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Photoreactivity of 4-chlorophenol in aqueous solution
363
L Photochem. Photobiol. A: Chem., 68 {1992) 36>373
Photoreactivity of 4-chlorophenol in’ aqueous solution Karima
Oudjehani
and Pierre
Boule
Laboratoire de Photochimie Moltkulaire et Macromoldculaire, Pascal {Clermont-Ferrand) 63I77 - AubiPre Ceder (France)
URA
CNRS 433, Universitt!
Blaise
(Received May 13, 1992; accepted June 4, 1992)
Abstract An air-saturated unbuffered solution of 4-chlorophenol (I) irradiated at 296 nm in the concentration range 2x 10m4-2x 10m3 M leads to benzoquinone (II) as the major photoproduct, but II is not the only product initially formed. In dilute solutions (2x 10m4 M) hydroquinone (III) accounts for about 30% of the initial transformation and in more concentrated solutions (2 X 10e3 M), the formation of 5-chloro 2,4’-dihydroxybiphenyl (V) is observed. In deoxygenated solutions (2X 10e4 M), III is the major photoproduct and the formation of V is the main reaction at 2X 10m3 M. Mechanisms are suggested which account for these photoproducts. Other products were identified: 2,5,4’-trihydroxy-biphenyl (VIII) results from a photoreaction between I and III or from the phototransformation of V; 4’-hydroxyphenylbenzoquinone (IX) can be formed either from V or from VIII. The formation of oligomers with three or four phenolic rings was also observed in deoxygenated solutions 2 X lo- 3 M. Phenol and 4,4’-dihydroxybiphenyl are minor photoproducts.
1. Introduction The phototransformation of chlorophenols in aqueous solution has received considerable attention because they are important xenobiotic micropollutants of the aquatic environment which are involved in the synthesis and in the transformation of many pesticides. More specifically 4-chlorophenol is involved in the synthesis of quinizarin (a dye), of clofibrate (a drug), of chlorphenesin and dichlorophen (fungicides). Moreover chlorophenols must be undetectable in drinkable water because of their unpleasant organoleptic properties. It is therefore important to determine their rates of phototransformation in neutral waters and the nature of the photoproducts. It is known that 4-chlorophenol can be phototransformed by sunlight, but, while several photoproducts have been identified, the mechanism of the reaction is not yet completely understood. In 1961, Grabowski [l] suggested that the phototransformation of chlorophenols in basic solution results from a substitution of chlorine atoms by hydroxyl groups. Joschek and Miller [2] observed the formation of hydroquinone (III) and 4,4’dihydroxybiphenyl (VI) from 4-bromophenol and they proposed a radical mechanism. Omura and Matsuura [3] identified also phenol, 5-chloro 2,4’-dihydroxybiphenyl (V) and 2,4’-dihydroxybiphenyl (VII) in the phototransformation of Cchlorophenol in basic medium. Later Boule ef al. [4, 51 observed the formation of products III, V, VI and polyphenolic oligomers on irradiating neutral degassed or air-saturated solutions of 4-chlorophenol in the concentration range (2 X 1O-4-1O-2) M. In air-saturated solutions, benzoquinone (II) and hydroxybenzoquinone (IV) were also formed. It was noted that
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0 1992 - Elsevier Sequoia. All rights reserved
364 V was the main primary photoproduct when the concentration was 1W2 M [5], but the reaction became rapidly very complex. Recently, Lipczynska-Kochany and Bolton [6, 71 proposed the coupling of flash photolysis and high performance liquid chromatography (HPLC) analysis to determine the primary products in the phototransformation of 4-chlorophenol. They concluded that, after a single flash on a degassed solution (6.3 X10e4 M), benzoquinone (II) was the only observed photoproduct. Hydroquinone (III) and hydroxybenzoquinone {IV) appeared only after several flashes, and their formations were attributed to the phototransformation of II. Argon bubbling reduced the yield of II implying that the reaction involves the oxidation of an intermediate species by molecular oxygen. It was suggested that this species was the 4-hydroxyphenyl radical, the formation of this radical being observed by EPR (electron paramagnetic resonance) spectrometry [8]. No significant effect of pH on the reaction rate was observed in the range 3.6-12.2. Above pH = 7 a modification of the products distribution was attributed to the instability of II and III in basic solution. The direct formation of II from I was somewhat unexpected, and new experiments were initiated under steady state UV irradiation to determine the influence of concentration and oxygenation on the photochemical behaviour of 4-chlorophenol, and to assess its fate under environmental conditions. OH
OH
0
0 6 I I
0
Cl
0
I
II
OH
III
H+-@QH
&@
VI
VII
OH
OH OH
/Q+ /
OH
0
0
0
OH
OH
IX
2. Experimental
VI I I
X details
