Langmuir 1991, 7, 3081-3089
3081
Pulse Radiolytic Studies of the Reaction of Pentahalophenols with OH Radicals: Formation of Pentahalophenoxyl, Dihydroxypentahalocyclohexadienyl, and Semiquinone Radicals Rita Terzian and Nick Serpone* Department of Chemistry, Concordia University, MontrQal, Quebec, Canada H3G 1 M8
R. Barton Draper and Marye Anne Fox Department of Chemistry, University of Texas, Austin, Texas 78712
Ezio Pelizzetti Dipartimento di Chimica Analitica, Universith di Torino, 10125 Torino, Italy Received March 14, 1991. In Final Form: May 8, 1991 The optical and kinetic characteristics of the transients formed by the reaction of OH*and N3*radicals with pentabromo- (PBP-OH), pentachloro- (PCP-OH), and pentafluorophenol (PFP-OH) have been examined by pulse radiolysis techniques in buffered (pH 8, phosphate) aqueous media. The principal products from the reaction of OH*with PBP-0- and PCP-0- are the corresponding pentahalophenoxyl radicals {ca.75 % (PBP-0.) and ca. 77 % (PCP-O.)},the OH' adducts dihydroxypentahalocyclohexadienyl radical anions, HO-PXP-0- (-18% for X = Br, -8% for X = Cl), and the semiquinone radicals bromanil (ca. 7 % ) and chloranil (ca. 15%). Their kinetics of formation via electron transfer and OH' addition have been assessed. Oxidation of PCP-0-and PBP-0- by N3*radicals also yields the following phenoxy1species: PBP-Om,ET = (6.1 f 0.9) X lo9 M-' s-l; PCP-O', ET = (3.3 f 0.6) X lo9 M-l s-l. The reaction of OH* with PFP-0- anion yields exclusively the dihydroxypentafluorocyclohexadienyl radical, HO-PFP-0-, k A D D = (4.6 f 0.9) X lo9 M-' s-l. Solvated electrons react with PBP-0- ( k , = 2.6 X 1Olo M-l s-l) to give a highly reactive dianionic electron adduct, which following loss of Br- and reaction with HzO yields a mixture of isomeric tetrabromophenoxylradicals. These observations find relevance in OH*radical photooxidative degradationsof these and analogous organic contaminants in homogeneous and heterogeneous aqueous media.
Introduction The photocatalyzed oxidation of halogenated phenols has been carried out successfully in aqueous dispersions of TiO2.l Noteworthy examples include the mineralization of such environmental contaminants as 2,4,5-trichlorophen01,~-~ pentachl~rophenol,~ 2-, 3-, and 4-fluorophenol, and 2,4-difl~orophenol.~ The hydroxyl radical has been implicated as a significant oxidant in the TiO2-assisted photomineralization of many organic compounds in aqueous environments,2+since hydroxylated intermediates can be detected during the photodegradation of many of these species. One example is detection of hydroquinone, catechol, 1,2,4-benzenetriol, benzoquinone, pyrogallol, and 2-hydroxy-1,4-benzoquinone intermediates in the photooxidative mineralization of phenol.'^^ However, since these same adducts could also be formed by hydration of a singly (1) Matthews, R. W. In Proceedings of the I.P.S.8 Conference; Pelizzetti, E., Schiavello, M., Eds.; Kluwer Publications: Dordrecht, The Netherlands, 1991. (2) Barbeni, M.; Morello, M.; Pramauro, E.; Pelizzetti, E.; Vincenti, M.; Borgarello, E.; Serpone, N. Chemosphere 1987,16,1165. (3) Al-Ekabi, H.; Serpone, N. J. Phys. Chem. 1988,92,5727. (4) Al-Ekabi, H.; SerDone. N.: Pelizzetti, E.; Minero. C.: Fox, M. A.: Draper, R. B. Langmui; 1989,5, 250. (5) Barbeni, M.; Pramauro, E.; Pelizzetti, E.; Borgarello, E.; Serpone, N. Chemosphere 1985, 14, 195. (6) Minero, C.; Aliberti, C.; Pelizzetti, E.; Terzian, R.; Serpone, N. Langnuir 1991, 7, 928. (7) (a) Okamoto, K.; Yamamoto, Y.; Tanaka, H.; Tanaka, M.; Itaya, A. Bull. Chem. SOC.Jpn. 1985,58, 1985. (b) Okamoto, K.; Yamamoto, Y.; Tanaka, H.; Itaya, A. Bull. Chem. SOC.Jpn. 1985,58, 2023. (8) Augugliaro, V.; Palmisano, L.; Sclafani, A.; Minero, C.; Pelizzetti, E. Torzcol. Enuiron. Chem. 1988, 16, 89.
