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Electron paramagnetic spin trapping detection of free radicals generated in direct photolysis of 4-bromophenol in aqueous solution
J. Phorochem. PhotobioI. A: Chem., 73 (1993) 23-33
23
Electron paramagnetic spin trapping detection of free radicals generated in direct photolysis of 4-bromophenol in aqueous solution Ewa
Lipczynska-Kochany” and Jan Kochanyb*+
Ykiversity of Waterloo, Waterloo Centre for Groundwater Research, Waterloo, Ont., N2L 3Gl %stitt& for Environmental Protection, ul. Krucza Z/11, 00548 Warsaw (Poland} (Received
September
22, 1992; accepted
February
(Canada)
25, 1993)
Abstract of 4-bromophenol in aqueous solution in the presence of the spin trap 5,5-dimethylpyrroline-Nresults in the formation of four distinct electron paramagnetic resonance (EPR) spectra. Three of these have been identified as arising from the DMPO spin adducts of an aryl radical (probably the 4hydroxyphenyl radical, spectrum A), a hydrated electron plus later protonation (spectrum B) and the hydroxyl radical (spectrum C). Spectrum D is that of the p-benzosemiquinone anion. Spectra A and B appear immediately on initiation of photolysis in both deoxygenated and oxygenated solutions. Spectra C and D arise from secondary products and require the presence of oxygen. The component EPR spectra do not depend on the pH value in the pH range 7.0X 10.5, but the overall appearance of the spectra changes with increasing pH, showing a larger contribution of both adducts A and B compared with adduct C. The results are compared with those of previous EPR investigations of 4-chlorophenol and with photochemica1 studies of 4-substituted halogenophenols using conventional sources of irradiation and the flash photolysis technique.
The photolysis oxide (DMPO)
The mechanism
of aqueous
photolysis
of the compounds
1. Introduction In the last decade, significant progress has been made in the aquatic photochemistry of halogenophenols and other organic pollutants. However, the detailed mechanism of the direct photolysis of many compounds in aqueous solution still remains unclear. It is well known that the photolysis of halogenols in organic non-polar solvents leads to the homolytic fission of a carbon-halogen bond [l-3]. Yet, the mechanism of the photochemical reaction of halogenophenols in water is still unclear. In an early report Grabowski [4] suggested that the photolysis of aqueous chlorophenols results in the replacement of a chlorine atom with a hydroxyl group. Omura and Matsuura [5] reported that the preparative-scale photochemistry of 4-chlorophenol (I) in concentrated aqueous alkaline solutions yields a mixture of products including hydroquinone and 2,4_dihydroxybiphenyl. Subsequently, the photochemical transformation of I in water was reexamined by Boule et al. [6], who found that the +Present address: Canada Centre for Inland Waters, P. 0.5050, 867 Lakeshore Rd., Burlington, Ont., L7R 4A6, Canada.
lOlO-6030/93/$6.00
is discussed.
reaction is non-specific giving p-benzoquinone, hydroquinone, biphenyls and polyphenolic oligomers. The results of recent studies on the photochemistry of I [7, 81 and 4-bromophenol (II) [9] using flash (xenon) photolysis, with the analysis of intermediate products by high performance liquid chromatography (HPLC) have suggested that p-benzoquinone (III) is the main primary photoproduct in aqueous solutions containing oxygen. The technique of spin trapping, with electron paramagnetic resonance (EPR) detection of free radical intermediates [lo], was also used in the direct photolysis of I [ll]. It was found that the irradiation of I in the presence of the spin trap 5,5-dimethylpyrroline-N-oxide (DMPO) resulted in the formation of the DMPO spin adducts of the hydrogen atom (or a hydrated electron plus later protonation), an aryl radical (probably 4-hydroxyphenyl radical) and a hydroxyl radical. The presence of oxygen did not influence the rate of disappearance of I [8]. Since the yield of III decreased in nitrogen-saturated compared with aerated solutions, it was suggested that the second oxygen atom in p-benzoquinone originated from dissolved molecular oxygen [ll]. However, the
D 1993 - Elsevier Sequoia.
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24
E. Lipczymka-Kochany, J. Kochony I EPR detection
proposed free radical mechanism did not explain the formation of hydroquinone (IV) as the main product of the reaction of I in degassed solution. This, together with the rather unexpected direct formation of III, has prompted two European groups [12, 131 to initiate studies on the aqueous photochemistry of I. Thus Brown and coworkers [12] have re-investigated the flash-induced photolysis of I with HPLC analysis. They confirmed that the main primary photoproduct of the transformation is III, but questioned the role of molecular oxygen in the formation of this compound, and suggested a mechanism based on a nucleophilic substitution. Unfortunately, their attempt to detect any expected transient species using microsecond flash photolysis was unsuccessful, possibly because of the time scale of the applied method [12]. Oudjehani and Boule [13] studied the reaction under steady state conditions at A = 296 nm, using a monochromator and a high-pressure mercury lamp. They observed that irradiation of an airsaturated aqueous solution of I gave III as the main photoproduct. In dilute solutions, hydroquinone IV was also detected, whereas in more concentrated solutions, 5-chloro-2,4-dihydroxybiphenyl was identified. Hydroquinone was the main product of the reaction in deoxygenated solution [13], suggesting that it was not formed solely by oxidation of the 4-hydrowhenyl radical. Since the yield of IV was not influenced by pH in the range l-4, the possibility of the photosubstitution of a chlorine atom by -OH was eliminated, and a photohydrolysis mechanism was proposed to explain the formation of IV [13]. As our recent results [9] reveal that the photochemical behaviour of aqueous II is very similar to that of I [8], we decided to extend our EPR studies to the investigation of II, hoping to obtain new evidence which may be helpful in understanding the mechanism of direct photolysis of 4substituted halogenophenols in aqueous media. Since its discovery [lo], the technique of spin trapping with EPR detection of free radicals has been used to observe a wide range of reactive free radicals generated in chemical transformations. We recently used this technique to investigate the reactions of carbonate, bicarbonate and phosphate anions with hydroxyl radicals generated by the photolysis of hydrogen peroxide [14]. In this work we have employed this method, using DMPO as spin trap, to detect and identify the radical intermediates generated in the direct photolysis of aqueous II in the pH range 7.0-10.5. We also investigated the effect of nitrous oxide, oxygen and
of free radicals in photolysis of Cbromophenol
argon on radical formation. The results have been compared with those of our previous EPR studies of I [ll] and with photochemical studies performed by other workers and ourselves.
