N otizen 1337

OH-Radical-Induced Oxidation of rapidly attained that it can be measured by pulse Benzene in the Presence of Oxygen: radiolysis without too much interference of subse­ R ’+ 0 2^ R 0 2‘ Equilibria in quent reactions. Aqueous Solution. A Pulse Radiolysis Study In the present study OH radicals have been ge­ nerated radiolytically (reactions (1) and (2), for de­ Xian-Ming Pan and Clemens von Sonntag* tails see ref. [19]). Max-Planck-Institut für Strahlenchemie, Stiftstraße 34-36, D-4330 Mülheim an der Ruhr, H 0 ionizing_^.QH h -5 h o n 2j H+ (1) G erm any radiation Z. Naturforsch. 45b, 1337-1340(1990); e;q + N20 OH + N2 + OH- (2) received April 5, 1990 The radiolytic yields of OH and H formed are Benzene, Hydroxyl Radical, known (G('OH) = 5.8* 10-7 mol J~] and G(H') = Hydroxycyclohexadienyl Radical, Peroxyl Radical, Pulse Radiolysis 0.58 x 10“7 mol J~'). The OH radicals react with benzene to give the hydroxycyclohexadienyl radical The reaction of hydroxycyclohexadienyl radi­ (1) (reaction (3)). In the presence of oxygen, 1, cal with oxygen was studied by pulse radiolysis. At room temperature the establishment of the which exists in two mesomeric forms, is converted equilibria between the hydroxycyclohexadienyl into the peroxyl radicals 3 and 4 (reactions (5) and radical (1), oxygen and the hydroxycyclohexa- (7)). The H atom plays only a minor role. It reacts dienperoxyl radicals 5-hydroxy-1,3-cyclohexa- both with benzene (reaction (4), k4 = 9.1 x 108 dm3 diene-6-peroxyl radical (3) and 6-hydroxy-1,4-cy- mol-1 s-1) and oxygen (reaction (10), k ]0 = clohexadiene-4-peroxyl radical (4), was observed. From the kinetic analysis of the decay of radical 1 2.1 x IO10 dm 3 m ol-1 s“1) [16]. At a typical benzene at 310 nm overall forward and reverse rate con­ concentration o f 2x 10-3 mol dm -3 the contribu­ stants o f k = 3.1 x 108 dm 3 m ol -1 s_1 and k = tion of radical 2 and hence radicals 5 and 6 is even 1.2* 10 4 s“1 were obtained. From the analysis of less (ca. 20%) than suggested by the primary H the transient spectra and the products of pulse-ir­ atom yield. In addition, cyclohexadienyl peroxyl radiated solutions it is concluded that in the equi­ radicals reform benzene in 60% yield by H 0 2'-eli- librium radical 4 is much favoured over radical 3. mination [20]. Thus, only radicals 3 and 4 (and some H 0 2702*-) have to be considered as inter­ mediates. Introduction There is a long-standing interest in the degrada­ tion of benzene by OH radicals in the presence of oxygen [1-15]. In aqueous solution the OH addi­ tion to benzene is fast and irreversible at room tem­ perature (reaction (3), k3 = 7.8 x 109 dm3 m o l'1 s“1) [16]. This also usually holds for the reaction of ox­ ygen with carbon-centered radicals [17], and it has been believed also to be the case for the hydroxy­ cyclohexadienyl radical formed in reaction (3) (reactions (5) and (7)) [9]. There is experimental evidence that the open- chain analogs of the hydroxycyclohexadienyl radi­ cal such as the pentadienyl radicals formed as intermediates in the autoxidation of polyunsatur­ ated fatty acids undergo reversible oxygen addi­ tion [18] (for a review see ref. [19]). Here we will show that this also holds for the hydroxycyclo­ hexadienyl radical and that the equilibrium is so