4-Chlorophenol (I) was provided by Carlo-Erba (RPE grade) and sublimed before use. In order to control the influence of the purity of this compound, some experiments were performed with 4-chlorophenol Fluka puriss (purity greater than 99%). No difference was noted between the reactivities of both samples. The UV spectrophotometric properties of 4-chlorophenol are given in Table 1. In unbuffered solution 4-chlorophenol is in the molecular form. But it can be noted that the UV spectrum is slightly modified by the deoxygenation of the solution which increases the pH from 5.5 to roughly 7.4. Hydroquinone (III) was Merck Fotopur (purity greater than 99.5%). l&Benzoquinone (II) was Merck zur synthese and was sublimed before use. The purified
365 TABLE
1
UV absorption of 4-chlorophenol Anionic form
Molecular form A1= 279 nm q = 1470 f30
M-’
s-l
A,=298 nm ~~=2340flOO
M-Is-I
9.4 AZ=224 nm ~,=8500~200
M-l s-l
&=244 nm l~=11000f300
M-’
s-l
product had a golden yellow color. 4,4’-Dihydroxybiphenyl (VI), phenol (X) and 4chlorocatechol (XI) were also commercial products. Hydroxybenzoquinone (IV) is an unstable product. A soIution was obtained by oxidizing an air-saturated solution of 1,2,4_trihydroxybenzene (hydrowhydroquinone) at pH = 8. After a few minutes the solution was acidified to prevent further transformation. The pKa of IV is rather low (3.5) and the anionic form can be identified by its UV absorption at 485 nm (~“2500 M-l cm-‘). (VII) were isolated 5-Chloro-2,4’-dihydroxybiphenyl (V) and 2,4’-dihydroxybiphenyl from an irradiated basic solution of 4-chlorophenol by preparative HPLC. 2,5,4’Trihydroxybiphenyl (VIII) was obtained in similar conditions by irradiating a mixture 4-chlorophenolkhydroquinone. The structures of these three products were determined from mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectra. 4’-Hydroxyphenylbenzoquinone (IX) was prepared according to the general method proposed by Brassard and L’Ekuyer [9]: 4-aminophenol was diazotized and then condensed with benzoquinone. The crude product was purified by chromatography on silica. The deoxygenation of solutions was effected either by argon bubbling or by four cycles of freezing-pumping-thaw on a vacuum line. Both methods gave similar results. The residual concentration of oxygen was evaluated to be in the range (7 x lo-‘)-(7 x 10m6) M- ’ from the inhibition of phosphorescence of 2,3-butanedione (biacetyl) on a solution used as a test. Solutions were irradiated under steady state conditions at 296 nm using a monochromator equipped with a high pressure mercury lamp. The irradiated solutions were analyzed by HPLC on a reverse phase Cl8 column using MeOH/& mixture as eluent. Three chromatographs were used: a Beckman chromatograph equipped with a W spectrophotometric detection, a Waters and a Gilson chromatograph with photodiode array detectors, Mass spectra and 300 MHz NMR spectra were recorded respectively in Service d’Analyse du Centre National de la Recherche Scientifique (CNRS) and in Centre Regional de Mesures Physiques.
3. Results In order to determine the influence of concentration and oxygen on the photochemical behaviour of 4-chlorophenol, air-saturated or deoxygenated solutions (2x 10e4 M or 2~ 10e3 M) were irradiated under steady state conditions at 296 nm. Typical chromatograms are given in Fig. 1 for an air-saturated solution (2 x 10m4 M)
366
Fig. 1. HF’LC chromatogram of an air-saturated solution of 4-chlorophenol 2 for 45 min at 296 nm. Detection 280 nm - MeOH& 42/S. I: 4chlorophenol; III: hydroquinone.
x
10m4 M irradiated II: benzoquinone;
Fig. 2. HPLC chromatogram of a deoxygenated solution of 4-chlorophenol 2~ IO-’ M irradiated for 36 min at 296 nm. Detection 290 nm-MeOH/HsO 57/43. I: 4-chlorophenol; III: hydroquinone; V: Schloro 2,4’-dihydroxybiphenyl; VIII: 2,5,4’-trihydroxybiphenyl; IX: 4’-hydroyphenylbenzoquinone XII: given in the text.
IV
0 3s
II Ill IX I VI
:
Fig. 3. HF’LC chromatogram of an air-saturated solution of 4-chlorophenol 2 x 10m3 M irradiated at 296 run. Detection 290 nm. (a) Irradiation time 64 minutes MeOH/HZO 57/43; (b) Irradiation time 190 minutes MeOH/H,O 42/58. I: 4-chlorophenol; II: benzoquinone; III: hydroquinone; IV: hydroxybenzoquinone; V: 5-chloro 2,4’-dihydroxybiphenyl; VI: 4,4’-dihydroxybiphenyl; IX: 4’-hydroxyphenylbenzoquinone; X: phenol; XI: 4-chlorocatechol.
and in Figs. 2 and 3 for solutions (2x 10 -’ M) respectively deoxygenated and airsaturated. 3.2. Soiutio?ts 2xlo-4 M Benzoquinone (II) and hydroquinone (III) are the only primary photoproducts formed in air-saturated solution (Fig. I). II is the main product, but III accounts for about 30% of the conversion, as it appears on Fig. 4(a). When the conversion extent is lower than 5%, both products accumulate in the solution,
20
30
irradiation A I reacted
II formed
n
40
time (min) l
III formed
2 CxlO
C b
5h4
/ , 1’
/
,
/ /
/
t’
,‘.
9’
/
q -.-.-.-0,. 0 0
10
20
30
irradiation formation
of:
.
(III)
u
w
Fig. 4. Kinetics of formation of the main photoproducts M irradiated
at 296 nm. (a} air-saturated;
40
5Q
time (min) m (VIII)
in solutions of 4-chlorophenol (b) deoxygenated.
