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HPLC method for studying the sequence of photochemical reactions: applications to 4-chlorophenol in aerated aqueous solution
J. Photochem. Photobiof. A:
31.5
Chem., 58 (1991) 315-322
Flash photolysis/HPLC method for studying the sequence of photochemical reactions: applications to 4-chlorophenol in aerated aqueous solution Ewa Lipczynska-Kochany?
and James R, Bolton
Department of Chemistry, The Univetsity of Western Ontario, London, (Canada)
Ontario ASA 5B7
(Received August 14, 1990)
Abstract A new method involving the use of flash photolysis followed by rapid high performance liquid chromatography analysis is described and applied to the examination of the photodegradation of 4-chlorophenol (I) in aerated aqueous solution (6.3~ lop4 M). Analysis after a single flash indicates that p-benzoquinone (II) is the onIy observed photoproduct. Irradiation with a more intense flash or with multiple flashes yields other products (principally hydroxy-p-benzoquinone (III) and hydroquinone (IV)), which are known photoproducts of the photolysis of II. The photoreaction of I is relatively independent of pH, except that above pH 7 the product distribution changes in accord with the known dark chemistry of II at high pH. Oxygen is a reagent in the photoreaction of I, as bubbling with argon signiscantly decreases the yield of II. This new method should be particularly useful for studying photochemical reactions in dilute solutions, since photolysis of intermediate products can be avoided.
1. Introduction in spite of its universal availability, water is seldom used as a solvent in organic photochemistry. However, aqueous photochemistry has recently received considerable attention, because of the growing interest in processes involving photochemical transformations of organic pollutants in the natural environment. Mechanistic investigations of these complex reactions are of great importance, since there are cases where intermediates and/or photoproducts are more toxic than the starting compounds. It is usually very difficult to observe the progress of these photochemical reactions using conventional steady state photolysis, since their primary products often undergo secondary photochemical transformations, which can occu even at a relatively low degree of photoconversion of the starting materials. The usu: solution to this problem is to work at very low conversion fractions and low concentmtion however, the sensitivity of detection methods available in the past has been too low under these conditions. In order to circumvent this problem, we have developed 1 new technique involving the use of flash photolysis with the analysis of intermediate products by high performance liquid chromatography (HPLC). *On leave from Warsaw Technical University, Department of Chemistry, uL Koszykowa 75, 00-662 Warsaw, Poland.
lOlO-6030/91/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
316
4-Chlorophenol (I) was chosen as a model compound for testing the method, since the aqueous photoreaction of this fungicide has been studied previously using conventional methods [l-6]. Photolysis of halogenols in non-polar organic solvents is well known to lead to the homoiytic fission of a carbon-halogen bond. For example, irradiation of monohalogenophenols in benzene gives phenylphenols [l-3]. In an early report, Grabowski [4] described the results of the polarographic analysis of mixtures of chlorophenols after photolysis in aqueous alkali and suggested that photolysis of these compounds resulted in the replacement of chlorine atoms with hydroxyl groups. Omura and Matsuura [5] examined the preparative scale photochemistry of chlorophenols in alkaline aqueous solutions. They found that after 5 h of irradiation (at 40 “C), a concentrated (approximately 0.1 M) solution of I yielded a mixture of products including hydroquinone and 2,4’-dihydroxydiphenyl. The photochemical reaction of I in water was re-examined by Boule er al. [6]. It was found that the photochemical reaction of this compound in aqueous solution (2 x 10e4 to 1 x lop2 M) was non-specificgiving hydroquinone,p-benzoquinone, biphenyls They estimated that the quantum yield for and polyphenolylic oligomers as products. disappearance of I was 0.4 +O.l. 2. Experimental
details
Materials 4-Chlorophenol, hydroquinone and Aldrich Chemical Inc. (U.S.A.); methanol were obtained from Omnisolv (U.S.A.); Chemicals (Toronto). 2.1.