* Reprint requests to Prof. Dr. C. v. Sonntag. 5 6 Verlag der Zeitschrift für Naturforschung, D-7400 Tübingen 0932-0776/90/0900-1337/$ 01.00/0 H* + 0 2 ------► H 02' (10) 1338 Notizen

In this paper it will be shown that our present 25 view of the 0H/benzene/02 system has to be re­ vised: In oxygenated solutions the hydroxycyclo- 20 hexadienyl radicals 1 are in equilibrium with their corresponding peroxyl radicals 3 and 4 (reactions (5)~(8)). 15

10 Experimental

Benzene (Merck) was used as supplied. Solu­ 5 tions were made up in triply distilled or Millipore Milli-Q filtered water presaturated with N20 /0 2 0 (4:1 v:v) (Messer Griesheim) prior to adding the 260 280 300 320 340 required amount of benzene through a serum cap. ------X/nm ------► Different N20 /0 2 mixtures were prepared with the help of a Brooks gas mixer. Pulse radiolysis was Fig. 1. UV spectra of pulse-irradiated N:0-saturated carried out using a Van de Graff 2.8 MeV electron 2 //s after the pulse (...; right scale) and N 2Ö /0 2-saturat- generator delivering 0.4- 1 /.is electron pulses of ed aqueous solution of benzene (left scale): (•) 10/is, 3-30 Gy. The pulse radiolysis setup as well as the ( x ) 20 ps, ( A ) 30 jus, (□ ) 40 ps and (O) 170 /us after the pulse. data processing procedures have been described elsewhere [21, 22].

icals 2 shows a 2:1 preference of the reaction from Results and Discussion their central vs. their terminal position(s) [24], The hydroxycyclohexadienyl radical 1 is charac­ There is strong evidence that oxygen is not quite as terized by a strong absorption at 310 nm. Our selective and adds to 2 with about equal probabili­ value agrees well with that reported in the litera­ ty at the central position and the two terminal po­ ture (e(310) = 3600 dm3 mol-1 cm“1) [23]. The un­ sitions [20]. Hence it can be expected that also in substituted cyclohexadienyl radical 2 also absorbs the case of 1 reactions (5) and (7) will be about at this wavelength (£(310 nm) = 4400 dm3 mol equally fast. However, the equilibrium concentra­ cm-1) [24], In the presence of oxygen 2 rapidly tions of 3 and 4 are not only determined by the for­ decays yielding 5 and 6 (reaction (9); k9 = ward reactions (5) and (7) but also by the reverse 1.2xl09dm3 mol-1 s“1) [20]. Because of very fast reactions (6) and (8). There is strong evidence that subsequent unimolecular reactions of the peroxyl in the equilibrium 4 is much favoured over 3. radicals 5 and 6 the reverse of reaction (9) cannot Peroxyl radical 3 has a 1,3-cyclohexadienyl be monitored [20], structure while 4 has a 1,4-cyclohexadienyl struc­ When the hydroxycyclohexadienyl radical 1 ture. 1,3-Cyclohexadienes have high extinction reacts with oxygen its absorption at 310 nm rapid­ coefficients at 260 nm (e ~ 104 dm3 mol-1 cm-1), ly decays, but after this rapid decay a considerable but 1,4-cyclohexadienes do not absorb at this absorption at this wavelength persists (cf. the “pla­ wavelength. Assuming that peroxyl radical 4 has teau” at around 30-40 //s in Fig. 1). This residual an extinction coefficient of around 1000 dm3 mol-1 absorption at 310 nm which subsequently decays cm“1 at 260 nm (as many peroxyl radicals have) it only slowly has been attributed to be due to the ab­ is calculated from the data shown in Fig. 1 that sorption of the resulting peroxyl radicals 3 and 4 peroxyl radical 3 can only contribute a few per­ [9], It will be shown below that this is only partly cent. Supporting evidence that in the equilibrium correct. In fact, a considerable part of the remain­ mixture of 1, 3 and 4 peroxyl radical 3 can only be ing absorption at 310 nm is due to hydroxycyclo­ a minor component comes from a product study of hexadienyl radicals 1 in equilibrium with its corre­ pulse-irradiated solutions [20]. sponding peroxyl radicals, the reactions (5) and (7) It can be shown that for the simple case, i.e. that being reversible (reactions (6) and (8)). there is only one peroxyl radical (R' + 0 2 R 02') In cyclohexadienyl radicals the highest spin the kinetics of the decay of R' into R 02' can be de­ density is at their central position (spin density scribed by kobsd = £fonvard[0 2] + £reverse. In such a 0.51) and considerably less at their terminal posi­ case a plot of k obsd vs. [0 2] will yield a straight line tions (spin density 0.35) [25]. The dimer distribu­ whose slope equals A'forward and an intercept which tion from the combination of cyclohexadienyl rad­ shows the value of /rreverse. The present system is N otizen 1339 more complex, but since peroxyl radical 4 predom­ cm“1. When the right e(4) is chosen a plot of A' vs. inates, this formulation also holds for the present [0 2] will yield a straight line, when e(4) is chosen system. too high the curve will bend upward, and when In Fig. 2 the observed first-order rate constant chosen too low, downward. As can be seen from of the decay of the 310 nm absorption is plotted as Fig. 3 a good fit is obtained when e(4) = 500 dm 3 a function of the oxygen concentration. A straight mol“1 cm“1 is assumed. With this value K = line with a slope of k = 3.1 x 108 dm 3 mol-1 s“1 is 3.2 x 104 dm 3 mol“1 is calculated. This value agrees obtained. The pronounced intercept indicates that well with the one calculated from the data shown the reverse reactions (6) and (8) occur with an in Fig. 2. overall rate constant of k = 1.2><104s_1. From these values an equilibrium constant of K = 2.6 x 104 dm3 mol“1 is calculated.