2 x 1 0e4
Benzoquinone is not formed in deoxygenated solutions. The main photoproducts are hydroquinone (III), 2,5,4’-trihydroxybiphenyl (VIII) and 5-chloro 2,4’-dihydroxybiphenyl (V). The rate of formation of III is almost constant whereas the rates of formation of V and VIII respectively decrease and increase with increasing irradiation time (Fig. 4(b)). A minor formatiou of 4’-hydroxyphenylbenzoquinone (Ik) was also detected. 3.2. SoZutions 2 x 1O-3 M Products II, III, V and IX also appear in the phototransformation of an airsaturated solution (Fig. 3). At low conversion extent, benzoquinone accounts for more than 80% of the chlorophenol converted (Fig. 5(a)). It can be noticed by comparing Figs. 4(a) and 5(a) that the efficiency of benzoquinone formation increases with increasing initial concentration of 4-chlorophenol: the rate of formation of II is about 14 times higher at 2 X 10d3 M than at 2 x 10v4 M, whereas the absorbed photon flow is only six times higher. This means that benzoquinone is not only formed through a monomolecular reaction. Later on benzoquinone is oxidized into hydroxybenzoquinone (IV) but the quantitative titration of this product was not possible with HPLC. In addition to products III, V and IX, which were identified in the phototransformation of dilute deoxygenated solutions, some minor products such as 4,4’-dihydroxybiphenyl (VI), phenol (X) and 4-chlorocatechol (XI) were also detected. In deoxygenated solution, V is the main photoproduct in the first stage of the transformation. Its rate of formation decreases with increasing irradiation time whereas the formation of VIII increases. From Fig. 5(b), it clearly appears that VIII is formed as a secondary product. A new photoproduct (XII) was identified. Its HPLC retention time is much longer than the retention times of the other identified products (Fig. 2). Product XII was isolated by preparative HPLC. The values M/z=312 and 314 (ratio=3) on the mass spectrum can be associated to the formula C18H1303C1. The following structure was deduced from the 300 MHz NMR spectrum in CD30D. Another photoproduct XIII was detected by mass spectrometry. The value M/z 404406 can be associated to the formula &H1704Cl and corresponds probably to a derivative of product XII with four phenolic rings.
4. Discussion
and mechanisms
The formation of hydroquinone cannot be attributed only to a secondary phototransformation of benzoquinone as was previously suggested [6, 71 for two reasons:
H3 (d)
Cl
XII
6.92
H3-H4 8.6
H4 (dd)
7.16
HI-H6
2.5
H6 (d1
7.30
Hz,H,,
2.4
Hz,(d)
7.49
H5‘;H6.
8.2
H 5' (d)
7.00
H2;;H3”
8.6
H 6, (ddl
7.38
H5rHb,,
8.6
H ,,,.HJdI
7.52
H3,,,H5,Jd) 6.91
369
20
30
irradiation time (min) formation of:
(11)
n
l
.
(III)
(VIII)
0.4
0
b
0
10
formation
20
30
40
irradiation time (min) of:
l
Cm)
0
(V)
q
50
(VIII)
Fig. 5. Kinetics of formation of the main photoproducts in solutions of 4-chlorophenol 2~ 10m3 M irradiated at 296 nm. (a) air-saturated; (b) deoxygenated.
(i) In air-saturated solution (2~ 10m4 M), hydroquinone is formed with kinetics of a primary photoproduct (Fig. 4(a)) and both hydroquinone and benzoquinone linearly accumulate in the solution during the first stage of the reaction; (ii) In deoxygenated solution (2 X 10m4 M), hydroquinone is the main photoproduct whereas benzoquinone is not formed (Fig. 4(b)).
370
Hydroquinone cannot result only from the oxidation of 4-hydroxyphenyl radical which was detected in the photolysis of 4-chlorophenol [8], because its formation mainly occurs in deoxygenated solution (Fig. 4(b)). A photosubstitution of chlorine by OH- with elimination of chloride ion can also be ruled out, the formation of hydroquinone being not significantly affected by the pH in the range pH= l-6. The photohydroIysis mechanism, which was previously suggested [5], is still consistent with the present work. OH
0
+
H,O -
$o_H-
6
0 Cl
Cl
+
H+
+
cl-
OH
;
It is most likely that the photohydrolysis involves the triplet state since it was observed [5] that the formation of hydroquinone can be sensitized by phenol. The formation of hydroquinone from benzoquinone suggested by Lipczynska-Kochany and Bolton is not to be ruled out when benzoquinone is the main photoproduct, i.e. in aerated solutions, and when the extent of the reaction exceeds about 30%. Under these conditions, the absorption of benzoquinone cannot be neglected and its phototransformation leads to an equimolecular mixture of hydroquinone and hydroxybenzoquinone. 0
OH
0
OH
In dilute sqlutions (approximately 10m4 M) hydroquinone is a primary photoproduct resulting from photohydrolysis. In environmental conditions (air saturated) it accounts for about 30% of the transformation whereas in degassed solution hydroquinone is the main photoproduct. In air-saturated solutions of concentration higher than 10m3 M, hydroquinone mainly results from the photolysis of benzoquinone. The formation of benzoquinone needs the presence of oxygen. The oxidation of hydroxyphenyl radical was suggested, but this reaction leads to the release of a chlorine atom out of the solvent cage and no chlorinated product was detected in air-saturated dilute solution. A direct oxidation of the triplet state associated with the formation of hydrochloric acid should account better for the experimental results. The formation of product V is favoured by increased concentration and it is inhibited by oxygen. It was previously suggested by Omura and Matsuura [3] that this product may result from a reaction of 4-hydroxyphenyl radical on 4-chlorophenol with elimination of an atom of hydrogen. But 4-hydroxyphenyl radical is also expected to lead to 4,4’-dihydroxybiphenyl (VI) according to the following scheme.