0743-7463/91/2407-3081$02.50/0
oxidized cation radical, produced by direct interfacial electron transfer to an adsorbed organic substrate! details concerning the primary process(es) initiated by photon activation of Ti02 in aqueous media are still a matter of debate.1° Hydroxyl radicals may originate from several possible routes.11-18 Irradiat,ion of Ti02 particles (Ebg. N 3.2 eV) generatesvalence band holes (h+w)and conduction band electrons (e-CB),eq 1,which are subsequentlytrapped on the particle surface where interfacial electron transfer to appropriate adsorbates can take place. Reaction of the holes with surface OH- groups or adsorbed water constitutes a major source of hydroxyl radicals," with a minor contribution from the cleavage of H2027aJ1J3-17J9 (formed (9) Draper, R. B.; Fox, M. A. Langmuir 1990,6, 1396. (10) Lawless, D.; Serpone, N.; Meisel, D. J. Phys. Chem. 1991, 95, 5166. (11) Matthews. R. W. J. Chem. Soc.. Faradav Trans. 1 1984.80.457. (12) Izumi, I.; Dunn, W. W.; Wilbourn, K. 0:;Fan, F. F.; B&d,'A. J. J. Phys. Chem. 1980,84, 3207. (13) Fujihira, M.; Satoh, Y.; Osa, T. Bull. Chem. SOC.Jpn. 1982,55, 666. (14) Izumi, I.; Fan, F. F.; Bard, A. J. J. Phys. Chem. 1981, 85, 218. (15) Cundall, R. B.; Rudham, R.; Salim, M. S. J. Chem. SOC.,Faraday Trans. 1 1976, 72, 1642. (16) Harvey, P. R.; Rudham, R.; Ward, S. J. Chem. SOC.,Faraday Trans. 1 1983, 79,1381. (17) Herrmann, J.-M.; Pichat, P. J. Chem. Soc., Faraday Trans. 1 1980, 76, 1138. (18) Kormann, C.; Bahnemann, D. W.; Hoffman, M. R. Enuiron. Sci. Technol. 1988, 22, 798. (19) (a) Matthews, F.. W. Water Res. 1986,20,569. (b) Matthews, R. W. J. Catal. 1986,97,565. (c) Matthews, R. W. J.Phys. Chem. 1987,91, 3328. (d) Matthews, R. W. Sol. Energy 1987,38,405. (e) Matthews, R. W. Aust. J. Chem. 1987, 40, 667.
0 1991 American Chemical Society
3082 Langmuir, Vol. 7, No. 12, 1991
Terzinn et al.
Table I. Observed Kinetics of Formation and Decay of Transients in Various Reaction Systems. formation reaction system PBP-0- + N3- + OH'
decay
A, nm
2klc, s-*
A, nm
2klr,
470
(3.5-12.7) X lo5
47OC 46OC 35OC 47or 460 330 460 715 4601 3301 480 380 440 440° 450 340 450 430 300
(1.3 f 0.3) X (1.2 f 0.2) x (3.7 f 0.7) X (1.1f 0.2) x (1.3 f 0.2) X (1.4 f 0.3) X (5.5 f 1.0) x 5.1 f lo6 8.6 X lo5 5.4 x 105 5.5 x 104 9 x 104 (7.6 f 1.6) X (7.4 f 1.6) x (6.5 f 1.0)x (3.0 f 0.6) X (2.1 0.6) x (7.0 f 1.4) X (2.8 f 0.3) X
*
PBP-0- + N3' + ascorbated PBP-0- + OH' g
390e 460
(1.0 0.2) x 105 1.6 X lo6
PBP-0- + OH' PBP-0- +
380
(6.9 f 2.0) x 104
480 330 -.
