12, and a RCA 1P28 photomultiplier tube coupled directly to Tektronix 454 and 564B scopes. A photomultiplier tube 1P28 with seven dynodes and a risetime of 0.3 I.LS was used in most experiments. In some cases a system with five dynodes and a 50-9 load resistor was used and the risetime was 10 ns. Stability of the lamp was controlled during observation of the slow processes. The total light path through the cell was 5.1 cm. The absorbed doses were in the range 0.5-7 krad/pulse. The absorbed dose was measured by using a potassium ferrocyanide dosimeter,13 taking t 4 2 0 ~ ~ ( F e ( C N ) 6as ~ - )1000 M-l cm-l and G(Fe(CN)63-) = 6.0. The temperature was 19 f 1 OC. Steady state irradiations were performed with a 6oCo y source. The total absorbed doses were in the range of 30180 krads and the dose rate was 1.6 krads min-l. The change in phenol concentration was measured using a simplified 4-aminoantipyrene method.14 Hydroquinone was determined directly by measuring absorbances of irradiated solutions a t 305 nm (€305nm 680 M-l cm-l). The total amount of hydroquinone and catechol was determined using o-phenantroline as a reagent.15 Results a n d Discussion
Spectrum and Decay of O z C a b O H ( 0 H ) Radicals. An aqueous phenol solution is expected to undergo the following reactions after the pulse in the presence of N20: HzO
----f
eaq-, H, OH,H,O,, H b H30+,OH-
(1)
About 90% of the total primary radicals should react with phenol to produce CsHbOH(0H) radicals. Reaction 3 is very fast, k 3 = 1.4 X 1Olo M-l s-l.I1 The nature and ab-
Peroxyhydroxycyclohexadienyl Radicals
941
0100
OdOO
0.050
u w
z 4
m a v) 0
m
0.075
4
0.0 50
0.025
280
320
360
h(nrn)
280
360
440
Figure 1. Absorption spectra of transients formed in phenol or hydroquinone aqueous solution, dose 3.5 krads. Each point is an average of three to six determinations. (a)5 X loe4 M phenol solution saturated with O2 and N20 (1:4):(0)pH 6.8(6 X M phosphate buffer); (0)pH 3.9 ( “ 3 0 4 solution). C6H&H(OH) radicals produced immediately (