2. Experimental
details
2.1. Materials All chemicals used in the experiments were ACS reagent grade (Aldrich Chemical Company). All solutions were prepared using doubly distilled water. Fresh bottles of DMPO were stored at - 20°C in a freezer under nitrogen. Open bottles of DMPO were also stored in the freezer and only used if the liquid was clear and colourless. Prepurified compressed gases (argon, oxygen and nitrous oxide) were obtained from Air Products, Alletown, PA, USA. 2.2. Apparatus EPR spectra were obtained using a Bruker model ESP 300 EPR spectrometer, coupled with a computer for data acquisition and instrument control. A TM102 cavity fitted with an aqueous solution sample holder was used. A 150 W mercury-xenon lamp (Conrad Hanovia 901 BOOl) was used as the light source. During irradiation, a quartz water filter was placed in the light beam to protect the cavity from heating. The pH values of the sample sohrtions were adjusted using a model pH1 10 pH-meter (Beckman, USA) with a pH combination electrode. 2.3. Methods Freshly prepared samples {always 1.0 mM in II and 2.0 mM in DMPO) were purged with argon, oxygen or nitrous oxide for about 20 min. DMPO was added to the sample (in darkness) just a few minutes before measurements. The solutions were pumped through Teflon tubing into an EPR flat quartz cell and irradiated with the W lamp. EPR spectra were detected after a fixed period of photolysis (3-90 s) of the solution. In some experiments, the time course of the growth of the EPR signal for various lines in the spectrum was measured as described in ref. 11. The pH effect (pH 7.0-10.5) on the photochemical reaction of aqueous II (c = 1.0 mM) was studied using potassium phosphate buffer solutions, prepared from 0.2 M Na,HPO, stock solution and 0.1 M NaOH solution and adjusted to a suitable pH using the pH meter. To remove metal ion impurities, phosphate buffers were passed through
E. Lipclynska-Kochany, J. Kochany / EPR detection
of free radicals in photo&s af I-bmmophenol
25
a Chelex-100 column, as suggested by Filkenstein et al. [1.5]. No radical adducts, which could have been formed in the photochemical reaction of the spin trap, were observed during a blank/control experiment. The fact that hydroxyl radical trapping really occurred was verified by the test with ethyl alcohol as described in the literature [16]. Simulated EPR spectra were calculated to second order using the Bruker simulation program EPRCALC.ESP (1600 SOFTWARE) provided with the Bruker spectrometer. 3. Results 3.1. Analysis of the spin adduct electron paramagnetic resonance When II is photolyzed by UV light in the EPR cavity in the presence of DMPO at pH 7.0-8.5, several EPR spectra can be detected (see Fig. l(a) for the spectra at pH 8.5). Figure l(b) is a computer simulation of the three simulated spectra A, B and C (Fig. 2) in the ration 3.3:1.0:2.0. These simulated EPR spectra correspond to the following radicals: an aryl (phenyl-type)-DMPO spin adduct (spectrum A) based on the parameters a”(1) = 2.44 mT, a”(1) = 1.58 mT and g=2.0053 [17], the hydrogen atom-DMPO spin adduct (spectrum B) based on the parameters aH(2)=2.25 mT, aN(1)=1.66 mT and g=2.0054 [lg] and the ‘OH radical-DMPO spin adduct (spectrum C) based on the parameters nH(l) =a”(l)= 1.49 mT and g= 2.0061 [19]. All the spectral features are well accounted for by the simulated EPR spectrum in Fig. l(b). We have found that we can account for almost all of the EPR spectra under various conditions by simulations made up of varying proportions of the three base EPR spectra. In addition, at higher pH values, the EPR spectrum of the pbenzosemiquinone anion radical (D) is observed {see Fig. 3(d)). The generation of this species is independent of the presence of DMPO as shown in Fig. 3(d). The EPR spectra of radicals A-D detected during the photolysis of II are identical with those observed during the irradiation of I [ 111. However, no decomposition of DMPO, which occurred during experiments with I [ll] (and resulted in a radical with a single interacting nitrogen nucleus with aN= 1.56 mT and g=2.0064), was observed during irradiation of II. This was probably due to the fact that the intensity of the EPR signals obtained for II was much higher than that of those recorded for I, and so a shorter time of irradiation was required to obtain a good spectrum of II.
Fig. 1. (a) EPR spectrum of DMPO spin adducts recorded after 60 s of UV irradiation of 11 in aqueous solution at pH 85; EPR spectrometer settings: centre field, 349.505 mT; sweep width, 10.0 mT, scan time, 5.243 s; number of scans, 50; microwave frequency, 9.77 GKz, modulation amplitude, 0.090 mT, microwave power, 20 mW; spectrometer gain, 1.0X 10s. (b) Composite computer simulation of three simulated spectra: an aryl radical-DMPO spin adduct (A), the DMPO-H spin adduct (B) and the DMPO4)H spin adduct (C) in the ratio 3.3 : 1.0 : 2.0 respectively.
3.2. EPR spectra for various times of irradiation Figure 4 illustrates the EPR spectra at pH 8.5 taken after 3, 15, 45 and 90 s of UV irradiation of II. The spectra of the adducts A and B rise immediately on initiation of UV irradiation, indicating that these are obtained from primary radicals. It can be seen from a comparison of these spectra (and confirmed by simulation, not shown in Fig. 4) that the ‘OH spin adduct spectrum increases relative to the others as the time of irradiation is increased. 3.3. Effects of oqgen, argon and nitrous oxide When solutions of II (at pH 7-8.5) were irradiated in the presence of oxygen, all three adducts A, B and C were observed (Figs. 5(a) and 6(b)). On purging with argon, the intensity of A (DMPO-aryl) and B (DMPO-H) increased (Figs.
26
E. Lipqvmka-Kochany, J. K&any
I EPR detection of free radicals in photo&s
of Cbromophenol
b)
k cl
Fig. 2. (A) Simulated spectrum of an atyl radical-DMPO spin adduct (a”(l) = 2.44 mT. ~~(1) = 1.58 mT; gz2.0053). (B) Simulated spectrum of the hydrogen atom-DMPO spin adduct (aH(2)= 2.25 mT, a”(l)= 1.66 mT, g=2.0054). (C) Simulated spectrum of the DMPO-OH spin adduct (aH(l) =aN(1)=1.49 mT; g= 2.0061).
5(b) and 6(a), whereas that of C (DMPO-OH) and D (p-benzosemiquinone anion) decreased (Fig. 6(a))When samples were gassed with NzO, the EPR signal of adduct DMPO-H (B) disappeared, and the intensity of the adduct DMPO-OH (C) was enhanced significantly (Fig. S(c)). 3.4. Eflect of pH As can be seen from Fig. 3, the component EPR spectra do not depend on pH in the investigated pH range 7.0-10.5, except that, at higher pH (in the solutions containing oxygen), the EPR spectrum ofp-benzosemiquinone anion (D) is more pronounced. However, the overall appearance of the EPR spectra changes with increasing pH, showing a larger contribution of both adducts A and B compared with that of C (Figs. 6(b) and 7(b)). In the presence of argon, the increase in the intensity of signals A and B on alkalinization
Fig. 3. Comparison of the EPR spectra of the DMPO spin adducts recorded after 60 s of WV irradiation of 11 in aqueous solutions at pH 7.0 (a), pH 85 (b) and pH 10.5 (cc). (d) EPR spectrum of the p-benzosemiquinone anion (D) detected after analogous irradiation of II (pH 10.5) without DMPO. EPR spectrometer settings as in Fig. l(a).
E. Lipczynska-Kochany,
J. Kochany I EPR detection of free radic&
in photo&sti
of I-bromophenol
27
b)
Fig. 4. EPR spectra from the photolysis of II in the presence of DMPO in an undegassed solution at pH 8.5, recorded after 3 s (a), 15 s (b), 45 s (c) 90 s (d). EPR spectrometer settings
as in Fig. I(a).
of the solution is more visible than that observed in the presence of oxygen (Figs. 6 and 7). The pK, value of II in the ground state is 9.3 [20]. On excitation to the singlet state, the pK, value drops to pK,‘* =2.9 [20]. Since, in the case of phenols, pKaT’ (in the triplet state) is usually much closer to the pK, value in the ground state than to that in the singlet excited state (pKas’)
Fig. 5. EPR spectra of DMPO spin adducts recorded after 60 s of UV irradiation of II in aqueous solution (pH 8.5), bubbled with oxygen (a), argon (b) and nitrous oxide (c). EPR spectrometer
settings as in Fig. l(a).
[21], it seems reasonable to assume that the pKaTW value of II is approximately 9. It follows that, at pH 10.5, most of the molecules exist in the anionic form. 4. Discussion These observations lead us to the conclusion that the only primary free radicals generated during
E. Lipcgw&t-Kochany,J. Kochany I EPR d&Won ofJiee radicalsin photo& 3_iZPR signal Intensity
of 4.bromophenoC
30EPR slgnel intensity
a)
4 25 -
10
20
30
irrsdiation
40
50
60
time [SW]
irrtiiation
VI signal intensity
time [set]
PR signal intensity b) /
0
10
20
30
irrediatlon
40
50
25 -
60
time [secl
0
IO
20 irradiation
30
40
50
60
time [ssc]
recorded during photoirradiation of II at pH 8.5; (a) in the presence of argon; (b) in the presence of oxyfp. (A) Aryl radicaLDMP0 spin adduct; (8) DMPO-H addud; (c) DMPO-OH adduct; (d) p-bcnzosemiquinone anion.