371
OH
c&-@oH H’
+
OH
OH
/
+
Cl’
Unfortunately, VI is in every case, a minor photoproduct. Besides, the formation of Chydroxyphenyl radical implies the simultaneous formation of a chlorine atom. When the formation of V is the main pathway (Fig. 5(b)), hydroquinone is also formed and the formation of benzoquinoue is very low. Thus, it might be supposed that chlorine atoms released in the solution mainly react with another species than hydroquinone, whereas hydroquinone is known to be a good radical quencher. It can be concluded that the formation of hydroxyphenyl radical is most likely a minor pathway. Nevertheless this radical, which was observed by Lipczynska-Kochany et al., can explain the minor formations of 4,4’-dihydroxybiphenyl and phenol. An alternative mechanism, involving a triplet exciplex of Cchlorophenol, can be suggested to expiain the formation of product V. Product V results from the elimination of HCl, and the influence of concentration of 4-chlorophenol on the formation of benzoquinone can be explained by the oxidation of this exciplex.
OH
H+ + CI-
@+oH+ OH
OH
Cl [exciplexl
+
2HCI
Product VIII is only formed in deoxygenated solutions as a secondary photoproduct (Figs. 4(b) and 5(b)). Its formation is enhanced by the addition of hydroquinone to the solution. Through excitation of mixtures of Cchlorophetiol and hydroquinone in aqueous solution at various wavelengths (254, 296, 313 nm), product VIII was shown to result from the excitation of 4-chlorophenol and also from the excitation of hydroquinone. Since it was recently established that the intersystem crossing yield S, +T, of hydroquinone is equal to 0.39 and that the life-time of the triplet state is 0.9 ~LS[lo], we suggest a mechanism involving the triplet state of hydroquinone or 4chlorophenol.
372
+ 6 0 OH
0
(T&
Cl
OH
OH
+ i
(TJ Cl
OH + OH
0
HCI
VIII
d
OH
Product VIII can also result from the photohydrolysis of V, as was shown by irradiation of a deoxygenated solution of an authentic sample of V. OH &OH Cl
+
HCI
OH
Product IX can be formed through two different pathways depending on the oxygen concentration: in air-saturated solution, it results from the photo-oxidation of V. This reaction was observed by irradiating a solution of isolated product V, Thus product V and 4-chlorophenol have similar reactivities. In deoxygenated solutions, product IX is formed from VIII. Product XII results from a reaction between compounds V and I, according to the same mechanism as the formation of V from I. This oligomerisation can continue, since it was previously noticed that the following series of products appear by irradiating a degassed solution of 0.01 M 4-chlorophenol [4]. M/Z= 186 + 92 n; 110 + 92 n; 128 + 92 n with n 94. Product XIII is included in the third series. The photochemical behaviour of 4-chlorophenol can be summarized by the following scheme.
5. Conclusions solutions, benzoquinone is the major product resulting from the In air-saturated excitation of 4-chlorophenol. At low concentration (e.g. 2X lo-’ M) it mainly results from a direct photo-oxidation of 4-chlorophenol. Thus this reaction is expected to occur in environmental conditions. But benzoquinone is not the only product formed: hydroquinone, resulting from a photohydrolysis, accounts for about 30% of the conversion_ In deoxygenated solution, hydroquinone is the main photoproduct, and benzoquinone is not a primary product. The formation of 2,5,4’-trihydroxybiphenyl (VIII) results from the reaction of excited hydroquinone with 4-chlorophenol, or excited 4-chlorophenol with hydroquinone in deoxygenated solution. Thus it is not expected in natural waters. It can also be formed from 5-chloro, 2,4’-dihyroxybiphenyl (V). The photochemical behaviour is modified by increasing concentration. In aerated solution (2 x 10S3 M) benzoquinone is the major photoproduct and its formation can
373
OH
0
+ H+ClOH
2x
+ H+Cl-
OH‘*A
0 /
minor
0
0
+ H+CI-
l
2(H+Cl-1
result from both molecular and bimolecular reactions. In deoxygenated solution, the formation of (V) is the major pathway. This reaction probably does not occur in environmental conditions. Minor formations of 4,4’-dihydroxybiphenyl (VI) and phenol (X) are attributed to reactions of hydroxyphenyl radicals. The reactivities of product V and 4-chlorophenol are similar, since the phototransformation of V leads to VIII or IX, depending on the conditions. The formation of oligomers is favoured by increasing concentration and the complexity of the product mixture also increases with irradiation time.
References 1 Z. R. Grabowski, 2. Phys. Chtm., 27 (1961) 239. 2 H. I. Joschek and S. I, Miller, J. Am. Chem. Sot.,
88 (1966) 3269. K. Omura and T. Matsuura, Tetmhedrun, 27 (1971) 3101. P. Boule, C. Guyon and J. Lemaire, Chemosphere, I1 (1982) 1179. P. Boule, C. Guyon and J. Lemaire, Tadcol. Environ. Chem., 7 (1984) 97. E. Lipczynska-Kochany and J. R. Bolton, _7.Chem. Sot., Chem. Comma., (1990) 1596. E. Lipczynska-Koehany and J. R. Bolton, J. Photochem. Photobiol. A: Chem, 58 (1991) 315. E. Lipczynska-Kochany, J. Kochany and J. R. Bolton, J. Photochem. Photobid A: Chem., 62 (1991) 229. 9 P. Brassard and P. L’JZcuyer, Can. J. Chem., 36 (1958) 700. 10 P. Boule, A. Rossi, J.-F. Pilichowski and G. Grabner, to be published. 3 4 5 6 7 8
L Photochem. Photobiol. A: Chem., 68 {1992) 36>373
Photoreactivity of 4-chlorophenol in’ aqueous solution Karima
Oudjehani
and Pierre
Boule
Laboratoire de Photochimie Moltkulaire et Macromoldculaire, Pascal {Clermont-Ferrand) 63I77 - AubiPre Ceder (France)
URA
CNRS 433, Universitt!