1,2,3_trihydroxybenzene were obtained from and acetonitrile (for liquid chromatography) other chemicals were obtained from BDH
2.2. Apparatus Samples were irradiated using a PRA International model FP 1000 flash photolysis system. The light was provided by two 100 J model FX 14lC-3.5 xenon lamps @G&G Inc., Electra-Optics Division, U.S.A.) located on either side of the sample chamber. The flash duration was approximately 10 ps (full width at half-maximum (FWHM)) at 49 J discharge (7 kV and 2 PF capacitor), but the lamp profile showed a long tail extending to approximately 30 p.s. HPLC was performed with a Waters model 501 solvent delivery system, equipped cl8 column, a Waters model 441 absorbance detector set at with a p BondapakTM A = 254 nm, a Waters Lambda-Max model 481 LC spectrophotometer set at A= 280 nm and a Fisher Recordall Series 5000 dual channel recorder. The flow rate was 2.0 ml min-I. Preparative liquid chromatography was performed using a LiChroprep Si 60 (40-63 pm) Lobar column (E. Merck, Darmstadt) with a basic outfit for liquid chromatography (E. Merck, Darmstadt) equipped with a Masterfex pump (Cole Parmer, U.S.A.) and a Hewlett-Packard model 8450A diode array UV-visible spectrometer as a detector. IR spectra were recorded with a model IFS 32 Bruker Fourier transform IR (FTIR) spectrometer with an IBM system 9000 computer; proton nuclear magnetic resonance (lH NMR) spectra were run on a Varian XL-200 NMR spectrometer and WV-visible spectra were measured on a Hewlett-Packard model 8450A diode array spectrometer. pH values were adjusted using a model E520 pH meter (Metrohm Herisau, Switzerland).
317
2.3. Methods 2.3.1. Irradiations
Irradiations were performed using a cylindrical quartz cell (length, 10.0 cm; diameter, 1.0 cm). Progress of the photochemical degradation of I in aerated water, methanol and acetonitrile was observed with the concentration maintained at 6.3 X lob4 M, while the number of flashes (45 J) was varied. The effect of concentration was determined by varying the concentration between 6 X lo-’ M and 1.2~ lo-’ M and keeping the energy of the flash irradiation constant (2X 50 J). The influence of the flash energy on the photodegradation of aqueous aerated solutions of I was studied with the concentration maintained constant (6 X 10m4 M) and the flash energy varied from 16 to 100 J. In order to observe any effect of oxygen, the solutions of I in water and methanol (approximately 6 x 10d4 M) were bubbled before flashing for approximately 1 h with oxygen or argon. Bubbling was also maintained during irradiation. Any pH effect (in the pH range 3.6-12.2) on the photochemical reaction of aqueous I (6.3X 10e4 M) was studied using a 1.0 mM potassium phosphate monobasic buffer, adjusted to the required pH value with sodium hydroxide (10%) or concentrated orthophosphoric acid. The pH of unbuffered aqueous solutions of I (6.3X 10e4 M) was 4.7. All flashed solutions were analysed by HPLC immediately after the flash irradiation sequence. In order to identify the main primary product of the photochemical degradation of I, the reaction was performed on a preparative scale. Thus samples (15 ml) of aerated solutions (approximately 8 x 10m4 M) of I in water were flashed (1 X lOO_J) in a quartz cell as described above. The procedure was repeated 100 times. Solutions were collected in the dark at 0 “C and extracted with ethyl ether. The ethyl ether was then removed on a rotary evaporator at room temperature; the main photoproduct was isolated on a Lobar column using chloroform as eluent and analysed immediately by FI’IR, W-visible and ‘H FTNMR spectroscopy. The spectral data of the photoproduct obtained, and the HPLC retention times, were identical with those of an authentic sample of p-benzoquinone (II). 2.3.2. HPLC analysis All the samples were analysed immediately after the reaction using the HPLC method. An acetonitrile-water mixture (Z&80), acidified to pH 4.5 with orthophosphoric acid, was used as eltrent. Details of the separations of chlorophenols will be reported elsewhere [7].
3. Results and discussion 3.1. Eflect of number of flashes and flash energy on the photochemicaI reaction of 4 chlorophenol in water
Aerated solutions of I (5 x 10v5 to 6.3 x 10m4 M) were repeatedly flashed (45 J) and analysed immediately by HPLC. Analysis of the solution following a single flash indicated the presence of only one primary photoproduct (Fig. l(a)), which had a retention time identical with that of II. The compound was isolated and identified by FTIR, ‘H FI’NMR, W and mass spectroscopy. After 5-10 flashes, new compounds begin to appear: hydroquinone (III) and 2-hydroxy-p-benzoquinone (IV), known products from the W irradiation of II [8]. After 20-30 flashes, traces of 1,2,4-trihydroxybenzene (V) and 4-chlorocatechol (VI) are also observed. The depletion of I and the formation of the photoproducts as a function of the number of flashes are shown in Fig. 2. The
318
(a)
I 0
C:5x
I
I
I
5
lo
15
TIME (min) Fig.