Fig. 7. Changes in the relative intensity of the EPR sip& recorded during photoirradiation of II at pH 10.5; (a) in the presence of argon; (b) in the Presence ti oxygen. (A) Aryl radical-DMPO spin adduct; (B) DMW-H adduct; (C) DMPO-OH adduct; (D) p-benzosemiquinone anion.
the photolysis of II are hydroxyphenyl) radical followed by protonation. ment with the results of
of I 1111 as well as with those reported for fentichlor ([bis(2-hydroxy-5-chlorophenyl)sulphide], V) and bithionol ([I&(2-hydroxy-?+dichlorophenyl)sulphide], VI) 1221. Photoionization and photo-
Fig. 6. Changes in Ihe relative intensity of the EPR signals
an aryl (probably the 4and a hydrated electron They are in good agreethe previous EPR studies
E. Lipcqmka-Kochmy,
J. Kochany / EPR detection of f?ee radicals in phoro&sis of I-bromophenol
29
dechlorination of the compounds I, V, and VI, as well as the generation of DMPO-OH adducts, were observed [ll, 221. OH
+
H,O
0 0
OH It should be noted that excited bromophenols were not expected to give aqueous electrons, since no such species was detected directly by flash spectroscopy [24], probably because its reaction with these compounds is very fast (k = 7 X lo9 M-’ s-‘forIIandk=2.9x10-gM-‘s-1foritsanionic form [26, 291). The hydrated electron acts as a powerful nucleophile in its reaction with aromatic compounds. Its reactivity is increased when the molecule contains a halogen atom. It is believed that the carboanion VII is the primary product of the reaction between e,, and halogenoarenes
-
+
H+
Cl-
+
OH
Homolytic halogen-carbon (where X is a halogen atom) Ar-x
t
-5
[Ar--x1*
-_)
bond is known
[AC*xj
-_)
cleavage to occur Ar’ + X’
1
recombination
(1)
products
in many halogenoaromatic compounds [23], including bromophenols irradiated in non-polar solvents [24]. The formation of hydroquinone and sym-dihydroxybiphenyl was observed during steady irradiation of II in water, and so a free radical mechanism, involving a photolytic bromine-carbon bond cleavage [25], was also suggested. Thus, the carbon-centred radical, trapped as the adduct A (Figs. 1 and 2(a)) during the photolysis of II, arises primarily from the homolytic fission of the carbon-bromine bond of II. As shown in Fig. 5, in the presence of nitrous oxide, the EPR signal of adduct DMPO-H (B) disappears and the intensity of the DMPO-OH (C) adduct increases. Since it is known that N,O undergoes a fast (k = 9.1 X 10’ M-’ s-’ [2.5, 261) reaction with the aqueous electron giving hydroxyl radicals (eqn. (2)), we conclude that the observed adduct B arises from the aqueous electron plus protonation N,O+e-+H,O-
HO’+HO-+N,
(2)
Electron ejection from excited aqueous phenols [27] has been studied in detail [28]. The results of the investigations reveal that electron ejection (b) occurs before deprotonation of the molecule (c)
ArX + eaq --+
[ArX]‘VII
(4)
Formation of such a radical anion would initiate a radical chain mechanism of nucleophilic substitution (SR-N1), as was suggested by Bunnett [30]. In the next step, radical anion VII would fragment to form radical Ar’ (in our experiment trapped by DMPO as the adduct A) and halide anion X[ArX]‘_ -
Ar’=x-
(5)
In deaerated dilute solutions, radicals Ar’ may undergo a nucleophilic substitution with another nucleophile, for instance -OH Ai + HO[ArOHj’-
-
[ArOH]‘-
+ ArX -
or react with water, Ai+
H,O --)
(6)
ArOH + [ArX]--
(7)
giving hydroquinone
[ArOH,]’
-
ArOH + H
(8)
Another explanation for the formation of hydroquinone as a main product in oxygen-free solutions of 4-halogenophenols could be similar to that proposed by Dulin et al. [31] for the aqueous photolysis of monochloroaromatic compounds. According to this mechanism, the initial step is the homolytic carbon-halogen cleavage to the radical pair [Ar . . X], which may diffuse either as a radical species as shown in eqn. (1) or as an ionic species (in aqueous media) as shown in eqn. (9). The aryl cation (VIII) would then be rapidly scavenged by water
30
Ar-X-
E. tipczynska-Kochany,
[Ar-*Xl%
J. Kochany I EPR detection
Al-+-l-X-+H,OVIII
ArOH,++X-
-
ArOH+H++X(9)
To our knowledge, direct experimental proof for the formation of VIII during photolysis of halogenophenols is still lacking. As can be seen from Figs. 6(a) and 7(a), alkalinization of the oxygen-free solution leads to an increase in the intensity of signal B, which is consistent with the fact that the yield of the eag ejection from phenols increases on alkalinization [28]. The enhanced intensity of the adduct B is accompanied by an increase in the intensity of A, which suggests that the reaction with aqueous electron (eqn. (4)) contributes to the formation of the aryl-centred radical in the direct photolysis (eqn. (1)) of II. In concentrated solutions, aryl radicals Ai can react with themselves or with the molecules of halogenophenol. A coupling product, namely 4,4’dihydroxybiphenyl, was detected during the steady irradiation of II (c =2X 10m3 M) in deaerated water [24], whereas 4,4’-dihydroxybiphenyl and 5chloro-2,4’-dihydroxybiphenyl were identified among other products of photolysis of aqueous I (c=2x lop3 M) [13]. In the presence of oxygen, the concentration of the adduct B (DMPO-H) decreases (Figs. 6 and 7), probably due to the fast reactions of aqueous electron and hydrogen with 0, (eqns. (lo)-(12) [26, 321 and with hydroxyl radicals and benzoquinone III (eqns. (13)-(15) [26], products of the photolysis of II e,,+O,--+
OZ.+
H-+0,-
HO;
K + HO; -
H,O,
e,,+HO’-
HO-
eaq + III -
k=2xlO’” k=l.B~lO’~
M-’
k= l(J’O M-l k=3X101’
(10)
SK’
(11)
s-’
(12)
M-‘s-r
(13)
4-(‘OC,&O-) k=2.3x101’
II- + III -
M-‘s-l
M-’
s-’
(14)
M-’
s-’
(15)
4-(‘OC,H,-OH) k=8.3x10r0
On bubbling the solution of II with oxygen, the concentration of the aryl-centred radical (A) also decreases, which may be related to the fact that its generation in reactions (4) and (5) is less efficient as some aqueous electrons are scavenged in r-eactions (lo), (13) and (14). However, since the
of freeradicals in photoiysb of I-bromophenol decline of the concentration of A is accompanied by an increase in the concentration of hydroxyl radicals (C) and p-benzosemiquinone anion (III, D) (Figs. 6 and 7), we can suggest the following mechanism for the decay of aryl radicals Ar’ + O2 ArOo’ ArOOH
ArOO
(16)
--!% ArOOH
(17)
z
ArO’+‘OH
(18)
We have previously observed [ll] that the intensity of the DMPO-OH andp-benzosemiquinone EPR signals, generated during the photolysis of I, are enhanced with increasing oxygen concentration. A similar effect of oxygen on the EPR spectra of V and VI was observed by Li and Chignell [22, 331, who employed “02 and H,l’O to demonstrate that the trapped hydroxyl radical arose from dissolved oxygen and not from water. They also showed that hydroxyl radicals did not derive from superoxide or hydrogen peroxide. Thus the formation of the aryldioxyl radical (ArOO’) as a key intermediate was taken into consideration [22]. The aryldioxyl radical formed during the photodechlorination of chloropromazine was trapped by DMPO at pH 4 where it was more stable. Formation of p-benzoquinone from irradiated 4-halogenophenols was also demonstrated by the EPR studies reported by Delahanty et al. [34] and Evans et al. [35]. Results of our experiments, using flash photolysis/HPLC, showed that p-benzoquinone was the main primary product of the photolysis of I [S] and II [9] in oxygen-containing solutions. We also observed that, on bubbling with argon, the yield of III dropped off indicating that the photoreaction of I involved oxygen as a reactant [8]. A similar effect of oxygen on the photochemistry of I was also reported by Oudjehani and Boule [13], who applied a conventional source of photoirradiation. We have shown that hydroxyl radicals arise from a secondary photochemical reaction of I and II. Since the main primary photoproduct, p-benzoquinone III, is photolabile [37-391, and its excited molecule abstracts hydrogen from water releasing a free hydroxyl group as shown in eqn. (19) [39], the photolysis of III may be an important source of the detected ‘OH radicals. III=’ + H,O -
4 - (HO-C,II,O-)
+ ‘OH
(19)
However, the results of our present and former [ll] EPR studies are very similar to those reported by Li and Chignell [22], who provided strong evidence that molecular oxygen is the source of hydroxyl radicals generated during the photolysis
E. Lipczyn.k-Kochany,
J. Kochany I EPR detection of free radicals in phofolysis of 4-brmwphenol
of 4-halogenophenols. Thus it seems likely that aryl peroxide photolysis, as shown in reaction (18), is also a source of these radicals. The detection of free radicals, generated during the photolysis of IL (and I), does not rule out the possibility of other non-radical routes. Previous studies have shown [40] that hydroquinones as well as quinones generate semiquinones on TJV irradiation. The autoxidation of hydroquinones and quinones in alkaline solution gives semiquinones as the primary products of oxidation [40]. Thus we could argue that p-benzoquinone, detected in irradiated aerated solutions of 4-halogenophenols, is not the primary product of their photolysis, but arises from the photo-oxidation of hydroquinone, generated in a heterolytic-type reaction. The formation of hydroquinone from I, in a concerted reaction with a transition state (IX), was taken into consideration [41a].
