J. Phys. Chem. 1995, 99, 11447-11451
11447
Formation and Reactions of Halogenated Phenylperoxyl Radicals in Aqueous Alcohol Solutions G. I. Khaikin? Z. B. Alfassi,b and P. Neta* Chemical Kinetics and Thermodynamics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received: March 15, 1995; In Final Form: May 15, 1995@ Halogenated phenylperoxyl radicals were produced in irradiated aqueous alcohol solutions by reductive dehalogenation of dihalo- and polyhalobenzenes with solvated electrons and subsequent reaction of the halophenyl radicals with oxygen. Phenylperoxyl radicals oxidize 2,2'-azinobis(3-ethylbenzothiazoline-6sulfonate ion) (ABTS2-) with rate constants between 3 x lo7 and 3 x lo9 L mol-' s-I, depending on the structure of the peroxyl radical and the alcohol concentration. For monohalogenated phenylperoxyl radicals, the reactivity changed in the order F < C1 < Br and p < m < 0. The reactivity increased on going from the (monohalopheny1)- to the (dihalopheny1)- and (trihalopheny1)peroxyl radicals. The rate constants were correlated with the substituent constants and with the pKa values of similarly halogenated phenols. The reduction potential for P h O O P h 0 0 - was estimated to be near 0.7 V vs NHE; that for the trichloro derivative, near 0.9 V. The rate constants in various solvent mixtures were correlated with the cohesive pressure of the medium.
with oxygen (k2 -lo9 L mol-' s - ' ) . ~ - ~For example,
Introduction
Halogenated alkylperoxyl radicals are known to be much more reactive than their nonhalogenated analogues.'-4 Recent ~tudies~ have - ~ shown that arylperoxyl radicals are more reactive than alkylperoxyl, and there is an indication5 that halogen substitution on the aromatic ring further increases the reactivity. In the present study, we compare the reactivities of phenylperoxyl and various halogenated phenylperoxyl radicals as oxidants. We use A13TS2- (2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate ion)) as a model organic reductant* and measure the rate constants for its oxidation by the three isomeric (monofluoropheny1)-, (chloropheny1)-, and (bromopheny1)peroxyl radicals and by several di- and trihalogenated phenylperoxyl radicals. Because of the limited solubility in water of the aryl halides used as precursors for the peroxyl radicals, we carried out only part of these measurements in predominantly aqueous solutions; most of the results were obtained in mixtures of water and methanol, and some in neat MeOH. Electron-withdrawing substituents on aliphatic peroxyl radicals increase the reactivity of these radicals as oxidant^.^ Increased reactivity in electron transfer reactions generally parallels increased reactivity in addition reaction^.^ Hydrogen abstraction reactions of peroxyl radicals are relatively slow' and have a smaller contribution to the fate of peroxyl radicals in complex systems. The present results are expected to shed light on the influence of the number and positions of halogen atoms on the reactivity of arylperoxyl radicals in general and may provide a model for predicting the behavior of peroxyl radicals that may be formed in the process of decomposition of polychlorinated and polybrominated biphenyls.
Experimental Section Halogenated phenylperoxyl radicals were produced in irradiated solutions by reductive dehalogenation of di- or polyhalogenated benzenes with solvated electrons (kl -lo9- l O I o L mol-' s - ' ) , ~ O followed by the rapid reaction of the phenyl radical t On leave from the Institute of Electrochemistry, Russian Academy of Sciences, Moscow. Permanent address: Ben-Gunon University of the Negev, Beer Sheva, Israel. @Abstractpublished in Advance ACS Abstracts, July 1, 1995.