I

------[02l / 10" mol dm-3

Fig. 3. Plots of A' vs. [02]. ( x ) e(4) = 650, (A ) e(4) = 500 and (O) e(4) = 400 dm 3 m ol -1 cm ' 1 (see text). [0 21 / 10~5 mol dm'3

Fig. 2. Observed first-order rate constant of the decay of the 310 nm absorption of pulse-irradiated N 20 / 0 2- Additional remarks. The hydroxycyclohexadien- saturated aqueous benzene solution as a function of yl radical presents us with the rather fortunate op­ oxygen concentration. portunity to study the reversibility of the oxygen addition reaction, because the equilibrium is estab­ lished at times when the subsequent reactions, bi- This equilibrium constant can be obtained by an molecular decay, HOV-elimination and intramole­ additional approach. The equilibrium constant as cular cyclization of the peroxyl radical and fixa­ defined by K = [4]eq/[l]eq[0 2j can be reformulated tion of this endoperoxyl radical by oxygen do not as K = ([l]0-[l]eq)/[l]eq[02] where the subscript yet play a decisive role (details of these reactions “eq” and “o” indicate “equilibrium'’ and “initial”, will be published elsewhere). In the unsubstituted respectively. The initial absorbance is A0 = cyclohexadienyl radical these unimolecular proc­ e(l)x[l]0 (e(l) = 3600 dm3 mol“1 cm“1, see above). esses are so fast (e.g. the H 02‘-elimination is The absorbance in the equilibrium is given by >8xl05s_1) that it was not possible to observe A = £(1) x [l]eq + «(4) x [4] = £(l)[l]eq + £(4) x such an equilibrium. In the case of the hydroxy- ([IJo - [I]«,)- From these basic equations the fol­ cyclohexadienyl radicals derived from benzoic acid lowing equation is derived the equilibrium appears to ly much more on the side of the biallylic radical than in the present sys­ 1 + K x [O,] = f (4)} * ft* = A' (a) tem and with hydroxycyclohexadienyl radicals de­ 2 e(l)x Aeq-e(4)x A0 rived from some other substituted benzenes (e.g. In this equation there are two unknowns, K and nitrobenzene and benzonitrile) a reaction with e(4). From Fig. 1 it may be estimated that e(4) may oxygen is barely observable on the pulse radiolysis range somewhere between 400 and 800 dm3 mol“1 time scale [26]. 1340 N otizen