Blaise
(Received May 13, 1992; accepted June 4, 1992)
Abstract An air-saturated unbuffered solution of 4-chlorophenol (I) irradiated at 296 nm in the concentration range 2x 10m4-2x 10m3 M leads to benzoquinone (II) as the major photoproduct, but II is not the only product initially formed. In dilute solutions (2x 10m4 M) hydroquinone (III) accounts for about 30% of the initial transformation and in more concentrated solutions (2 X 10e3 M), the formation of 5-chloro 2,4’-dihydroxybiphenyl (V) is observed. In deoxygenated solutions (2X 10e4 M), III is the major photoproduct and the formation of V is the main reaction at 2X 10m3 M. Mechanisms are suggested which account for these photoproducts. Other products were identified: 2,5,4’-trihydroxy-biphenyl (VIII) results from a photoreaction between I and III or from the phototransformation of V; 4’-hydroxyphenylbenzoquinone (IX) can be formed either from V or from VIII. The formation of oligomers with three or four phenolic rings was also observed in deoxygenated solutions 2 X lo- 3 M. Phenol and 4,4’-dihydroxybiphenyl are minor photoproducts.
1. Introduction The phototransformation of chlorophenols in aqueous solution has received considerable attention because they are important xenobiotic micropollutants of the aquatic environment which are involved in the synthesis and in the transformation of many pesticides. More specifically 4-chlorophenol is involved in the synthesis of quinizarin (a dye), of clofibrate (a drug), of chlorphenesin and dichlorophen (fungicides). Moreover chlorophenols must be undetectable in drinkable water because of their unpleasant organoleptic properties. It is therefore important to determine their rates of phototransformation in neutral waters and the nature of the photoproducts. It is known that 4-chlorophenol can be phototransformed by sunlight, but, while several photoproducts have been identified, the mechanism of the reaction is not yet completely understood. In 1961, Grabowski [l] suggested that the phototransformation of chlorophenols in basic solution results from a substitution of chlorine atoms by hydroxyl groups. Joschek and Miller [2] observed the formation of hydroquinone (III) and 4,4’dihydroxybiphenyl (VI) from 4-bromophenol and they proposed a radical mechanism. Omura and Matsuura [3] identified also phenol, 5-chloro 2,4’-dihydroxybiphenyl (V) and 2,4’-dihydroxybiphenyl (VII) in the phototransformation of Cchlorophenol in basic medium. Later Boule ef al. [4, 51 observed the formation of products III, V, VI and polyphenolic oligomers on irradiating neutral degassed or air-saturated solutions of 4-chlorophenol in the concentration range (2 X 1O-4-1O-2) M. In air-saturated solutions, benzoquinone (II) and hydroxybenzoquinone (IV) were also formed. It was noted that
lOlO_6030/92/$5.00
0 1992 - Elsevier Sequoia. All rights reserved
364 V was the main primary photoproduct when the concentration was 1W2 M [5], but the reaction became rapidly very complex. Recently, Lipczynska-Kochany and Bolton [6, 71 proposed the coupling of flash photolysis and high performance liquid chromatography (HPLC) analysis to determine the primary products in the phototransformation of 4-chlorophenol. They concluded that, after a single flash on a degassed solution (6.3 X10e4 M), benzoquinone (II) was the only observed photoproduct. Hydroquinone (III) and hydroxybenzoquinone {IV) appeared only after several flashes, and their formations were attributed to the phototransformation of II. Argon bubbling reduced the yield of II implying that the reaction involves the oxidation of an intermediate species by molecular oxygen. It was suggested that this species was the 4-hydroxyphenyl radical, the formation of this radical being observed by EPR (electron paramagnetic resonance) spectrometry [8]. No significant effect of pH on the reaction rate was observed in the range 3.6-12.2. Above pH = 7 a modification of the products distribution was attributed to the instability of II and III in basic solution. The direct formation of II from I was somewhat unexpected, and new experiments were initiated under steady state UV irradiation to determine the influence of concentration and oxygenation on the photochemical behaviour of 4-chlorophenol, and to assess its fate under environmental conditions. OH
OH
0
0 6 I I
0
Cl
0
I
II
OH
III
H+-@QH
&@
VI
VII
OH
OH OH
/Q+ /
OH
0
0
0
OH
OH
IX
2. Experimental
VI I I
X details