IX Brown and coworkers [12] suggested that the mechanism may be similar to that of the SR+N reaction, proposed for the photolysis of halogenoanisoles [42]. It involves the formation of a radical carbocation X, which is subsequently attacked by a nucleophile N- (eqn. (21)) ArX ---+ ArX* -
ArX’+N-
-
AI-N+ t eag -
ArX+ X ArXNXI A-N
+ eas
ArN’* +X-
(20)
(21)
(22)
However, thus far, no direct evidence has been reported to demonstrate the short-lived intermediate species XI generated during photoirradiation of halogenophenols. Since the formation of hydroquinone from I is not influenced by the pH in the range l-6, the hypothesis of the photosubstitution of chlorine by -OH has been criticized, and a mechanism involving the attack of water, as shown in eqn. (23), has been suggested [13]
On the basis of theoretical considerations, we can expect that S,(T, r*) states, which commonly have a zwitterionic character, will result in nucleophilic or electrophilic additions. Unless the product is formed in the triplet state or a good spin-orbit coupling mechanism is available, T,(z-, T*) states usually undergo primary photoreactions characteristic of radicals, including homolytic fragmentation [21]. The primary photochemical processes of both S,(n, n*) and T,(n, v*) also produce radicals [21]. Thus, it is the r, rTT*state rather than the n, rTT* state from which nucleophilic substitution may easily start. Further experiments are necessary to determine the nature of the excited state from which photolysis of 4-halogenophenols begins. However, it is known that the substitution of Br (or Cl) for H in an aromatic ring results in a significant increase in the rate (kST) and efficiency (c#+) of intersystem crossing S,-T,. Therefore it is unlikely that the photolysis of II (and also of I) involves molecules in the S1(r, 77”) state. Photonucleophilic aromatic substitution reactions often involve an interaction between the aromatic T,(r, T*) state and a nucleophile [42]. Consequently, if the photolysis of 4-halogenophenols involves molecules in the T,(r, v*) state, the S,2 mechanism is theoretically possible. However, the S,2 mechanism usually operates for molecules containing an electron-withdrawing substituent, such as a nitro group. Therefore the S,-,l mechanism (eqn. (4)) to form an axyl radical (eqn. (5)), trapped in our experiments as the DMPO adduct A, seems more probable in the case of 4-halogenophenols. We should also take into consideration the possibility of the formation of the aromatic cation VIII (eqn. (9)), which could be attacked by water (or -OH) to give hydroquinone, the principal primary product observed in deoxygenated solutions. The quantum yield of photochemical transformation of hydroquinone in the presence of oxygen is relatively low (4 = 0.05 [43] compared with 4 = 0.5 for p-benzoquinone [36] and 4 ~0.24 for I [6]). Thus it does not appear to be very plausible that p-benzoquinone., reported as a main primary pho-
32
E. Lipuynska-Kochany,
J. Kochany / EPR detection of free radicals in photo&sti of I-bromophenol
toproduct in oxygenated solutions, is actually a product of hydroquinone oxidation. As already indicated, the observation of the generation of free radicals does not mean that they have a direct relationship with the main route of the reaction. Further investigation is required to conclude the role which may be played by the species observed by the EPR method. However, in the case of halogenoaromatic compounds, intersystem crossing followed by homolytic cleavage (eqn. (1)) often predominates over other mechanisms of photosubstitution [21, 231. While an energy of 114 kcal mall’ (A=254 nm) (or 95.3 kcal mol-’ (A= 300 nm)) [21] may be accepted by a molecule of II, it would require approximately 85 kcal mol-’ to eject an electron (eqn. (3)) and only about 67 kcal mol-’ to break a bromine-carbon (eqn. (1)) [24]. Thus oxidation of aryl radicals (formed in reactions (1) (4) and (5)) should be taken into consideration as a meaningful route in the photochemical transformation of II in dilute oxygenated solutions i.e. under environmentally meaningful conditions. Comparison of the results of the EPR studies and photochemical investigations of II and I suggests a similar mechanism for both reactions, Since the main photoproduct of the reaction, p-benzoquinone, is very photolabile, the photolysis of 4-halogenophenols should be regarded as an important route for their environmental degradation.
5. Conclusions Our spin trapping EPR experiments indicate that the photolysis of 4-bromophenol(I1) in aqueous (oxygenated and deoxygenated) solutions in the pH range 7.0-10.5 produces two primary radicals. One is an aryl radical (A), probably the 4-hydroxyphenyl radical, and the other (B) arises from the photoionization of II followed by PTOtonation of the hydrated electron. The aryl radical is formed in two routes: (i) by the photolytic cleavage of the carbon-bromine bond; (ii) in the reaction between photoejected aqueous electrons and II. In the presence of oxygen, two spectra of secondary products are also observed; the spin adduct of the hydroxyl radical (C) and the pbenzosemiquinone radical anion (D). Since the spectra of C and D can be detected only in oxygenated solutions, they are both concluded to be the products of a reaction involving molecular oxygen. These results are analogous with those obtained during our previous EPR investigations of 4-chlo-
rophenol (I), and suggest that the formation and oxidation of aryl radicals may play an important role in the aqueous photolysis of 4-halogenophenols in aerated solutions.
Acknowledgments The authors are grateful to Dr. T. Muller for access to the EPR spectrometer and to Mr J. Smitt for technical assistance. Referees’ comments are greatly ZippreCiated.
References 1 W. Wolf and N. Kbarasch, J. Org. Chem., 30 (1965) 2493. 2 T. Matsuura and K. Omura, Bull. Chem. Sot. Jpn., 39 (1966) 944. 3 G. E. Robinson and J. M. Vermon, Chem. Commun., (1969) 977. 4 Z. R. Grabowski, Z. Phys. Chem., 27 (1961) 239. 5 K. Omura and T. Matsuura, Tetrahedron, 27 (1971) 3101. 6 (a) P. Boule, C. Guyon and J. Lemaire, Chemosphere, 11 (1982) 1179; (b) P. Boule, C. Guyon, A. Tissot and J. Lemaire, L Chim. Phys, 82 (1985) 513. and J. R. Bolton,J. Chem. Sot., Chem. 7 E. Lipczynska-Kochany Commun.. (1990) 1596. and J. R. Bolton, 1. Photochem. Pho8 E. Lipcaynska-Kochany robiol. A: Chem., 58 (1991) 315. Chemosphere, 24 (1992) 911. 9 E. Lipczynska-Kochany, 10 E. G. Janzcn, Ace. Chem. Res., 4 (1971) 31. 11 E. Lipczynska-Kochany, J. Kochany and J. R. Bolton. J. Photochem. Photobiol. A: Chem., 62 (1992) 229. 12 A.-P. Y. Durand, D. Brattan and R. G. Brown, Chemosphere, 25 (1992) 783. 13 K. Oudjehani and P. Boule,l. Phorochem. Photobiol. A: Chem., 68 (1992) 363. Chemosphere, 25 14 J. Kochany and E. Lipczynska-Kochany, (1992) 1769. 15 E. Filkenstein, G. M. Rosen and E. J. Rauckman, J Am. Chem. Sot., 102 (1980) 4994. 16 E. G. Adams and P. Wardman, in W. A. Pryor (ed.), Free Radicals in Eio&v, Vol. III, Academic Press, New York, 1977, pp. 53-95. 17 H. Kaur, K. H. Leung and M. J. Perkins, 1. Chem. Sot., Chem. Commun., (1981) 142. Detection of toxic free radicals in biology 18 B. Kalyanaraman, and medicine, in R. A. Floyd, (ed.), Reviews in Biochemical Toxicology and Cancer, Marcel Dekker, New York, 1982, p. 397. 19 J. R. Harbour and J. R. Bolton, Photochem. Photobiol., 28 (1978) 231. 20 W. Bartok, R. B. Hartman and P. J. Lucchesi, Photochem. Photobiol., 4 (1965) 499. 21 N. J. Turro, Modern Molecular Photochemistry, Benjamin/ Cummings, Menlo Park, CA, 1978. 22 A. S. W. Li and C. F. Chignell, Photochem PbotobioI.. 46 (1987) 445. 23 J. Grimshaw, Chem. Sot. Rev., 20 (1981) 181. 24 H. I. Joschek and S. I. Miller, J. Am. Chem. Sot., 88 (1966) 3269.
