172 J. CHEM. RESEARCH (S), 1997 J. Chem. Research (S), 1997, 172–173† Kinetics of the Reactions of O.µ and HO. with a,a,a-Trifluorotoluene and 4-Fluorotoluene† Laura S. Villata, Janina A. Rosso, M�onica C. Gonzalez* and Daniel O. M�artire*‡ Instituto de Investigaciones Fisicoqu�ýmicas Te�oricas y Aplicadas (INIFTA), Casilla de Correo 16, sucursal 4, (1900) La Plata, Argentina The absolute rate constants for the reactions of O.µ and HO. with a,a,a-trifluorotoluene and 4-fluorotoluene were obtained by flash photolysis.The photolysis of strong alkaline solutions (pHa12.7) of hydrogen peroxide (pKa=11.61) yields HO./O.µ radicals (pKa211.9) [eqns. (1) and (µ1) in Table 1]. In the presence of molecular oxygen, O.µ radicals reversibly react with O2 yielding ozonide radical ions, O3 .µ [eqns. (2) and (3)] .2–4 Ozonide itself is thought to be rather unreactive towards non-radical substrates; however its decay rate is extremely sensitive to small quantities of HO./O.µ scavengers.5–7 In previous investigations4 it has been shown that in solutions containing H2O2 concentrations of the order of 5Å10µ5 mol dmµ3 or larger and added substrates (S), competition between H2O2, S and O2 for O.µ/HO.radicals rules the formation efficiency and decay rate of O3 .µ. A detailed kinetic study on the O3 .µ concentration profiles following flash photolysis of alkaline hydrogen peroxide solutions in the presence of scavengers yields kinetic information on the reactions between O.µ/HO. and the substrates [eqns.(6) and (7)].4 Based on this methodology, we report here a kinetic study of the reactions of O.µ/HO. radicals with a,a,a-tri- fluorotoluene (TFT) and 4-fluorotoluene (4-FT). Experimental Hydrogen peroxide (Riedel-de Ha�en), TFT and 4-FT (Fluka) were used as received. The flash-photolysis set-up and experimental procedures are described elsewhere.4 All the experiments were carried out at 25�1 °C. In order to study the effect of substrates on the O3 .µ decay kinetics, a series of experiments with 1.16Å10µ4 mol dmµ3 hydrogen peroxide and systematic variations in the amounts of substrate (0–4Å10µ3 mol dmµ3) and pH were performed.Results and Discussion The time-resolved absorption curves obtained in the range 350–700 nm show the presence of a single intermediate identified as O3 .µ by its characteristic absorption spectrum.4 The traces can be well described by a first-order law over more than three lifetimes under all the experimental conditions (Fig. 1). The observed rate constant, kapp, linearly depends on the analytical concentration of added substrate at constant pH as shown in Fig. 2 (inset). Previous investigations on the photolysis of alkaline solutions of hydrogen peroxide, have shown that reactions (1)–(5) in Table 1 are the main reactions leading to ozonide formation and decay.4 If organic substrates such as 4-FT and TFT are added, they will also contribute to the depletion of O.µ/HO.according to reactions (6) and (7). Any possible reaction of O.µ/HO. and O3 .µ with the radical products, S., formed from eqns. (6) and (7) should also be considered. The total amount of S. formed can be estimated from the ratio of the areas of the ozonide decay profiles in the presence and absence of scavenger, respectively. For experiments with the highest concentrations of scavenger, [S.]total ss8Å10µ7 mol dmµ3. Consequently, S. cannot compete for O.µ/HO.radicals with the efficient scavenging by much larger concentrations of H2O2 and substrates, and eqn. (8) is negligible under our experimental conditions. However, the contribution of the reaction of S. with O3 .µ [eqn. (9)] cannot be evaluated a priori without a complete kinetic analysis. In a simplified analysis, reaction (9) will not be considered. O.µ/HO.+S.hProducts (8) O3 .