4.3 x 105 8 X lo5
440"' 360
(6.1-83) X lo4 (7.4 1.5) x 104
PBP-0- + e-,, PCP-0-
+ ascorbateh + ascorbatek
+ N3- + OH' '
PCP-0- + N3. + ascorbate" PCP-0- + OH' P PCP-0- + OH' PFP-0- + OH'
+ ascorbateq
PFP-0- + N3- + OH'
'
360 4308 30Ot 320
*
(3.0 f 0.9) x 104 (3.c-24) x 105 (3.1-21) X lo5 1.9 x 104
*
s-1
lo6 106 lo6
105 lo6 lo6 104
lo6 104
105 lo6 104 lo5
lo5
M phosphate; [NaN3] = 0.01 M; [t-BuOH] = 0.2 M. a General conditions: NzO-saturated solutions (25 mM); pH 8 buffered with M; [OH'] = 1.88 X lo4 M. [PBP-0-1 = 2.0 X lo4 M; [PBP-0-1 = (5-20) X 10-5 M; [OH'] = 1.97 X lo4 M. [PBP-0-1 = 2.0 X [ascorbate] = 5 X lW5 M. e [OH'] = 2.85 X 1VM. f [OH'] = 7.37 X lV M. g [PBP-O-]= 2.0 X 10-4 M [OH'] = 4 X 1V M. [PBP-0-1 = 2.0 X 10-4 M; [OH'] = 3.38 X M; [ascorbate] = 2.5 X M. [PBP-0-1 = 2.0 X lo-* M; [e-,,] = 2.2 X M. j [e-,,] = 1.15 X lo4 M. M; [OH'] = 2.95 X M. k [PBP-0-1 = 2.0 X 10-4 M; [e-,,] = 2.5 X 10-6 M; [ascorbate] = 5 X loF5 M. [PCP-0-1 = 2.5 X M. m [PCP-0-1 = (1.3-2.5) x 10-4 M; [OH'] =.1.14 X lo4 M. [PCP-0-1 = 2.5 X M; [OH'] = 3.21 X lo4 M; [ascorbate] = 5 X 0 [OH'] = 3.41 x 10-6M. p [ P C P O ] = 2.5 X 10-4 M; [OH'] = 5.1 X 10-6M. 4 [PCP-0-1 = 2.5 X 10-4M, [OH'] = 2.91 X 1V M, [ascorbate] = 2.5 X 106 M. [PFP-0-1 = 5.0 X 10-4 M; [OW] = 3.92 X 10-6 M. [PFP-0-1 = (5-50) X 10-5 M; [OH'] = 1.83 X lo4 M. [PFP-0-1 = (5-50) X M, [OH'] = 1.52 X 10-6 M. u [PFP-0-1 = 5.0 X W4M [OH'] = 3.43 X lo4 M. b
via the superoxide
since [HzOz] is smalLZ1
The relative importance of direct oxidation of organic substrates by the photogenerated holes will depend on the magnitude of the preadsorption equilibria and may be insignificant for weakly adsorbed substrates.22 A recent study9 on 2,4,5-trichlorophenol by diffuse reflectance nanosecond flash photolysis suggests h+VB oxidation of this and several other organic and inorganic substrates. Involvement of 'OH radical as a redox precursor to these adsorbed oxidized species is not precluded by such observations. In fact, these suggest that hydration of surface-bound cation radicals may be slow and that formation of hydroxylated products at intermediate stages of the oxidation is likely to occur significantly via direct OH' attack. The question of the role of h+vB vs OH' in photooxidations by light-activated Ti02 in aqueous media has recently been addressed.l0 The role of electrons produced concurrently with the holes has not been established. The present view is that electrons are consumed via eq 2 to produce the superoxide radical anion, 0 2 * - , since the degradation reaction requires 0xygen,~0,~~ and since 0 2 ' - radicals have been detected.24 I t is not inconceivable, however, that electrons react via a parallel process with easily reduced organic substrates. Reductive dehalogenation of haloaromatics is known to occur rapidly in steady-state r a d i o l y ~ i s . ~ ~ (20) Both these mechanisms (Le. 'OH formation via H202 or via adsorbed HzO or OH-)are consistent with the fact that no photodegradation of 4-chlorophenoloccurs in the absence of either 02or H20 or both.23 (21) Serpone, N.; Borgarello,E.; Barbeni, M.; Pelizzetti, E.; Pichat, P.; Herrmann, J.-M.; Fox, M. A. J. Photochem. 1987, 36, 373. (22) Turchi, C. S.; Ollis, D. F.J. Catal. 1990, 122, 178. (23) Barbeni, M.; Pramauro, E.; Pelizzetti, E.; Borgarello,E.; Grltzel, M.; Serpone, N. N o w . J. Chim. 1984, 8, 547. (24) Anpo, M.; Kubokawa, Y. Reu. Chem. Intermed. 1987,8,105, and references therein. (25) Getoff, N.; Solar, S. Radiat. Phys. Chem. 1986, 28, 443.