E. Lipuynska-Kochany, J. Kochany I EPR detection of Ji-ee radicals in photo&is of I-bromophenol 25 B. Hickel and K. H. Schmidt, /. Phys. Chem., 74 (1970) 2470. 26 G. Buxton, C. L. Greenstock, W. P. Helman and A. B. Ross, 1. Phys. Chem. Ref: Data, 17 (1988) 514. 27 E. J. Land, G. Porter and E. Strahan, Trans. Farodny Sot., 57 (1961) 1885. 28 (a) J. Jortner, M. Ottolenghi and G. Stein, I. Am. Chem. Sot., 85 (1963) 2712; (b) G. Grabner, G. Kohler, J. Zecher and N. Getoff, Phorochem, PhorobioL, 26 (1977) 449; (c) G. Grabner, G. Kohler, J. Zecher and N. Getoff, J. Phys. Chem., 84 (1980) 3000; (d) J. C. Mialocq, J. Sutton and P. Goujon, J Chem. Phys., 72 (1980) 6338; (e) N. Getoff, Radiur. Phys. Chem., 34 (1989) 711. 29 M. Anbar and E. J. Hart, J. Am. Chem. Sot., 86 (1964) 5633. 30 J. F. Bunnett, Act. Chem. Res., I1 (1978) 413. 31 D. Dulin, H. Drossman and T. Mill, Environ. Sci. Technol., 20 (1986) 72. 32 N. Getoff and S. Solar, Radiar. Phys. Chem., 31 (1988) 121. 33 A. S. W. Li and C. F. Chignell, Photochem. Photobiol., 49 (1989) 2.5. 34 J. N. Delahanty, J. C. Evans, C. C. Rowlands and M. D. Barratt, .I. Chem. Sot., Faraday Trans. 1, 83 (1987) 135.
33
35 J. C. Evans, C. C. Rowlands, L. A. Turkson and M. D. Barratt, J. Chem. Sot., Faraday Trans. 1. 84 (1988) 3249. 36 S. Hashimoto, K. Kane and H. Okamoto, BuU. Chem. Sot. Jpn.. 45 (1972) 966. 37 A. Rossi, G. Guyot and P. Boule, C. R. Acad. Sci., Paris, 303 (1986) 1179. 38 (a) A. I. Onoye, A. R. McIntosh and J. R. Bolton, 1. Phys. Chem., 90 (1986) 6266; (b) A. I. Onoye and J. R. Bolton, /. Phys. Chem., 90 (1986) 6271. 39 Y. Kambara and H. Yoshida, Bu!l. Chem. Sot. Jpn., 50 (1977) 1367. 40 (a) P. Ashworth and W. T. Dixon, J. Chem. Sot., Perkin Trans. II, (1972) 1130; (b) J. Pedersen, J. Chem. Sot., Perkin Trans., II (1973) 424. 41 (a) J. Comelisse and E. Havinga, Chem. Rev., 75 (1975) 353; (b) J. Comelisse, G. P. De Gunst and E. Havinga, Adv. Phys. 0~. Chem., II (1975) 225. J. Konijnenberg, J. Comelisse and C. 42 (a) H. Lemmetyinen, A. G. 0. Varma, I. Photochem., 30 (1985) 315; (b) J. den Heijer, 0. B. Shadid. J. CornelisseandE‘. Ha&a, Ti%ahedron, 33 (1977) 779. 43 P. Boule, A. Rossi, J. F. Pilichowski and G. Grabner, New. J. Chem., in the press.
23
Electron paramagnetic spin trapping detection of free radicals generated in direct photolysis of 4-bromophenol in aqueous solution Ewa
Lipczynska-Kochany” and Jan Kochanyb*+
Ykiversity of Waterloo, Waterloo Centre for Groundwater Research, Waterloo, Ont., N2L 3Gl %stitt& for Environmental Protection, ul. Krucza Z/11, 00548 Warsaw (Poland} (Received
September
22, 1992; accepted
February
(Canada)
25, 1993)
Abstract of 4-bromophenol in aqueous solution in the presence of the spin trap 5,5-dimethylpyrroline-Nresults in the formation of four distinct electron paramagnetic resonance (EPR) spectra. Three of these have been identified as arising from the DMPO spin adducts of an aryl radical (probably the 4hydroxyphenyl radical, spectrum A), a hydrated electron plus later protonation (spectrum B) and the hydroxyl radical (spectrum C). Spectrum D is that of the p-benzosemiquinone anion. Spectra A and B appear immediately on initiation of photolysis in both deoxygenated and oxygenated solutions. Spectra C and D arise from secondary products and require the presence of oxygen. The component EPR spectra do not depend on the pH value in the pH range 7.0X 10.5, but the overall appearance of the spectra changes with increasing pH, showing a larger contribution of both adducts A and B compared with adduct C. The results are compared with those of previous EPR investigations of 4-chlorophenol and with photochemica1 studies of 4-substituted halogenophenols using conventional sources of irradiation and the flash photolysis technique.
The photolysis oxide (DMPO)
The mechanism
of aqueous
photolysis
of the compounds
1. Introduction In the last decade, significant progress has been made in the aquatic photochemistry of halogenophenols and other organic pollutants. However, the detailed mechanism of the direct photolysis of many compounds in aqueous solution still remains unclear. It is well known that the photolysis of halogenols in organic non-polar solvents leads to the homolytic fission of a carbon-halogen bond [l-3]. Yet, the mechanism of the photochemical reaction of halogenophenols in water is still unclear. In an early report Grabowski [4] suggested that the photolysis of aqueous chlorophenols results in the replacement of a chlorine atom with a hydroxyl group. Omura and Matsuura [5] reported that the preparative-scale photochemistry of 4-chlorophenol (I) in concentrated aqueous alkaline solutions yields a mixture of products including hydroquinone and 2,4_dihydroxybiphenyl. Subsequently, the photochemical transformation of I in water was reexamined by Boule et al. [6], who found that the +Present address: Canada Centre for Inland Waters, P. 0.5050, 867 Lakeshore Rd., Burlington, Ont., L7R 4A6, Canada.
lOlO-6030/93/$6.00
is discussed.
reaction is non-specific giving p-benzoquinone, hydroquinone, biphenyls and polyphenolic oligomers. The results of recent studies on the photochemistry of I [7, 81 and 4-bromophenol (II) [9] using flash (xenon) photolysis, with the analysis of intermediate products by high performance liquid chromatography (HPLC) have suggested that p-benzoquinone (III) is the main primary photoproduct in aqueous solutions containing oxygen. The technique of spin trapping, with electron paramagnetic resonance (EPR) detection of free radical intermediates [lo], was also used in the direct photolysis of I [ll]. It was found that the irradiation of I in the presence of the spin trap 5,5-dimethylpyrroline-N-oxide (DMPO) resulted in the formation of the DMPO spin adducts of the hydrogen atom (or a hydrated electron plus later protonation), an aryl radical (probably 4-hydroxyphenyl radical) and a hydroxyl radical. The presence of oxygen did not influence the rate of disappearance of I [8]. Since the yield of III decreased in nitrogen-saturated compared with aerated solutions, it was suggested that the second oxygen atom in p-benzoquinone originated from dissolved molecular oxygen [ll]. However, the
D 1993 - Elsevier Sequoia.
All rights reserved
24
E. Lipczymka-Kochany, J. Kochony I EPR detection
proposed free radical mechanism did not explain the formation of hydroquinone (IV) as the main product of the reaction of I in degassed solution. This, together with the rather unexpected direct formation of III, has prompted two European groups [12, 131 to initiate studies on the aqueous photochemistry of I. Thus Brown and coworkers [12] have re-investigated the flash-induced photolysis of I with HPLC analysis. They confirmed that the main primary photoproduct of the transformation is III, but questioned the role of molecular oxygen in the formation of this compound, and suggested a mechanism based on a nucleophilic substitution. Unfortunately, their attempt to detect any expected transient species using microsecond flash photolysis was unsuccessful, possibly because of the time scale of the applied method [12]. Oudjehani and Boule [13] studied the reaction under steady state conditions at A = 296 nm, using a monochromator and a high-pressure mercury lamp. They observed that irradiation of an airsaturated aqueous solution of I gave III as the main photoproduct. In dilute solutions, hydroquinone IV was also detected, whereas in more concentrated solutions, 5-chloro-2,4-dihydroxybiphenyl was identified. Hydroquinone was the main product of the reaction in deoxygenated solution [13], suggesting that it was not formed solely by oxidation of the 4-hydrowhenyl radical. Since the yield of IV was not influenced by pH in the range l-4, the possibility of the photosubstitution of a chlorine atom by -OH was eliminated, and a photohydrolysis mechanism was proposed to explain the formation of IV [13]. As our recent results [9] reveal that the photochemical behaviour of aqueous II is very similar to that of I [8], we decided to extend our EPR studies to the investigation of II, hoping to obtain new evidence which may be helpful in understanding the mechanism of direct photolysis of 4substituted halogenophenols in aqueous media. Since its discovery [lo], the technique of spin trapping with EPR detection of free radicals has been used to observe a wide range of reactive free radicals generated in chemical transformations. We recently used this technique to investigate the reactions of carbonate, bicarbonate and phosphate anions with hydroxyl radicals generated by the photolysis of hydrogen peroxide [14]. In this work we have employed this method, using DMPO as spin trap, to detect and identify the radical intermediates generated in the direct photolysis of aqueous II in the pH range 7.0-10.5. We also investigated the effect of nitrous oxide, oxygen and
of free radicals in photolysis of Cbromophenol
argon on radical formation. The results have been compared with those of our previous EPR studies of I [ll] and with photochemical studies performed by other workers and ourselves.