µ+S.hProducts (9) The reaction mechanism shown in Table 1 can be analytically resolved assuming steady-state conditions for the extremely reactive O.µ and HO.radicals. From this kinetic analysis, a first-order law is expected for the decay of ozonide radical ions.4 The first-order apparent rate constant kapp is given by eqn. (10). A more detailed discussion on the reaction mechanism and kinetic analysis can be found in the literature.4 kapp= k3{kp[HOµ2 ]+ks[S]} k2[O2]+kp[HOµ2 ]+ks[S] (10) *To receive any correspondence. †This is a Short Paper as defined in the Instructions for Authors, Section 5.0 [see J.Chem. Research (S), 1997, Issue 1]; there is therefore no corresponding material in J. Chem. Research (M). ‡E-mail: dmartire@volta.ing.unlp.edu.ar. Table 1 Manifold of important reactions leading to the formation and decay of ozonide radical ions in the presence of efficient O.µ/HO. radical scavengersa Reaction k/dm3 molµ1 sµ1 HO.+HOµhO.µ O.µ+H2OhHO.+HOµ O.µ+O2hO3 .µ O3 .µhO.µ+O2 HO.+HOµ2 hO2 .µ+H2O O.µ+HOµ2 hO2 .µ+HOµ O.µ+ShProducts OH.+ShProducts 1.3Å1010 1.8Å106 3.6Å109 3.6–6.0Å103 sµ1 b 9.0Å109 5.0Å108 (1) (µ1) (2) (3) (4) (5) (6) (7) aTaken from refs. 5, 9 and 11. bThe value 4.0Å103 sµ1 was used at 25 °C, in line with the reported value for the activation energy of this reaction (46 kJ molµ1).12 Fig. 1 Logarithmic plots for the decay of [O3 .µ] obtained for airsaturated alkaline 1.16Å10µ4 mol dmµ3 H2O2 solutions (pH=13.9) containing (a) no added scavengers, (b) 3.87Å10µ5 mol dmµ3 4-FT and (c) 1.61Å10µ3 mol dmµ3 TFT: solid lines represent computer simulations of the decays (see text)J.CHEM. RESEARCH (S), 1997 173 with ks=k6+k7Kµ1[H+] kp=k5+k4[H+] /K K=[O.µ][H+]/[HO.] =1.26Å10µ12 (11) If k2[O2]+kp[HO2 µ]aaks[S], then the linear dependence of kapp on [S] is expected as observed experimentally. The ratio of the slope (b) to the intercept (a) of the plots of kapp vs. [S] (Fig. 2), times [HOµ2 ]Åkp yields ks at a given pH. According to eqn. (11), ks should linearly depend on [H+] as experimentally observed (Fig. 2). A least-squares analysis of the plots shown in Fig. 2 allows determination of k6 and k7 for each substrate from the intercept and slope, respectively. The values k6=(4�2)Å108 dmµ3 molµ1 sµ1, k7=(6�1)Å109 dm3 molµ1 sµ1 and k6=(2�1)Å107 dm3 molµ1 sµ1, k7=(1.1�0.2)Å109 dm3 molµ1 sµ1 are retrieved for 4-FT and TFT, respectively. The errors in the rate constants were estimated from the dispersions observed for the intercepts and the slopes in Fig. 2. In order to support the simplified kinetic analysis, other literature reactions involving the species present in the irradiated system,4 and not shown in Table 1, were considered in a kinetic simulation program together with reactions (6) and (7). An excellent agreement between experimental and simulated data was obtained for the observed kinetics of O3 .µ, also shown in Fig. 1, using the values of k6 and k7 retrieved from the simplified analysis. Thus, the contribution of reactions other than those shown in Table 1 is negligible under our experimental conditions.Reaction (9) between O3 .µ and the S. radicals was not considered in a first approximation, though its participation in the overall mechanism is expected to accelerate the decay kinetics of ozonide radical ions. Consequently, the retrieved values of k6 and k7 are upper limits for these rate constants. The contribution of this reaction to the concentration pro- files of O3 .µ in the presence of added S was evaluated with the aid of the computer simulation program.The simulations show that taking k9=2Å1010 dm3 molµ1 sµ1, and in the extreme conditions where no other decay reaction for S. is considered, the values of k6 and k7 that best fozonide profiles are 50% lower than those estimated without considering eqn. (9). There is almost no contribution from this reaction to the ozonide decay if k9R5Å108 dm3 molµ1 sµ1. The reactions of substituted benzenes with O.