[1] M. Daniels, G. Seholes, and J. Weiss, J. Chem. Soc. Schuchmann, H.-P. Schuchmann, C. von Sonntag, 1956, 832. and H. Zegota, in M. G. Simic, K. A. Taylor, J. F. [2] E. V. Barelko, L. I. K artasheva, P. D. Novikov, and Ward, and C. von Sonntag (eds): Oxygen Radicals M. A. Proskurnin, Proc. 1st. All-Union Conf. Ra- in Biology and Medicine, Vol. 49, p. 55, New York: diat. Chem. Moscow (1957); New York: Consult­ Plenum, Basic Life Sciences. ants Bureau (1959), p. 81. [15] M. Al-Sheikhly and M. G. Simic, in C. Rice-Evans [3] I. Loeff and G. Stein, J. Chem. Soc. 1963, 2623. and B. Halliwell (eds): Free Radicals: Methodology [4] L. I. Kartasheva and A. K. Pikaev, Russ. J. Phys. and Concepts, p. 481, Richelieu Press, London Chem. 41, 1534,(1967). (1988). [5] T. K. K. Srinivasan, I. Balakrishnan, and M. P. [16] G. V. Buxton, C. L. G reenstock, W. P. H elm an, and Reddy, J. Phys. Chem. 73, 2071 (1969). A. B. Ross, J. Phys. Chem. Ref. Data 17, 513 (1988). [6] I. Balakrishnan and M. P. Reddy, J. Phys. Chem. [17] G.E. Adams and R. L. Willson, Trans. Faraday 74,850(1970). Soc. 65,2981 (1969). [7] I. Balakrishnan and M. P. Reddy, J. Phys. Chem. [18] N. A. Porter, L. S. Lehman, B. A. Weber, and K. J. 76,1273(1972). Smith, J. Am. Chem. Soc. 103, 6447 (1981). [8 ] L. I. Kartasheva and A. K. Pikaev, High Energy [19] C. von Sonntag, The Chemical Basis of Radiation Chem. 6, 76(1972). Biology, Taylor and Francis, London (1987). [9] G. Ramanan, J. Indian Chem. Soc. 53, 957 (1976). [20] X.-M. Pan and C. von Sonntag, to be published. [10] L. Castle, J. R. Lindsay Smith, and G. V. Buxton, J. [21] S. Steenken, J. Buschek, and R. A. McClelland, J. Mol. Catal. 7, 235 (1980). Am. Chem. Soc. 108, 2808 (1986). [11] N. Narita and T. Tezuka, J. Am. Chem. Soc. 104, [22] A. J. S. C. Vieira and S. Steenken, J. Am. Chem. 7316(1982). Soc. 109, 7441 (1987). [12] G. Cohen and S. E. Ofodile, in G. Cohen and R. A. [23] L. M. Dorfman, I. A. Taub, and R. E. Bühler, J. Greenwald (eds): Oxy Radicals and Their Scavenge Chem. Phys. 36, 3051 (1962). Systems, p. 74, New York: Elsevier, Vol. I, Molecu­ [24] X.-M. Pan, E. Bastian, and C. von Sonntag, Z. Na- lar Aspects (1983). turforsch. 43b, 1201 (1988). [13] L. Latriano, A. Zaccaria, B. D. Goldstein, and [25] R. W. Fessenden and R. H. Schuler, J. Chem. Phys. G. Witz, J. Free Radicals Biol. Med. 1, 363 (1985). 39,2147(1963). [14] S. Das, O. J. Mieden, X.-M. Pan, M. Repas, M. N. [26] X.-M. Pan and C. von Sonntag, unpublished results.

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