Radiolysis of water quantitatively produces either OH' or e-aq in homogeneous solution and thereby affords a means of examining the reactivity of each with a specified organic substrate. Previous steady-state radiolysis studies25p26 have shown that OH' radicals and e-aqinduce the decomposition of 2-, 3-, and 4-chlorophenol;e.g., reactions of OH' and e-aqwith 2-chlorophenol occur with ADD = 1.2 X 10'0 M-' s-l and k, = 2.0 X lo8 M-l s-l, respectively. Draper et aLZ7found that OH' adds to 2,4,54richloropheno1 = 1.2 X 1Olo M-ls-l) to give thedihydroxytrichlorocyclohexadienyl species. The main focus of the present study was to identify the products and assess the rates of reaction of OH' and the hydrated electron with a series of pentahalophenols: pentabromophenol (PBP-OH),pentachlorophenol (PCPOH), and pentafluorophenol (PFP-OH). We were particularly interested in establishing whether the nature of the halogen for an analogous series of haloaromatics might affect the reaction pathway and whether addition of OH* to the aromatic ring, to give hydroxycyclohexadienyl species, is a principal reaction pathway as present evidence would These studies are also needed to identify the primary oxidized species from surface-bound OH' radical attack on these pentahalophenols,whose photomineralization over Ti02 aqueous dispersions was investigated earlier for PCP-OH5 and recently for PFP-OH6 to demonstrate the photooxidation process and to address the mechanistic details underlying it. Herein, we report the nature, the spectroscopic properties, and the kinetics of formation and decay of radicals produced by the reaction of OH', N3*,and solvated e- with the three pentahalogenated substrates in a homogeneous phase. (26) Getoff, N.; Solar, S. Radiat. Phys. Chem. 1988,31, 121. (27) Draper, R. B.; Fox, M. A.; Pelizzetti, E.; Serpone, N. J. Phys. Chem. 1989, 93, 1938.
Reaction of Pentahalophenols with OH Radicals
Langmuir, Vol. 7,No. 12, 1991 3083 Table 11. Comparison of the Optical and Kinetic Properties of PXP-0' with Values Published for Similar Radicals
b"
radical
nm 400
?-
05 -0
11"
emam
M-' cm-l
kf,M-' s-l
2.20 x 103
4.3 x 109
ko, M-l
8-l
CI
q1 0.
CI
430
3.60 x 103
4.3 x 109
390
2.92 X lo3
4.6 X
420
3.70 X lo3
440
2.39 x 103
330 470
3.88 x 103 3.20 X lo3
7.7 x 108
ref 34 37 27
CI
lo9
34
F
300
400 500 Wavelength (nm)
1 x 108
35
3.7 x 109
9.1 x
a
5.9 x 109
2.8 x 109
600 Br
Figure 1. Transient absorption spectra of PBP-0' at 40, 90, 245,475, and 745 ~s after irradiation of 2 X lo4 M pentabromophenoxide at pH 8 (buffered) in a 0.01 M NaN3 aqueous solution; the solution was N2O-saturated. [ O H ] = 1.97 x lo* M. Insets show the decays of the optical density at 350 and 470 nm. Experimental Section Chemicals. Pentafluorophenol(99+ % ), pentachlorophenol (99%),and pentabromophenol(96%)were purchased from Aldrich. PFP-OH was used as received,while PBP-OH and PCPOHwere further purified by two successivesublimations. Sodium azide, potassium phosphate (MCB), tert-butyl alcohol (Fisher Scientific), potassium thiocyanate, L-ascorbic acid, and other chemicals were reagent grade and were used as received. Electron Accelerator. Pulse radiolysis experiments were performed with a 4-MeV (1-3 A) van de Graff accelerator. Pulse widths of 50, 100, 250, and 500 ns provided radiation doses of 2.5-20 Gy per pulse.28 Aqueous solutions (20 2 OC) were irradiated in a single-pass flow cell with either a 2.2- or 3.3-cm analyzingpath length. Transient absorptions were measured by using a xenon lamp, grating monochromator, and photomultiplier tube arrangement. The absorbed radiation dose was determined by thiocyanate dosimetry: 0.01 M KSCN in N2Osaturated water; [G(OH)] = [G(SCN)z'-] = 6.0 molecules/100 eV;€480 = 7600 M-l cm-1).29Absorptionvs time