2. Experimental
details
2.1. Materials All chemicals used in the experiments were ACS reagent grade (Aldrich Chemical Company). All solutions were prepared using doubly distilled water. Fresh bottles of DMPO were stored at - 20°C in a freezer under nitrogen. Open bottles of DMPO were also stored in the freezer and only used if the liquid was clear and colourless. Prepurified compressed gases (argon, oxygen and nitrous oxide) were obtained from Air Products, Alletown, PA, USA. 2.2. Apparatus EPR spectra were obtained using a Bruker model ESP 300 EPR spectrometer, coupled with a computer for data acquisition and instrument control. A TM102 cavity fitted with an aqueous solution sample holder was used. A 150 W mercury-xenon lamp (Conrad Hanovia 901 BOOl) was used as the light source. During irradiation, a quartz water filter was placed in the light beam to protect the cavity from heating. The pH values of the sample sohrtions were adjusted using a model pH1 10 pH-meter (Beckman, USA) with a pH combination electrode. 2.3. Methods Freshly prepared samples {always 1.0 mM in II and 2.0 mM in DMPO) were purged with argon, oxygen or nitrous oxide for about 20 min. DMPO was added to the sample (in darkness) just a few minutes before measurements. The solutions were pumped through Teflon tubing into an EPR flat quartz cell and irradiated with the W lamp. EPR spectra were detected after a fixed period of photolysis (3-90 s) of the solution. In some experiments, the time course of the growth of the EPR signal for various lines in the spectrum was measured as described in ref. 11. The pH effect (pH 7.0-10.5) on the photochemical reaction of aqueous II (c = 1.0 mM) was studied using potassium phosphate buffer solutions, prepared from 0.2 M Na,HPO, stock solution and 0.1 M NaOH solution and adjusted to a suitable pH using the pH meter. To remove metal ion impurities, phosphate buffers were passed through
E. Lipclynska-Kochany, J. Kochany / EPR detection
of free radicals in photo&s af I-bmmophenol
25
a Chelex-100 column, as suggested by Filkenstein et al. [1.5]. No radical adducts, which could have been formed in the photochemical reaction of the spin trap, were observed during a blank/control experiment. The fact that hydroxyl radical trapping really occurred was verified by the test with ethyl alcohol as described in the literature [16]. Simulated EPR spectra were calculated to second order using the Bruker simulation program EPRCALC.ESP (1600 SOFTWARE) provided with the Bruker spectrometer. 3. Results 3.1. Analysis of the spin adduct electron paramagnetic resonance When II is photolyzed by UV light in the EPR cavity in the presence of DMPO at pH 7.0-8.5, several EPR spectra can be detected (see Fig. l(a) for the spectra at pH 8.5). Figure l(b) is a computer simulation of the three simulated spectra A, B and C (Fig. 2) in the ration 3.3:1.0:2.0. These simulated EPR spectra correspond to the following radicals: an aryl (phenyl-type)-DMPO spin adduct (spectrum A) based on the parameters a”(1) = 2.44 mT, a”(1) = 1.58 mT and g=2.0053 [17], the hydrogen atom-DMPO spin adduct (spectrum B) based on the parameters aH(2)=2.25 mT, aN(1)=1.66 mT and g=2.0054 [lg] and the ‘OH radical-DMPO spin adduct (spectrum C) based on the parameters nH(l) =a”(l)= 1.49 mT and g= 2.0061 [19]. All the spectral features are well accounted for by the simulated EPR spectrum in Fig. l(b). We have found that we can account for almost all of the EPR spectra under various conditions by simulations made up of varying proportions of the three base EPR spectra. In addition, at higher pH values, the EPR spectrum of the pbenzosemiquinone anion radical (D) is observed {see Fig. 3(d)). The generation of this species is independent of the presence of DMPO as shown in Fig. 3(d). The EPR spectra of radicals A-D detected during the photolysis of II are identical with those observed during the irradiation of I [ 111. However, no decomposition of DMPO, which occurred during experiments with I [ll] (and resulted in a radical with a single interacting nitrogen nucleus with aN= 1.56 mT and g=2.0064), was observed during irradiation of II. This was probably due to the fact that the intensity of the EPR signals obtained for II was much higher than that of those recorded for I, and so a shorter time of irradiation was required to obtain a good spectrum of II.
Fig. 1. (a) EPR spectrum of DMPO spin adducts recorded after 60 s of UV irradiation of 11 in aqueous solution at pH 85; EPR spectrometer settings: centre field, 349.505 mT; sweep width, 10.0 mT, scan time, 5.243 s; number of scans, 50; microwave frequency, 9.77 GKz, modulation amplitude, 0.090 mT, microwave power, 20 mW; spectrometer gain, 1.0X 10s. (b) Composite computer simulation of three simulated spectra: an aryl radical-DMPO spin adduct (A), the DMPO-H spin adduct (B) and the DMPO4)H spin adduct (C) in the ratio 3.3 : 1.0 : 2.0 respectively.
3.2. EPR spectra for various times of irradiation Figure 4 illustrates the EPR spectra at pH 8.5 taken after 3, 15, 45 and 90 s of UV irradiation of II. The spectra of the adducts A and B rise immediately on initiation of UV irradiation, indicating that these are obtained from primary radicals. It can be seen from a comparison of these spectra (and confirmed by simulation, not shown in Fig. 4) that the ‘OH spin adduct spectrum increases relative to the others as the time of irradiation is increased. 3.3. Effects of oqgen, argon and nitrous oxide When solutions of II (at pH 7-8.5) were irradiated in the presence of oxygen, all three adducts A, B and C were observed (Figs. 5(a) and 6(b)). On purging with argon, the intensity of A (DMPO-aryl) and B (DMPO-H) increased (Figs.
26
E. Lipqvmka-Kochany, J. K&any
I EPR detection of free radicals in photo&s
of Cbromophenol
b)
k cl
Fig. 2. (A) Simulated spectrum of an atyl radical-DMPO spin adduct (a”(l) = 2.44 mT. ~~(1) = 1.58 mT; gz2.0053). (B) Simulated spectrum of the hydrogen atom-DMPO spin adduct (aH(2)= 2.25 mT, a”(l)= 1.66 mT, g=2.0054). (C) Simulated spectrum of the DMPO-OH spin adduct (aH(l) =aN(1)=1.49 mT; g= 2.0061).
5(b) and 6(a), whereas that of C (DMPO-OH) and D (p-benzosemiquinone anion) decreased (Fig. 6(a))When samples were gassed with NzO, the EPR signal of adduct DMPO-H (B) disappeared, and the intensity of the adduct DMPO-OH (C) was enhanced significantly (Fig. S(c)). 3.4. Eflect of pH As can be seen from Fig. 3, the component EPR spectra do not depend on pH in the investigated pH range 7.0-10.5, except that, at higher pH (in the solutions containing oxygen), the EPR spectrum ofp-benzosemiquinone anion (D) is more pronounced. However, the overall appearance of the EPR spectra changes with increasing pH, showing a larger contribution of both adducts A and B compared with that of C (Figs. 6(b) and 7(b)). In the presence of argon, the increase in the intensity of signals A and B on alkalinization
Fig. 3. Comparison of the EPR spectra of the DMPO spin adducts recorded after 60 s of WV irradiation of 11 in aqueous solutions at pH 7.0 (a), pH 85 (b) and pH 10.5 (cc). (d) EPR spectrum of the p-benzosemiquinone anion (D) detected after analogous irradiation of II (pH 10.5) without DMPO. EPR spectrometer settings as in Fig. l(a).