µ/HO.are known to yield reactive hydroxycyclohexadienyl and/or benzyl radicals.8 Consequently, S. may undergo other reactions in competition with reaction (9), and the effect of the latter reaction on the estimated values of k6 and k7 will be much lower than 50%. Thus, any possible effect of reaction (9) is already included within the reported error bars for k6 and k7, even in the case that eqn. (9) is diffusion-controlled. The rate constant obtained for the reaction of HO.and TFT is lower than the reported ones for toluene [(3–7)Å109 dm3 molµ1 sµ1] and benzene (5.0Å109 dm3 molµ1 sµ1), and similar to that for nitrobenzene (2Å109 dm3 molµ1 sµ1).9 This trend is expected from a radical addition of HO. to the aromatic ring, considering the electron-withdrawing ability of the CF3 group. On the other hand, the rate constant for the reaction of 4FT and HO. is of the same order as the reported values for other toluenes with para electron-withdrawing substituents (i.e. 7Å109, 2.9Å109 and 5.5Å109 dm3 molµ1 sµ1 for p-nitrotoluene, p-bromotoluene and p-chlorotoluene, respectively).9,10 The rate constant obtained for the reaction of O.µ and TFT is two orders of magnitude smaller than that observed for toluene (2.1Å109 dm3 molµ1 sµ1), but of the same order of magnitude than the reported ones for benzene (7.5Å107 dm3 molµ1 sµ1), benzonitrile (7Å107 dm3 molµ1 sµ1) and nitrobenzene (5Å107 dm3 molµ1 sµ1). The higher reactivity of O.µ with toluene indicates the preferential reaction of this radical by H abstraction from aliphatic side chains of the aromatic molecules, rather than by addition to the aromatic ring.9 On the other hand, the rate constant obtained for the reaction of O.µ and 4-FT is of the same order as the reproted values for other toluenes containing electron-withdrawing substituents, such as nitrotoluene (7.6Å108 dm3 molµ1 sµ1).9 M.C. G. and D. O. M. are research members of CONICET and CICPBA, respectively. This research was partially supported by grants number A-13218/1-000062 and A-13359/1-000084 of Fundaci�on Antorchas (Argentina) Received, 6th January 1997; Accepted, 4th February 1997 Paper E/7/00126F References 1 K.Chelkowska, D. Grasso, I. F�abi�an and G. Gordon, Ozone Sci. Eng., 1992, 14, 33. 2 J. Hunt and H. Taube, J. Am. Chem. Soc., 1952, 74, 5999. 3 J. H. Baxendale and J. A. Wilson, Trans. Faraday Soc., 1957, 53, 344. 4 M. C. Gonzalez and D.O. M�artire, Int. J. Chem. Kin., in press. 5 K. Sehested, J. Holcman, E. Bjerbakke and E. J. Hart, J. Phys. Chem., 1982, 86, 2066. 6 B. L. Gall and L. M. Dorfman, J. Am. Chem. Soc., 1969, 91, 2199. 7 G. Czapski, Annu. Rev. Phys. Chem., 1971, 22, 171. 8 (a) C. von Sonntag and H.-P. Schuchmann, Angew. Chem., Int. Ed. Engl., 1991, 30, 1229; (b) P. Neta, J. Grodkowski and A. B. Ross, J. Phys. Chem. Ref. Data, 1996, 25, 709. 9 (a) G. V. Buxton, C. L. Greenstock, W. P. Helman and A. B. Ross, J. Phys. Chem. Ref. Data, 1988, 17, 513; (b) Farhataziz and A. B. Ross, Selected Specific Rates of Reactions of Transients from Water in Aqueous Solutions. II. Hydroxyl Radical and Perhydroxyl Radical and Their Radical Ions, NSRDS-NBS 59, National Bureau of Standards, Washington DC, 1977. 10 G. Merga, B. S. M. Rao, H. Mohan and J. P. Mittal, J. Phys. Chem., 1994, 98, 9158. 11 P. Neta, R. E. Huie and A. B. Ross, J. Phys. Chem. Ref. Data, 1988, 17, 1027. 12 D. Behar, J. Phys. Chem., 1974, 78, 2660. Fig. 2 Plots of (b/a)Å[HO– 2]kp vs. [H+] for (d) TFT and (s) 4-FT: the error in each data point was estimated from the dispersion in the calculated values of a and b. For 4-FT, the error bars are of the same size as the symbols. Inset: Plots of kapp vs. [S] for air-saturated alkaline solutions containing 1.16Å10µ4 mol dmµ3 H2O2 and (a) TFT, pH=12.9; (b) TFT, pH=13.3; (c) 4-FT, pH=12.9 (upper