4-Chlorophenol (I) was provided by Carlo-Erba (RPE grade) and sublimed before use. In order to control the influence of the purity of this compound, some experiments were performed with 4-chlorophenol Fluka puriss (purity greater than 99%). No difference was noted between the reactivities of both samples. The UV spectrophotometric properties of 4-chlorophenol are given in Table 1. In unbuffered solution 4-chlorophenol is in the molecular form. But it can be noted that the UV spectrum is slightly modified by the deoxygenation of the solution which increases the pH from 5.5 to roughly 7.4. Hydroquinone (III) was Merck Fotopur (purity greater than 99.5%). l&Benzoquinone (II) was Merck zur synthese and was sublimed before use. The purified
365 TABLE
1
UV absorption of 4-chlorophenol Anionic form
Molecular form A1= 279 nm q = 1470 f30
M-’
s-l
A,=298 nm ~~=2340flOO
M-Is-I
9.4 AZ=224 nm ~,=8500~200
M-l s-l
&=244 nm l~=11000f300
M-’
s-l
product had a golden yellow color. 4,4’-Dihydroxybiphenyl (VI), phenol (X) and 4chlorocatechol (XI) were also commercial products. Hydroxybenzoquinone (IV) is an unstable product. A soIution was obtained by oxidizing an air-saturated solution of 1,2,4_trihydroxybenzene (hydrowhydroquinone) at pH = 8. After a few minutes the solution was acidified to prevent further transformation. The pKa of IV is rather low (3.5) and the anionic form can be identified by its UV absorption at 485 nm (~“2500 M-l cm-‘). (VII) were isolated 5-Chloro-2,4’-dihydroxybiphenyl (V) and 2,4’-dihydroxybiphenyl from an irradiated basic solution of 4-chlorophenol by preparative HPLC. 2,5,4’Trihydroxybiphenyl (VIII) was obtained in similar conditions by irradiating a mixture 4-chlorophenolkhydroquinone. The structures of these three products were determined from mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectra. 4’-Hydroxyphenylbenzoquinone (IX) was prepared according to the general method proposed by Brassard and L’Ekuyer [9]: 4-aminophenol was diazotized and then condensed with benzoquinone. The crude product was purified by chromatography on silica. The deoxygenation of solutions was effected either by argon bubbling or by four cycles of freezing-pumping-thaw on a vacuum line. Both methods gave similar results. The residual concentration of oxygen was evaluated to be in the range (7 x lo-‘)-(7 x 10m6) M- ’ from the inhibition of phosphorescence of 2,3-butanedione (biacetyl) on a solution used as a test. Solutions were irradiated under steady state conditions at 296 nm using a monochromator equipped with a high pressure mercury lamp. The irradiated solutions were analyzed by HPLC on a reverse phase Cl8 column using MeOH/& mixture as eluent. Three chromatographs were used: a Beckman chromatograph equipped with a W spectrophotometric detection, a Waters and a Gilson chromatograph with photodiode array detectors, Mass spectra and 300 MHz NMR spectra were recorded respectively in Service d’Analyse du Centre National de la Recherche Scientifique (CNRS) and in Centre Regional de Mesures Physiques.
3. Results In order to determine the influence of concentration and oxygen on the photochemical behaviour of 4-chlorophenol, air-saturated or deoxygenated solutions (2x 10e4 M or 2~ 10e3 M) were irradiated under steady state conditions at 296 nm. Typical chromatograms are given in Fig. 1 for an air-saturated solution (2 x 10m4 M)
366
Fig. 1. HF’LC chromatogram of an air-saturated solution of 4-chlorophenol 2 for 45 min at 296 nm. Detection 280 nm - MeOH& 42/S. I: 4chlorophenol; III: hydroquinone.
x
10m4 M irradiated II: benzoquinone;
Fig. 2. HPLC chromatogram of a deoxygenated solution of 4-chlorophenol 2~ IO-’ M irradiated for 36 min at 296 nm. Detection 290 nm-MeOH/HsO 57/43. I: 4-chlorophenol; III: hydroquinone; V: Schloro 2,4’-dihydroxybiphenyl; VIII: 2,5,4’-trihydroxybiphenyl; IX: 4’-hydroyphenylbenzoquinone XII: given in the text.
IV
0 3s
II Ill IX I VI
:
Fig. 3. HF’LC chromatogram of an air-saturated solution of 4-chlorophenol 2 x 10m3 M irradiated at 296 run. Detection 290 nm. (a) Irradiation time 64 minutes MeOH/HZO 57/43; (b) Irradiation time 190 minutes MeOH/H,O 42/58. I: 4-chlorophenol; II: benzoquinone; III: hydroquinone; IV: hydroxybenzoquinone; V: 5-chloro 2,4’-dihydroxybiphenyl; VI: 4,4’-dihydroxybiphenyl; IX: 4’-hydroxyphenylbenzoquinone; X: phenol; XI: 4-chlorocatechol.
and in Figs. 2 and 3 for solutions (2x 10 -’ M) respectively deoxygenated and airsaturated. 3.2. Soiutio?ts 2xlo-4 M Benzoquinone (II) and hydroquinone (III) are the only primary photoproducts formed in air-saturated solution (Fig. I). II is the main product, but III accounts for about 30% of the conversion, as it appears on Fig. 4(a). When the conversion extent is lower than 5%, both products accumulate in the solution,
20
30
irradiation A I reacted
II formed
n
40
time (min) l
III formed
2 CxlO
C b
5h4
/ , 1’
/
,
/ /
/
t’
,‘.
9’
/
q -.-.-.-0,. 0 0
10
20
30
irradiation formation
of:
.
(III)
u
w
Fig. 4. Kinetics of formation of the main photoproducts M irradiated
at 296 nm. (a} air-saturated;
40
5Q
time (min) m (VIII)
in solutions of 4-chlorophenol (b) deoxygenated.