data were recorded on a Biomation 8100 digitizer interfaced with a PDP 11/70 computer for data analysis.30 Solutions. Millipore filtered water was used for all solutions. These were NaN3 (0.01 M) or buffered pH 8 solutions M phosphate buffer) owing to the low solubility of PCP-OH (8mg/ 100 mL) and PBP-OH (insoluble) in water. The three pentahalophenols examined in this work should exist as the phenoxidesat this pH (pK, = 4.5,pK,,,, = 5.53).31Deoxygenation was accomplishedby bubbling with either N20 or N2, as indicated. The concentrations of solutes were chosen to ensure 90-100% capture of the desired primary radicals. Additional details of the experimental conditions are reported in Table I and in the figure captions. Results and Discussion Radiolysis of water by high-energy electrons produces hydrogen atoms, hydrogen gas, protons, hydroxide, hy-
*
(28)Rodgers, M.A. J.; Foyt, D. C.; Zimek, Z. A. Radiat. Res. 1977,75, 296. (1 Gy = 1 Gray = 1 J/kg, the SI unit for adsorbed dose.) (29)Baxendale, J. H.; Bevan, P. L. T.; Stott, D. A. Trans. Faraday SOC.1968,64, 2389. (30)Foyt, D. C. Comput. Chem. 1981,5,49. (31)Serjeant, E. P.; Dempsey, B. Ionization Constants of Organic Acids in Aqueous Solution; IUPAC Chemical Data Series No. 23;Pergamon Press: Oxford, 1979. The value for PFP-OH is at 25 OC, the temperature for PCP-OH was not stated.
cl$
108
ci
CI CI
Br$Br Br
a
Br Br
This work. drogen peroxide, and solvated electrons, in addition to the desired OH' radical.32 To ensure scavenging of the other species, the aqueous solutions were saturated with nitrous oxide ([NzO] = 25 mM), which converts e-sq to OH' (eq 3). The quantity of H' radicals produced a t pH 8 via eq 4 is negligible.
N,O
+ e-aq+ H,O
-
OH'
k = 9.1 X lo9 M-' H 3 0 + e-aq
H'
+ N, + OHs-'
33
+ H,O
k = 2.3 X 10" M-'
(3)
(4)
s-l 33
Hydroxyl radicals have been used to prepare less reactive oxidizing radicals, e.g., the azide (N3.) radical via reaction with NaN3 (eq 5 ) . Azide radicals are more selective and OH'
+ N3-
-
OH- + N,'
k = 1.2 x 1 o ' O M-'s-'
(5)
33
normally react with aromatic compounds via electron transfer, unlike OH' radicals where addition to the ring (32)Swallow, A. J. In The Study of Fast Processes and Transient Spectes by Electron Pulse Radiolysis; Baxendale, J. H.,Busi, F., Eds.; D. Reidel Publishing Company: Boston, MA, 1982;pp 289-315. (33)Buxton, G. V.;Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1988,17 (2),513.
Terzian e t al.
3084 Langmuir, Vol. 7, No. 12, 1991
2 . 5 PS 0 0
R
7.75 cs 15.5 cs
PBP-0-
+
*OH
0
25 us 75 us
o 23.25 PS A 52.25 CIS
W-0. t Ascorbate
1
350
400
450
500
Wavelength, nm Figure 2. Changes in the optical spectrum of PBP-0' in the presence of ascorbate. The spectra were recorded at 2.5, 7.75, 15.5,23.25,and 52.25 ps following irradiationof 2 X M pentabromophenoxide in an NzO-saturated aqueous solution at pH 8 containing 0.01 M NaN3 and 5 X M ascorbate. [OH'] = 3.17 x lo4 M. The right inset shows the decay of the 470-nm absorbance; [OH] = 7.37 X 10-6M. The left inset shows the growth observed at 390 nm; [OH'] = 2.85 X lo+ M. usually competes significantly.% In addition, the absorption spectrum of N3' has a narrow band at 270 nm and little absorption at wavelengths above 300 nm, thereby facilitating the optical analysis.34 Electrons can also be used as reducing radicals. Upon addition of tert-butyl alcohol as a scavenger, OH' radicals are trapped from solution, producing a relatively inert radical (eq 6). OH'
+ (CH,),COH
-
H 2 0 + (CH,),(CH,)COH (6)