E. Lipczynska-Kochany,
J. Kochany I EPR detection of free radic&
in photo&sti
of I-bromophenol
27
b)
Fig. 4. EPR spectra from the photolysis of II in the presence of DMPO in an undegassed solution at pH 8.5, recorded after 3 s (a), 15 s (b), 45 s (c) 90 s (d). EPR spectrometer settings
as in Fig. I(a).
of the solution is more visible than that observed in the presence of oxygen (Figs. 6 and 7). The pK, value of II in the ground state is 9.3 [20]. On excitation to the singlet state, the pK, value drops to pK,‘* =2.9 [20]. Since, in the case of phenols, pKaT’ (in the triplet state) is usually much closer to the pK, value in the ground state than to that in the singlet excited state (pKas’)
Fig. 5. EPR spectra of DMPO spin adducts recorded after 60 s of UV irradiation of II in aqueous solution (pH 8.5), bubbled with oxygen (a), argon (b) and nitrous oxide (c). EPR spectrometer
settings as in Fig. l(a).
[21], it seems reasonable to assume that the pKaTW value of II is approximately 9. It follows that, at pH 10.5, most of the molecules exist in the anionic form. 4. Discussion These observations lead us to the conclusion that the only primary free radicals generated during
E. Lipcgw&t-Kochany,J. Kochany I EPR d&Won ofJiee radicalsin photo& 3_iZPR signal Intensity
of 4.bromophenoC
30EPR slgnel intensity
a)
4 25 -
10
20
30
irrsdiation
40
50
60
time [SW]
irrtiiation
VI signal intensity
time [set]
PR signal intensity b) /
0
10
20
30
irrediatlon
40
50
25 -
60
time [secl
0
IO
20 irradiation
30
40
50
60
time [ssc]
recorded during photoirradiation of II at pH 8.5; (a) in the presence of argon; (b) in the presence of oxyfp. (A) Aryl radicaLDMP0 spin adduct; (8) DMPO-H addud; (c) DMPO-OH adduct; (d) p-bcnzosemiquinone anion.
Fig. 7. Changes in the relative intensity of the EPR sip& recorded during photoirradiation of II at pH 10.5; (a) in the presence of argon; (b) in the Presence ti oxygen. (A) Aryl radical-DMPO spin adduct; (B) DMW-H adduct; (C) DMPO-OH adduct; (D) p-benzosemiquinone anion.
the photolysis of II are hydroxyphenyl) radical followed by protonation. ment with the results of
of I 1111 as well as with those reported for fentichlor ([bis(2-hydroxy-5-chlorophenyl)sulphide], V) and bithionol ([I&(2-hydroxy-?+dichlorophenyl)sulphide], VI) 1221. Photoionization and photo-
Fig. 6. Changes in Ihe relative intensity of the EPR signals
an aryl (probably the 4and a hydrated electron They are in good agreethe previous EPR studies
E. Lipcqmka-Kochmy,
J. Kochany / EPR detection of f?ee radicals in phoro&sis of I-bromophenol
29
dechlorination of the compounds I, V, and VI, as well as the generation of DMPO-OH adducts, were observed [ll, 221. OH
+
H,O
0 0
OH It should be noted that excited bromophenols were not expected to give aqueous electrons, since no such species was detected directly by flash spectroscopy [24], probably because its reaction with these compounds is very fast (k = 7 X lo9 M-’ s-‘forIIandk=2.9x10-gM-‘s-1foritsanionic form [26, 291). The hydrated electron acts as a powerful nucleophile in its reaction with aromatic compounds. Its reactivity is increased when the molecule contains a halogen atom. It is believed that the carboanion VII is the primary product of the reaction between e,, and halogenoarenes
-
+
H+
Cl-
+
OH
Homolytic halogen-carbon (where X is a halogen atom) Ar-x
t
-5
[Ar--x1*
-_)
bond is known
[AC*xj
-_)
cleavage to occur Ar’ + X’
1
recombination
(1)
products
in many halogenoaromatic compounds [23], including bromophenols irradiated in non-polar solvents [24]. The formation of hydroquinone and sym-dihydroxybiphenyl was observed during steady irradiation of II in water, and so a free radical mechanism, involving a photolytic bromine-carbon bond cleavage [25], was also suggested. Thus, the carbon-centred radical, trapped as the adduct A (Figs. 1 and 2(a)) during the photolysis of II, arises primarily from the homolytic fission of the carbon-bromine bond of II. As shown in Fig. 5, in the presence of nitrous oxide, the EPR signal of adduct DMPO-H (B) disappears and the intensity of the DMPO-OH (C) adduct increases. Since it is known that N,O undergoes a fast (k = 9.1 X 10’ M-’ s-’ [2.5, 261) reaction with the aqueous electron giving hydroxyl radicals (eqn. (2)), we conclude that the observed adduct B arises from the aqueous electron plus protonation N,O+e-+H,O-
HO’+HO-+N,
(2)
Electron ejection from excited aqueous phenols [27] has been studied in detail [28]. The results of the investigations reveal that electron ejection (b) occurs before deprotonation of the molecule (c)
ArX + eaq --+
[ArX]‘VII
(4)
Formation of such a radical anion would initiate a radical chain mechanism of nucleophilic substitution (SR-N1), as was suggested by Bunnett [30]. In the next step, radical anion VII would fragment to form radical Ar’ (in our experiment trapped by DMPO as the adduct A) and halide anion X[ArX]‘_ -
Ar’=x-
(5)
In deaerated dilute solutions, radicals Ar’ may undergo a nucleophilic substitution with another nucleophile, for instance -OH Ai + HO[ArOHj’-
-
[ArOH]‘-
+ ArX -
or react with water, Ai+
H,O --)
(6)
ArOH + [ArX]--
(7)
giving hydroquinone
[ArOH,]’
-
ArOH + H
(8)
Another explanation for the formation of hydroquinone as a main product in oxygen-free solutions of 4-halogenophenols could be similar to that proposed by Dulin et al. [31] for the aqueous photolysis of monochloroaromatic compounds. According to this mechanism, the initial step is the homolytic carbon-halogen cleavage to the radical pair [Ar . . X], which may diffuse either as a radical species as shown in eqn. (1) or as an ionic species (in aqueous media) as shown in eqn. (9). The aryl cation (VIII) would then be rapidly scavenged by water
30
Ar-X-
E. tipczynska-Kochany,
[Ar-*Xl%
J. Kochany I EPR detection
Al-+-l-X-+H,OVIII
ArOH,++X-
-
ArOH+H++X(9)
To our knowledge, direct experimental proof for the formation of VIII during photolysis of halogenophenols is still lacking. As can be seen from Figs. 6(a) and 7(a), alkalinization of the oxygen-free solution leads to an increase in the intensity of signal B, which is consistent with the fact that the yield of the eag ejection from phenols increases on alkalinization [28]. The enhanced intensity of the adduct B is accompanied by an increase in the intensity of A, which suggests that the reaction with aqueous electron (eqn. (4)) contributes to the formation of the aryl-centred radical in the direct photolysis (eqn. (1)) of II. In concentrated solutions, aryl radicals Ai can react with themselves or with the molecules of halogenophenol. A coupling product, namely 4,4’dihydroxybiphenyl, was detected during the steady irradiation of II (c =2X 10m3 M) in deaerated water [24], whereas 4,4’-dihydroxybiphenyl and 5chloro-2,4’-dihydroxybiphenyl were identified among other products of photolysis of aqueous I (c=2x lop3 M) [13]. In the presence of oxygen, the concentration of the adduct B (DMPO-H) decreases (Figs. 6 and 7), probably due to the fast reactions of aqueous electron and hydrogen with 0, (eqns. (lo)-(12) [26, 321 and with hydroxyl radicals and benzoquinone III (eqns. (13)-(15) [26], products of the photolysis of II e,,+O,--+
OZ.+
H-+0,-
HO;
K + HO; -
H,O,
e,,+HO’-
HO-
eaq + III -
k=2xlO’” k=l.B~lO’~
M-’
k= l(J’O M-l k=3X101’
(10)
SK’
(11)
s-’
(12)
M-‘s-r
(13)
4-(‘OC,&O-) k=2.3x101’
II- + III -
M-‘s-l
M-’
s-’
(14)
M-’
s-’
(15)
4-(‘OC,H,-OH) k=8.3x10r0
On bubbling the solution of II with oxygen, the concentration of the aryl-centred radical (A) also decreases, which may be related to the fact that its generation in reactions (4) and (5) is less efficient as some aqueous electrons are scavenged in r-eactions (lo), (13) and (14). However, since the
of freeradicals in photoiysb of I-bromophenol decline of the concentration of A is accompanied by an increase in the concentration of hydroxyl radicals (C) and p-benzosemiquinone anion (III, D) (Figs. 6 and 7), we can suggest the following mechanism for the decay of aryl radicals Ar’ + O2 ArOo’ ArOOH
ArOO
(16)
--!% ArOOH
(17)
z
ArO’+‘OH
(18)