2 x 1 0e4
Benzoquinone is not formed in deoxygenated solutions. The main photoproducts are hydroquinone (III), 2,5,4’-trihydroxybiphenyl (VIII) and 5-chloro 2,4’-dihydroxybiphenyl (V). The rate of formation of III is almost constant whereas the rates of formation of V and VIII respectively decrease and increase with increasing irradiation time (Fig. 4(b)). A minor formatiou of 4’-hydroxyphenylbenzoquinone (Ik) was also detected. 3.2. SoZutions 2 x 1O-3 M Products II, III, V and IX also appear in the phototransformation of an airsaturated solution (Fig. 3). At low conversion extent, benzoquinone accounts for more than 80% of the chlorophenol converted (Fig. 5(a)). It can be noticed by comparing Figs. 4(a) and 5(a) that the efficiency of benzoquinone formation increases with increasing initial concentration of 4-chlorophenol: the rate of formation of II is about 14 times higher at 2 X 10d3 M than at 2 x 10v4 M, whereas the absorbed photon flow is only six times higher. This means that benzoquinone is not only formed through a monomolecular reaction. Later on benzoquinone is oxidized into hydroxybenzoquinone (IV) but the quantitative titration of this product was not possible with HPLC. In addition to products III, V and IX, which were identified in the phototransformation of dilute deoxygenated solutions, some minor products such as 4,4’-dihydroxybiphenyl (VI), phenol (X) and 4-chlorocatechol (XI) were also detected. In deoxygenated solution, V is the main photoproduct in the first stage of the transformation. Its rate of formation decreases with increasing irradiation time whereas the formation of VIII increases. From Fig. 5(b), it clearly appears that VIII is formed as a secondary product. A new photoproduct (XII) was identified. Its HPLC retention time is much longer than the retention times of the other identified products (Fig. 2). Product XII was isolated by preparative HPLC. The values M/z=312 and 314 (ratio=3) on the mass spectrum can be associated to the formula C18H1303C1. The following structure was deduced from the 300 MHz NMR spectrum in CD30D. Another photoproduct XIII was detected by mass spectrometry. The value M/z 404406 can be associated to the formula &H1704Cl and corresponds probably to a derivative of product XII with four phenolic rings.
4. Discussion
and mechanisms
The formation of hydroquinone cannot be attributed only to a secondary phototransformation of benzoquinone as was previously suggested [6, 71 for two reasons:
H3 (d)
Cl
XII
6.92
H3-H4 8.6
H4 (dd)
7.16
HI-H6
2.5
H6 (d1
7.30
Hz,H,,
2.4
Hz,(d)
7.49
H5‘;H6.
8.2
H 5' (d)
7.00
H2;;H3”
8.6
H 6, (ddl
7.38
H5rHb,,
8.6
H ,,,.HJdI
7.52
H3,,,H5,Jd) 6.91
369
20
30
irradiation time (min) formation of:
(11)
n
l
.
(III)
(VIII)
0.4
0
b
0
10
formation
20
30
40
irradiation time (min) of:
l
Cm)
0
(V)
q
50
(VIII)
Fig. 5. Kinetics of formation of the main photoproducts in solutions of 4-chlorophenol 2~ 10m3 M irradiated at 296 nm. (a) air-saturated; (b) deoxygenated.
(i) In air-saturated solution (2~ 10m4 M), hydroquinone is formed with kinetics of a primary photoproduct (Fig. 4(a)) and both hydroquinone and benzoquinone linearly accumulate in the solution during the first stage of the reaction; (ii) In deoxygenated solution (2 X 10m4 M), hydroquinone is the main photoproduct whereas benzoquinone is not formed (Fig. 4(b)).
370
Hydroquinone cannot result only from the oxidation of 4-hydroxyphenyl radical which was detected in the photolysis of 4-chlorophenol [8], because its formation mainly occurs in deoxygenated solution (Fig. 4(b)). A photosubstitution of chlorine by OH- with elimination of chloride ion can also be ruled out, the formation of hydroquinone being not significantly affected by the pH in the range pH= l-6. The photohydroIysis mechanism, which was previously suggested [5], is still consistent with the present work. OH
0
+
H,O -
$o_H-
6
0 Cl
Cl
+
H+
+
cl-
OH
;
It is most likely that the photohydrolysis involves the triplet state since it was observed [5] that the formation of hydroquinone can be sensitized by phenol. The formation of hydroquinone from benzoquinone suggested by Lipczynska-Kochany and Bolton is not to be ruled out when benzoquinone is the main photoproduct, i.e. in aerated solutions, and when the extent of the reaction exceeds about 30%. Under these conditions, the absorption of benzoquinone cannot be neglected and its phototransformation leads to an equimolecular mixture of hydroquinone and hydroxybenzoquinone. 0
OH
0
OH
In dilute sqlutions (approximately 10m4 M) hydroquinone is a primary photoproduct resulting from photohydrolysis. In environmental conditions (air saturated) it accounts for about 30% of the transformation whereas in degassed solution hydroquinone is the main photoproduct. In air-saturated solutions of concentration higher than 10m3 M, hydroquinone mainly results from the photolysis of benzoquinone. The formation of benzoquinone needs the presence of oxygen. The oxidation of hydroxyphenyl radical was suggested, but this reaction leads to the release of a chlorine atom out of the solvent cage and no chlorinated product was detected in air-saturated dilute solution. A direct oxidation of the triplet state associated with the formation of hydrochloric acid should account better for the experimental results. The formation of product V is favoured by increased concentration and it is inhibited by oxygen. It was previously suggested by Omura and Matsuura [3] that this product may result from a reaction of 4-hydroxyphenyl radical on 4-chlorophenol with elimination of an atom of hydrogen. But 4-hydroxyphenyl radical is also expected to lead to 4,4’-dihydroxybiphenyl (VI) according to the following scheme.