k = 6.0 X 10' m-l s-l 33 Pentabromophenol. PentabromophenoxylRadical (PBP-0'). The absorption spectrum (Figure 1) of the product from the oxidation of PBP-O- anion by N3* radicals, pentabromophenoxyl radical (PBP-O'), is characterized by a band at -330 nm (€330 = (3.88 f 0.45) X lo3 M-' cm-' 35) and at X = 470 nm (€470 = (3.20 f 0.35) X 103M-' cm-139, Table 11. The long wavelength feature is similar to the one reported for the 2,4,5-trichlorophenoxyl radical.27 The oxidation (eq 7, X = Br) was monitored by observing the increase of the optical density at 470 nm as a function of [PBP-0-1 over the range from 5 X to 2 X M; the observed appearance rate constant kobsd ranging from 3.5 X 1 0 5 to 1.3 X lo6s-l (Table I) gave a bimolecular rate constant kET of (6.3 f 0.9) x lo9 (34) Alfassi, Z. B.; Schuler, R. H. J. Phys. Chem. 1985,89, 3359. (35) The extinction coefficient of PBP-0' was determined via two methods. One consisted of extrapolating the second-order decay data at four radiation doses to time zero. The optical density at time zero (OD0470) was then divided by the concentration of radical produced [G(N3) = G(PBP-O')] and the optical path length. The second set oft calculations was based on the maximum optical density attained while measuring the rate of increase of the optical density at 470 nm as a function of PBP-OH concentration (short timescales). These values were then treated as above. Both seta of calculations were in agreement yielding €470 = (3.20 f 0.35) X l o 3 M-l cm-1.
00
400 500 Wavelength (nm)
600
Figure 3. Transient absorption spectra of the product(s)of the reaction between PBP-O- and OH' at 25,75,220,615, and 1325 ps following irradiation of a NzO-saturated aqueous solution of 2 X 10-4 M PBP-O- at pH 8 (buffered);[OH'] = 4 X 104 M. The inset shows the decay of optical density due to products of the (PBP-O- + OH') reaction at 330 and at 460 nm. M-' s-l. The absorption at 470 nm (Figure 1,inset) decayed via second-order kinetics: k d = (2.0 f 0.4) X lo9 M-l s-l. 0-
0'
Monitoring the increase in the optical density at 350 nm over the same [PBP-0-1 gave kobsd = 3.1 X 1 0 5 to 1.2 X lo6s-l and ET = (6.0 f 0.9) X lo9M-l s-l. The transient absorbing at 350 nm36 also decayed via second-order kinetics (Figure 1 inset): k d = (3.6 f 1.0) X lo9 M-' s-l. The correspondenceof k d a t 350 nm and at 470 nm confirms that both bands originate from one species. Also the bimolecular rate constants for eq 7 at 350 and 470 nm are virtually identical. The kinetics of formation and decay of the PBP-0' radical correlate well with those of similar compounds (Table II).27p34*35p37 Further confirmation of the nature of the radical was obtained by generating the same species in the presence of ascorbate. Most phenoxy1 radicals rapidly oxidize the ascorbate anionB with rate constants typically in the range 4 X 108 to 20 X lo8 M-' s-1,39 This oxidized ascorbate radical has a characteristic absorption at ,A, = 360 nm (€360 = 3.30 (36)This wavelength (350 nm) was chosen because of minimal interference from ground-state absorption of PBP-O-. (37) Land, E. J.; Ebert, M. Trans. Faraday SOC.1967, 63, 1181. (38)Attempts to find another suitable reducing agent to react with PBP-0' in the same manner as ascorbate but which was transparent in the 360-nm region were unsuccessful: (i) triphenylamine is not water soluble and the water-soluble triethylamine proved ineffective. Attempts to oxidize the OH' adduct to remove its spectral contribution were also unsuccessful; the alkaline conditions used precluded Fe(CN)e3- as a potential oxidant and IrCl$, which oxidizes the 2,4,5-trichlorophenol OH adduct in acidic media proved ineffective. Persulfate had no noticeable effect on the absorbance decay (as noted for PBP-0' in the presence of ascorbate), not surprising as the oxidation of the OH' adduct to yield a quinone implicates the elimination of a bromine atom and would therefore not be expected to proceed at a rapid rate. (39) Schuler, R. H. Radiat. Res. 1977, 69, 417.