We have previously observed [ll] that the intensity of the DMPO-OH andp-benzosemiquinone EPR signals, generated during the photolysis of I, are enhanced with increasing oxygen concentration. A similar effect of oxygen on the EPR spectra of V and VI was observed by Li and Chignell [22, 331, who employed “02 and H,l’O to demonstrate that the trapped hydroxyl radical arose from dissolved oxygen and not from water. They also showed that hydroxyl radicals did not derive from superoxide or hydrogen peroxide. Thus the formation of the aryldioxyl radical (ArOO’) as a key intermediate was taken into consideration [22]. The aryldioxyl radical formed during the photodechlorination of chloropromazine was trapped by DMPO at pH 4 where it was more stable. Formation of p-benzoquinone from irradiated 4-halogenophenols was also demonstrated by the EPR studies reported by Delahanty et al. [34] and Evans et al. [35]. Results of our experiments, using flash photolysis/HPLC, showed that p-benzoquinone was the main primary product of the photolysis of I [S] and II [9] in oxygen-containing solutions. We also observed that, on bubbling with argon, the yield of III dropped off indicating that the photoreaction of I involved oxygen as a reactant [8]. A similar effect of oxygen on the photochemistry of I was also reported by Oudjehani and Boule [13], who applied a conventional source of photoirradiation. We have shown that hydroxyl radicals arise from a secondary photochemical reaction of I and II. Since the main primary photoproduct, p-benzoquinone III, is photolabile [37-391, and its excited molecule abstracts hydrogen from water releasing a free hydroxyl group as shown in eqn. (19) [39], the photolysis of III may be an important source of the detected ‘OH radicals. III=’ + H,O -
4 - (HO-C,II,O-)
+ ‘OH
(19)
However, the results of our present and former [ll] EPR studies are very similar to those reported by Li and Chignell [22], who provided strong evidence that molecular oxygen is the source of hydroxyl radicals generated during the photolysis
E. Lipczyn.k-Kochany,
J. Kochany I EPR detection of free radicals in phofolysis of 4-brmwphenol
of 4-halogenophenols. Thus it seems likely that aryl peroxide photolysis, as shown in reaction (18), is also a source of these radicals. The detection of free radicals, generated during the photolysis of IL (and I), does not rule out the possibility of other non-radical routes. Previous studies have shown [40] that hydroquinones as well as quinones generate semiquinones on TJV irradiation. The autoxidation of hydroquinones and quinones in alkaline solution gives semiquinones as the primary products of oxidation [40]. Thus we could argue that p-benzoquinone, detected in irradiated aerated solutions of 4-halogenophenols, is not the primary product of their photolysis, but arises from the photo-oxidation of hydroquinone, generated in a heterolytic-type reaction. The formation of hydroquinone from I, in a concerted reaction with a transition state (IX), was taken into consideration [41a].
IX Brown and coworkers [12] suggested that the mechanism may be similar to that of the SR+N reaction, proposed for the photolysis of halogenoanisoles [42]. It involves the formation of a radical carbocation X, which is subsequently attacked by a nucleophile N- (eqn. (21)) ArX ---+ ArX* -
ArX’+N-
-
AI-N+ t eag -
ArX+ X ArXNXI A-N
+ eas
ArN’* +X-
(20)
(21)
(22)
However, thus far, no direct evidence has been reported to demonstrate the short-lived intermediate species XI generated during photoirradiation of halogenophenols. Since the formation of hydroquinone from I is not influenced by the pH in the range l-6, the hypothesis of the photosubstitution of chlorine by -OH has been criticized, and a mechanism involving the attack of water, as shown in eqn. (23), has been suggested [13]
On the basis of theoretical considerations, we can expect that S,(T, r*) states, which commonly have a zwitterionic character, will result in nucleophilic or electrophilic additions. Unless the product is formed in the triplet state or a good spin-orbit coupling mechanism is available, T,(z-, T*) states usually undergo primary photoreactions characteristic of radicals, including homolytic fragmentation [21]. The primary photochemical processes of both S,(n, n*) and T,(n, v*) also produce radicals [21]. Thus, it is the r, rTT*state rather than the n, rTT* state from which nucleophilic substitution may easily start. Further experiments are necessary to determine the nature of the excited state from which photolysis of 4-halogenophenols begins. However, it is known that the substitution of Br (or Cl) for H in an aromatic ring results in a significant increase in the rate (kST) and efficiency (c#+) of intersystem crossing S,-T,. Therefore it is unlikely that the photolysis of II (and also of I) involves molecules in the S1(r, 77”) state. Photonucleophilic aromatic substitution reactions often involve an interaction between the aromatic T,(r, T*) state and a nucleophile [42]. Consequently, if the photolysis of 4-halogenophenols involves molecules in the T,(r, v*) state, the S,2 mechanism is theoretically possible. However, the S,2 mechanism usually operates for molecules containing an electron-withdrawing substituent, such as a nitro group. Therefore the S,-,l mechanism (eqn. (4)) to form an axyl radical (eqn. (5)), trapped in our experiments as the DMPO adduct A, seems more probable in the case of 4-halogenophenols. We should also take into consideration the possibility of the formation of the aromatic cation VIII (eqn. (9)), which could be attacked by water (or -OH) to give hydroquinone, the principal primary product observed in deoxygenated solutions. The quantum yield of photochemical transformation of hydroquinone in the presence of oxygen is relatively low (4 = 0.05 [43] compared with 4 = 0.5 for p-benzoquinone [36] and 4 ~0.24 for I [6]). Thus it does not appear to be very plausible that p-benzoquinone., reported as a main primary pho-
32
E. Lipuynska-Kochany,
J. Kochany / EPR detection of free radicals in photo&sti of I-bromophenol
toproduct in oxygenated solutions, is actually a product of hydroquinone oxidation. As already indicated, the observation of the generation of free radicals does not mean that they have a direct relationship with the main route of the reaction. Further investigation is required to conclude the role which may be played by the species observed by the EPR method. However, in the case of halogenoaromatic compounds, intersystem crossing followed by homolytic cleavage (eqn. (1)) often predominates over other mechanisms of photosubstitution [21, 231. While an energy of 114 kcal mall’ (A=254 nm) (or 95.3 kcal mol-’ (A= 300 nm)) [21] may be accepted by a molecule of II, it would require approximately 85 kcal mol-’ to eject an electron (eqn. (3)) and only about 67 kcal mol-’ to break a bromine-carbon (eqn. (1)) [24]. Thus oxidation of aryl radicals (formed in reactions (1) (4) and (5)) should be taken into consideration as a meaningful route in the photochemical transformation of II in dilute oxygenated solutions i.e. under environmentally meaningful conditions. Comparison of the results of the EPR studies and photochemical investigations of II and I suggests a similar mechanism for both reactions, Since the main photoproduct of the reaction, p-benzoquinone, is very photolabile, the photolysis of 4-halogenophenols should be regarded as an important route for their environmental degradation.
5. Conclusions Our spin trapping EPR experiments indicate that the photolysis of 4-bromophenol(I1) in aqueous (oxygenated and deoxygenated) solutions in the pH range 7.0-10.5 produces two primary radicals. One is an aryl radical (A), probably the 4-hydroxyphenyl radical, and the other (B) arises from the photoionization of II followed by PTOtonation of the hydrated electron. The aryl radical is formed in two routes: (i) by the photolytic cleavage of the carbon-bromine bond; (ii) in the reaction between photoejected aqueous electrons and II. In the presence of oxygen, two spectra of secondary products are also observed; the spin adduct of the hydroxyl radical (C) and the pbenzosemiquinone radical anion (D). Since the spectra of C and D can be detected only in oxygenated solutions, they are both concluded to be the products of a reaction involving molecular oxygen. These results are analogous with those obtained during our previous EPR investigations of 4-chlo-
rophenol (I), and suggest that the formation and oxidation of aryl radicals may play an important role in the aqueous photolysis of 4-halogenophenols in aerated solutions.
Acknowledgments The authors are grateful to Dr. T. Muller for access to the EPR spectrometer and to Mr J. Smitt for technical assistance. Referees’ comments are greatly ZippreCiated.
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