371
OH
c&-@oH H’
+
OH
OH
/
+
Cl’
Unfortunately, VI is in every case, a minor photoproduct. Besides, the formation of Chydroxyphenyl radical implies the simultaneous formation of a chlorine atom. When the formation of V is the main pathway (Fig. 5(b)), hydroquinone is also formed and the formation of benzoquinoue is very low. Thus, it might be supposed that chlorine atoms released in the solution mainly react with another species than hydroquinone, whereas hydroquinone is known to be a good radical quencher. It can be concluded that the formation of hydroxyphenyl radical is most likely a minor pathway. Nevertheless this radical, which was observed by Lipczynska-Kochany et al., can explain the minor formations of 4,4’-dihydroxybiphenyl and phenol. An alternative mechanism, involving a triplet exciplex of Cchlorophenol, can be suggested to expiain the formation of product V. Product V results from the elimination of HCl, and the influence of concentration of 4-chlorophenol on the formation of benzoquinone can be explained by the oxidation of this exciplex.
OH
H+ + CI-
@+oH+ OH
OH
Cl [exciplexl
+
2HCI
Product VIII is only formed in deoxygenated solutions as a secondary photoproduct (Figs. 4(b) and 5(b)). Its formation is enhanced by the addition of hydroquinone to the solution. Through excitation of mixtures of Cchlorophetiol and hydroquinone in aqueous solution at various wavelengths (254, 296, 313 nm), product VIII was shown to result from the excitation of 4-chlorophenol and also from the excitation of hydroquinone. Since it was recently established that the intersystem crossing yield S, +T, of hydroquinone is equal to 0.39 and that the life-time of the triplet state is 0.9 ~LS[lo], we suggest a mechanism involving the triplet state of hydroquinone or 4chlorophenol.
372
+ 6 0 OH
0
(T&
Cl
OH
OH
+ i
(TJ Cl
OH + OH
0
HCI
VIII
d
OH
Product VIII can also result from the photohydrolysis of V, as was shown by irradiation of a deoxygenated solution of an authentic sample of V. OH &OH Cl
+
HCI
OH
Product IX can be formed through two different pathways depending on the oxygen concentration: in air-saturated solution, it results from the photo-oxidation of V. This reaction was observed by irradiating a solution of isolated product V, Thus product V and 4-chlorophenol have similar reactivities. In deoxygenated solutions, product IX is formed from VIII. Product XII results from a reaction between compounds V and I, according to the same mechanism as the formation of V from I. This oligomerisation can continue, since it was previously noticed that the following series of products appear by irradiating a degassed solution of 0.01 M 4-chlorophenol [4]. M/Z= 186 + 92 n; 110 + 92 n; 128 + 92 n with n 94. Product XIII is included in the third series. The photochemical behaviour of 4-chlorophenol can be summarized by the following scheme.
5. Conclusions solutions, benzoquinone is the major product resulting from the In air-saturated excitation of 4-chlorophenol. At low concentration (e.g. 2X lo-’ M) it mainly results from a direct photo-oxidation of 4-chlorophenol. Thus this reaction is expected to occur in environmental conditions. But benzoquinone is not the only product formed: hydroquinone, resulting from a photohydrolysis, accounts for about 30% of the conversion_ In deoxygenated solution, hydroquinone is the main photoproduct, and benzoquinone is not a primary product. The formation of 2,5,4’-trihydroxybiphenyl (VIII) results from the reaction of excited hydroquinone with 4-chlorophenol, or excited 4-chlorophenol with hydroquinone in deoxygenated solution. Thus it is not expected in natural waters. It can also be formed from 5-chloro, 2,4’-dihyroxybiphenyl (V). The photochemical behaviour is modified by increasing concentration. In aerated solution (2 x 10S3 M) benzoquinone is the major photoproduct and its formation can
373
OH
0
+ H+ClOH
2x
+ H+Cl-
OH‘*A
0 /
minor
0
0
+ H+CI-
l
2(H+Cl-1
result from both molecular and bimolecular reactions. In deoxygenated solution, the formation of (V) is the major pathway. This reaction probably does not occur in environmental conditions. Minor formations of 4,4’-dihydroxybiphenyl (VI) and phenol (X) are attributed to reactions of hydroxyphenyl radicals. The reactivities of product V and 4-chlorophenol are similar, since the phototransformation of V leads to VIII or IX, depending on the conditions. The formation of oligomers is favoured by increasing concentration and the complexity of the product mixture also increases with irradiation time.
References 1 Z. R. Grabowski, 2. Phys. Chtm., 27 (1961) 239. 2 H. I. Joschek and S. I, Miller, J. Am. Chem. Sot.,
88 (1966) 3269. K. Omura and T. Matsuura, Tetmhedrun, 27 (1971) 3101. P. Boule, C. Guyon and J. Lemaire, Chemosphere, I1 (1982) 1179. P. Boule, C. Guyon and J. Lemaire, Tadcol. Environ. Chem., 7 (1984) 97. E. Lipczynska-Kochany and J. R. Bolton, _7.Chem. Sot., Chem. Comma., (1990) 1596. E. Lipczynska-Koehany and J. R. Bolton, J. Photochem. Photobiol. A: Chem, 58 (1991) 315. E. Lipczynska-Kochany, J. Kochany and J. R. Bolton, J. Photochem. Photobid A: Chem., 62 (1991) 229. 9 P. Brassard and P. L’JZcuyer, Can. J. Chem., 36 (1958) 700. 10 P. Boule, A. Rossi, J.-F. Pilichowski and G. Grabner, to be published. 3 4 5 6 7 8