*
XC,H,'
+ 0, - XC,H,O,'
(2)
where X and Y represent halogen atoms. To produce the various isomeric chloro- and bromophenyl radicals, we used the corresponding dichloro- and dibromobenzenes so that only one type of halophenyl radical is produced in the solution. For di- and polyhalogenated phenyl radicals, we used only those isomers that produce one type of phenyl radicals (1,3,5trichlorobenzene to produce 1,3-dichlorophenyl, 1,2,4,5-tetrachlorobenzene to produce 1,2,4-trichlorophenyl). For production of fluorophenyl radicals we utilized bromofluorobenzenes and expect that debromination will predominate over defluorination." The simpler compounds were sufficiently soluble in water so that they could be studied in aqueous solutions containing a small fraction of an alcohol as a scavenger for H and OH radicals. The more heavily halogenated compounds, however, were sufficiently soluble only with a large fraction of alcohol. Since the reactivity is solvent dependent, we studied each compound in several solvent mixtures (in most cases in 33% and 60% MeOH and in some cases also in neat MeOH). Absorption spectra of the peroxyl radicals were monitored in air-saturated aqueous alcohol solutions, using a high dose per pulse (generally in the range of 200 Gy) because the molar absorptivities of these radicals were relatively low ((1-2) x lo3 L mol-' ~ m - ' ) , ~and - ~ at high MeOH fraction the yield also is quite low (see below). Because of the low yield and the considerable uncertainties in estimating this yield, the molar absorptivities have not been determined and are assumed to be in the same range as those for other phenylperoxyl radicals. Since the absorption bands are fairly broad, we estimate the uncertainties in the positions of the peak as f 1 0 nm. Rate constants for reactions of arylperoxyl radicals with ABTS2-, XC&,O,'
4-ABTS2-
-
XC6H402-
+ ABTS'-
(3)
were measured by following the buildup of absorption of the product radical at 415 nm. Since A B T Y has high molar
0022-365419512099-11447$09.00/00 1995 American Chemical Society
11448 J. Phys. Chem., Vol. 99, No. 29, 1995
Khaikin et al.
TABLE 1: Rate Constants for Reactions of Halogenated Phenylperoxyl Radicals with ABTS2- in Aqueous Alcohol Solutions k, L mol-'
substituent H 4-F 3-F 2-F 4-C1 3-C1 2-c1 4-Br 3-Br 2-Br 3,5-F2 3,4-F2 3,4,5-F3 3.5-Cl7 2,4,5-C13 a
0.5% 2-PrOH 6.6 x lo8" (1.0 f 0.2) x (1.7 f 0.2) x (2.1 f 0.3) x 1.0 x 109a (2.7 f 0.3) x (2.3 f 0.3) x
109 109 109
lo9 lo9
33% MeOH (2.9 f 0.7) x (2.0 f 0.7) x (7.4 f 1.2) x (8.1 f 1.6) x (6.7 f 1.2) x (1.1 i 0.2) x (1.2 i0.2) x (9.9 i 1.8) x (1.4 5 0.2) x (1.3 f 0.2) x (9.3 f 1.5) x (6.2 f 1.5) x (8.5 i 1.4) x (2.1 f 0.4)
SKI
60% MeOH
lo8 lo8 108 lo8 lo8 109 109 108 109 109 lo8 lo8 lo8 109
(4.8 f 1.0) x (6.8 f 1.7) x (1.8 f 0.5) x (2.3 f 0.5) x (2.0 f 0.4) x (3.2 f 0.6) x (4.9 f 0.9) x (2.8 f 0.5) x (3.8 f 0.6) x (4.7 f 0.7) x (3.7 f 0.6) x (1.8 i 0.3) x (3.5 f 0.7) x (8.8 f 1.7) x
107 lo7 lo8 lo8 lo8 lo8 lo8 lo8 lo8 lo8 lo8 lo8 lo8 lo8
100% MeOH
(5.2 i 1.8) xi07 (3.3 i 1.5) x 107 (5.5 f 1.8) x 107 (3.2 f 1.5) x lo7 (4.1 f 1.6) x107
(1.2 i 0.2) x 10s
From ref 5.