Langmuir, Vol. 7, No. 12,1991 3085
Reaction of Pentahalophenols with OH Radicals lo3M - l ~ m - l ) which , ~ ~ can be used to identify phenoxyl radicals and to selectively remove the phenoxyl radical absorption from complex spectra of product(s) mixtures (see below). The spectrum illustrated in Figure 2 was obtained under otherwise identical conditions to those used above (see Figure 1). In the presence of 5 X M ascorbate, the absorption of PBP-0' a t 470 nm decayed rapidly with a concomitant growth of a transient at 360 nm, which we attribute to the ascorbate radical cation. Isosbestic points are seen at -410 nm and at about 340 nm. The PBP-0' absorption at 470 nm decayed via first= (1.1f 0.2) X lo5s-l) in the presence order kinetics of ascorbate, while the transient feature at 390 nm (no interference from the 330-nm band) grew in with kobsd = (1.0 f 0.2) X los s-l (Figure 2 inset). The congruence of the two k values confirms the oxidation of PBP-0- by azide radicals. Pentabromophenol OH' Radical Adduct. Hydroxyl radicals add to halogenated phenols (e.g., 2,4,5-trichlorophen01~~ and 2-chlorophen01~~) to form dihydroxycyclohexadienyl radicals, in competition with H atom abstraction and electron transfer.34 We, therefore, infer that OH' radicals react with PBP-0- as in eqs 8 (X= Br).
m
X
+-:ex f - :(OH 9'
x
I
0-
'@: X
X
CI: 77%
I
+
'OH
+
OH-
0-
(8b)
x-CI: X Br: 18% 8%
X
X
X I F: 1w9h
I
0-
X-Br: 7%XI CI: 15%
X
~ HO X
X
-HX
2
B
B
B
.04 -
v)
c 0)
0
42
0 0
"
350
400 450 500 550 Wavelength, nm Figure 4. Changes in the optical spectrum of the products of the (PBP-0-+ OH') reaction in the presence of ascorbate. The spectra were recorded at 6, 11.5, 22, 41, and 91 ps following irradiation of a NzO-saturated aqueous solution containing 2 X lo4 M pentabromophenoxide and 5 X M ascorbate at pH 8; [ O H ] = 2.78 X 1 PM. The left inset showsthe growth observed at 380 nm; [ O H ] = 3.38 X 10" M. The right inset shows the decay observed at 460 nm; [OH'] = 3.38 X lo4 M. 300
below). The overlap of the HO-PBP-0- absorption bands with those of PBP-O', together with the possible interference from the semiquinone radical (eq 8c, X = Br), precluded an estimate of the rate of decay of the OH' adduct. However, if the band at 320-330 nm were due solely to the HO-PBP-0- radical, €330 N 7.61 X lo3 M-l cm-l (Table III).25~27~40~41 The ratio of the absorbance of PBP-0' at 490 nm (extrapolated to time zero) to that at 470 nm, under the conditions of Figure 1(€470 = (3.20 f 0.35) X lo3M-l cm-l), gave €490 = (2.86 f 0.60) X lo3 M-l cm-l and a [PBP-0'1 of (3.0 f 0.7) X 10-6 M, i.e., 75% of the total products, which parallels the findings of Schuler et al.42for the reaction of p-bromophenol with OH'. Thus, 25% or the remaining (1 X 10-6 M) OH' radicals react with PBP-Oto produce HO-PBP-0- and/or other radical species. An OH' radical addition at the para position of PBP-0- would yield the tetrabromo-p-semiquinone anion radical (bromanil) which, by comparison with chloranil (see below and eq 812,X = Cl), absorbs around 450 nm. Semiquinone and semiquinone anion radicals typically have high extinction coefficients: for the p-benzosemiquinone anion radical, (425 = 7.30 X lo3 M-l cm-'; for the chloranil semiquinone radical, E448 = 6.00 X lo3 M-' cm-1.43 Semiquinone radicals have pK values in the range 3-6;43therefore the anion radical should form under our reaction conditions (pH 8 buffer). If we assume an extinction coefficient €450 6.00 X 103M-l cm-' (as in the protonated form of chlorani14y, we estimate that the amount of semiquinone M or -7 % of the total quantity of formed is -3.0 X radicals formed. The second major product (- 18%)of
xax 9-
X
X
(W
6'