absorptivity (3.5 x lo4 L mol-' cm-' at 417 nm),I2 these measurements were done with a lower dose per pulse (5-10 Gy). To correct for possible competing reactions (such as radical-radical decay or radical reaction with the alcohol), the first-order formation rates were measured with at least three different concentrations of ABTS2-, varying by about a factor of 4, and the second-order rate constants were derived from plots of the first-order rate constants vs concentration (under conditions where the rate of the oxidation reaction 3 is not limited by the rate of reaction 2). The HOCH202. radical, produced along with the phenylperoxyl, does not interfere with the spectral and kinetic measurements because it absorbs only in the UV and reacts considerably more slowly than phenylperoxyl radical^.^^^ There are two major sources of uncertainty in the derived rate constants for reaction 3: the statistical uncertainties in the first-order fits of the kinetic traces and in the plots of kobs vs concentration, which were generally between 5% and lo%, although in a few cases they were higher, and uncertainties in the measurements of volumes and weights, which we estimate as 5 10%. The overall estimated standard deviations are given in Table 1 along with the rate constants. Further details on the materials used (the halogenated aromatics and ABTS2- were from Aldrich13), solution preparation, the Febetron-based pulse radiolysis apparatus, and data processing were described b e f ~ r e . ~All - ~ measurements were carried out at room temperature, 20 f 2 "C. Results and Discussion
The absorption spectra of the (chloropheny1)peroxylradicals in the visible range (Figure 1) are very similar to that of the unsubstituted phenylperoxyl radical, for which Amm was reported to be at 490 nm.637 The spectrum of (3-chlorophenyl)peroxyl has an identical A,, (490 f 10 nm), whereas those of the (2chloropheny1)- and (4-chlorophenyl)peroxyl have peaks at 500 and 520 (f10) nm, respectively. Although the 2- and 4-substituents may be expected to exert a stronger effect on the spectrum than the 3-substituent, the peaks are almost the same within experimental uncertainty. The molar absorptivities at these maxima were not determined since the radiolytic yields of the phenylperoxyl radicals were not quantitative (a considerable fraction of the eaq- react with 0 2 instead of reacting with the dichlorobenzenes because of the limited solubilities of these compounds). These spectra were determined in aqueous solutions containing 0.5% alcohol. The spectrum of the (3,5dichloropheny1)peroxyl radical could not be measured in such aqueous solutions, because of the very low solubility of the parent compound, and was instead determined in 60% MeOH (Figure 2a). It was found to have A,, at 450 nm. Since
monochloro substitution at the meta position does not shift the peak of phenylperoxyl, we expect the second chlorine at the other meta position to have a negligible effect. To check whether the observed blue shift may be due to a solvent effect, we compared the spectrum of (3-chloropheny1)peroxylradical in 0.5% 2-PrOH, 60% MeOH, and 100% MeOH. For clearer comparison, the spectra were normalized to correct for the different yields in the various solvent mixtures (Figure 2b), and it appears that at high MeOH concentration A,, shifts slightly found for the to the blue. This suggests that the lower A,, (dichlorophenyl)peroxyl, as compared with the (monochlorophenyl)peroxyl, is due to the effect of the solvent and not the substituent. The spectrum of (3,4,5-trifluorophenyl)peroxyl radical could be monitored in both 60% and 2% MeOH (Figure 2c, normalized spectra), but any shift due to solvent effect in this case is within experimental uncertainties. Theoretical calculation^'^ on the spectrum of the phenylperoxyl radical predict a blue shift with a decrease in medium dielectric constant; our results appear to support that prediction, although not conclusively. Attempts to monitor the spectrum of (3chloropheny1)peroxylin lower polarity dioxane/water mixtures were inconclusive because the position of the peak in the 500 nm region was masked by an additional band with a peak at (350 nm but with a considerable tail which extended to 450 nm. A recent studyt5 indicated a 40 nm blue shift in the spectrum of (trichloroviny1)peroxyl on going from water to tetrachloroethene as solvents. The rate constants for oxidation of ABTS2- by the various halogenated phenylperoxyl radicals are summarized in Table 1. In aqueous solutions with only 0.5% alcohol, the rate constants measured were near or above 1 x lo9 L mol-' s-l. The radiolytic yields of peroxyl radicals, and thus the absorbance of ABTS'-, in these experiments were relatively high; therefore, the rate constants determined have estimated uncertainties of 3~10%.The peroxyl radicals derived from the alcohol have a much lower reactivity. We measured an upper limit of k 5 x lo6 L mol-' s-l for the reaction of HOCH202' with A13TS2-, and thus this reaction did not interfere with the measurements. It is important, however, to prevent interference by another reaction, which may become important if a considerable fraction of the eaq- reacts with ABTS2-. Reaction of ABTS2- with eaqis very rapid ( k = 5.1 x lo9 L mol-' s - I ) ~ and forms a species that oxidizes another molecule of ABTS2- with a rate constant of 5 x lo8 L mol-' s - ' . ~ In the present experiments, eaq- may react partly with 0 2 ( [ 0 2 ] 2.4 x mol L-I, k = 1.9 x 1OloL mol-' s-l)Io and partly with the aryl halide, depending on its concentration and reactivity. Since the rate constants for the peroxyl radicals determined in this medium (0.5% 2-ROH
-
J. Phys. Chem., Vol. 99, No. 29, 1995 11449
Reactions of Halogenated Phenylperoxyl Radicals
0.010 0.02
0
0
e
c
u
u
0
c3
0
c0
0.005
a a
a