The absorption spectrum of the adduct of OH' to PBP-0- (eq 8b, Figure 3) is characterized by a sharp band a t 320 nm, a broad band at -460 nm, and a poorly resolved feature at 430 nm. The transients absorbing a t 330 and 460 nm decayed via second-order kinetics (Table I): 2k/t = (1.4 f 0.2) X lo6 and (1.3 f 0.2) X lo6 s-l, respectively (Figure 3 insets). The identity of these decay constants with that of the second-order decay constant of the PBP0' radical (460 nm of Figure 1; 2 k / ~= (1.2 f 0.2) X 106 s-l) indicates that PBP-0' is formed in the reaction of PBP-OH- with OH'. In the presence of 5 X M ascorbate, the transient a t 460 nm decayed rapidly as a new band grew in at -370 nm (Figure 4) via first-order kinetics, how = (6.9 f 2.0) x 104 5-1. Subtraction of the contribution of the PBP-0' absorption from the (PBP-0- + OH') spectrum gives the spectrum shown in Figure 5. We attribute the major portion of the absorption in the difference spectrum to the OH' radical adduct of PBP-0-, the dihydroxypentabromocyclohexadienyl radical anion, after noting the similarity of this spectrum with that of the OH' adduct of PFP-0- (see
-
(40) Although the error in this value may be large, it is of the same order of magnitude as 6320 (=5300 M-l cm-l) for the dihydroxytrichlorocyclohexadienyl radical (Table II).26.27.41 (41) Matthews, R. W.; Sangster, D. F. J. Phys. Chem. 1965,69,1938. (42) Schuler,R. H.; Neta, P.; Zemel, H.; Fessenden,R. W. J.Am. Chem. SOC. 1976,98,3825. (43) Khudanov, I. V.; Kuz'min, V. A. Russ. Chem. Reu. 1975,44,801, and references therein.
Terzian et al.
3086 Langmuir, Vol. 7, No. 12, 1991
f i L
.020
1,-
1 3 .1
I 0
," ,015 .H
c v, m
0
15 1s 25 1s 55 1s 175 1s
.010
m
400 500 600 Wavelength, (nml Figure 5. Transient difference absorption spectrum of the dihydroxypentabromocyclohexadienylradical HO-PBP-0-(+ other species)(0)as calculated from the transient absorption spectrum of the products of the reaction between PBP-0- solution and OH' (m) at 75 ps following irradiation of an aqueous solution 2 X 10-4 M PBP-0-, buffered at pH 8. The solution was NzOsaturated. [OH'] = 4 X lo4 M. Solid circles ( 0 )represent the contribution to the product spectrum of the pentabromophenoxyl radical. "300
Table 111. Comparison of the Optical and Kinetic Properties of HO-PXP-0- with Values Published for Similar Radicals
pH 320
OH
CI
1.2 x 10'0
300
OH
5.30 X IO3
1.2 X 1010
25
5.6 X
108
27
CI
0-
Br
9.6 x 109
41
a
330
7.61 X
lo3
1.4 x 109
300 430
3.56 x 103 1.69X lo3
4.6 x 109
Br
F F%LH F
5.4 x
108
a
u .*
U
0" .005 0 300
400
500 Wavelength, nm
600
Figure 6. Transient absorption spectra of tetrabromophenoxyl
radical(s)resulting from the reaction of PBP-OH with e- at 15, 25, 55, 175, and 395 pa following irradiation of a 2 X lo-' M PBP-OH and 0.2 M t-BuOH aqueous Nz-saturated solution at pH 8; [e-] = 1.15 X 104 M. The left inset shows the decay at 330 nm and the right inset shows the decay at 460 nm. of H20 (or OH- for the PXP-0- species) following OH' addition, rather than by direct (outer sphere) electron transfer owing to an unfavorable reorganization energy for the OH'/OH- tran~formation.~~ Our results are silent on the details, but interestingly the amount of PBP-0' expected via OH- loss is about 40% contrary to the observed 75 % of the product mixture. Additional electron transfer via an inner-spheretransition state, different from the OH adduct, cannot be precluded. Dehalogenation to Phenyl Radical via Electron Attachment. The transient absorption spectrum (Figure 6) of the product(s) from the reaction between PBP-Oand e-aqshows two bands at -330 and -470 nm. These transient absorptions decayed concurrently via secondorder kinetics: 2k/e = 5.4 X lo5~ ~ ( 3nm) 3 0 and 8.6 X 105 s-l (460 nm), Figure 6 inset. Phenoxide ions react with electrons to give the corresponding electron adduct which loses halogen to produce isomeric hydroxyphenyl radicals (eq 9).25v42p45 Their bimolecular rate of formation, k, = 2.6 X 1Olo M-l s-l, was monitored as the decay of the transient absorption at 715 nm (e-*,), which is 1 to 2 orders of magnitude greater than those of the 2-bromophenoxide and 4-chlorophenoxide anions (Table IV).42145
F
F
-