J):
K)-%A
I
20 -
1. High performance liquid chromatograms (a) 5 X 10e5 M; (b) 5 x lo-’ M. Detection
of aqueous solutions of I after wavelength: A=280 nm.
two flashes
(50
concentration of II rises sharply with a small number of flashes and reaches a maximum as the photochemistry of this compound becomes important. Thus the postulated sequence of steps in the photodegradation of I is shown in Scheme 1. The mechanism of the formation of II is being investigated using electron paramagnetic resonance (EPR) spectroscopy. The results of these studies will be reported in the near future [9]. p-Benzoquinone is formed initially at a low flash energy. As the energy of the flash increases to 100 J, the substrate peak shows a decrease indicating that the extent of the reaction increases with the flash energy. Other products (III, IV, V and VI) can be observed by HPLC. This can easily be explained since a higher energy flash has a longer lifetime and thus secondary photochemical reactions can take place. 3.2. Effect of concentration Our results are at some variance with those reported in refs. 4-6. However, these workers investigated more concentrated solutions using a steady state UV irradiation source. No significant effect of concentration on the yield of the reaction of I in water was observed during our experiments. However, when the concentration of I was increased above lo- 3 M, II was still the major photoproduct, but evidence for many coupling products was obtained (Fig. l(b)). 3.3. Effect of oxygen When a solution of I was bubbled with argon for 15 min, flash photolysis of I produced II in less than 50% of the yield observed with aerated solutions. When a solution of I was bubbled with pure oxygen for 15 min, the yield of II on flash photolysis of I increased by approximately 20% over that observed with an aerated solution. These results indicate that the photoreaction probably involves the reaction of oxygen with the excited state of I.
319
80
I \
I remaining
60 II formed
IV formed
_
1
0
10
5
15
20
30
25
Number of flashes (45 J) Fig. 2. Mole fractions (5%) (by HPLC analysis) of reactant and products present in an aqueous solution (initially 6.3x.1Dm4 M) of I after being subjected to a number of flashes (45 J): 4-
chlorophenol (I);p-benmuinone (II); hydroquinone (III); 2-hydroxy-p-benzoquinone(IV). Smaller amounts of unidentified compounds were detected.
Cl
0
(1)
OH
0
Scheme 1.
3.4. Eflect of pH As in the photoreactions performed by conventional methods [6], no significant effect of pH on the rate of photodegradation of I was observed in the pH range 3.6-12.2. However, an increase in pH to greater than pH 7 influences the product distribution. As can Be seen from Fig. 3, the primary photoproduct begins to change from II to IV above piH 7. This is consistent with the known instability of II in alkaline solutions.
3.5. Eflect of solvent The
of I (6.3 x fOB4 M) was also studied using methanol or as solvent. The decrease in the concentration of I and the formation of
photodegradation
acetonitrile
320
0 Time (min)
5
10
Number
-
15
of flashes
20
(45
25
30
J)
Fig. 3. High performance liquid chromatograms of aqueous solutions (initially 6.3 x 10e4 M) of I after two flashes (50 J) at pH 6.9, 8.3, 11.3 and 12.8. Detection wavelength: A=254 nm. Fig. 4. Mole fractions (%) (by HPLC analysis) of reactant and products present in a methanol solution (initially 6.3 X lop4 m) of I after being subjected to a number of flashes (45 J): 4chlorophenol (I): p-benzoquinone (II); hydroquinone (III); 2-hydroxyp-benzoquinone (IV). products are shown in Figs. 4 and 5 as a function of the number of flashes (45 J). II is still the main photoproduct, but the reactions are more “messy” and the yields of the photodegradation of I are lower (0.59 for methanol and 0.29 for acetonitrile of the yield for an aqueous solution). Comparison of the results presented in Figs. 2, 4 and 5 shows that the formation of II is favoured in protic solvents, especially in water. The effect of oxygen on the formation of II in methanol and acetonitrile was similar but less pronounced than that observed in water. 3.6.
Photochemical
reaction
of p-benzoquinone
in waier
Aerated aqueous solutions (3 x 10S4 M) of II were flashed and analysed by HPLC in a similar way to that described for solutions of I. III and IV were the primary products of the reaction caused by two flashes (45 J). When the solution was flashed ten times (45 J), V was also detected. Thus the extra products observed after multiple flashes of solutions of I can be attributed to the photolysis of II.
4. Conclusions The results of our experiments show aerated dilute aqueous solutions proceeds
that the photoreaction of 4-chlorophenol very cleanly to produce p-benzoquinone
in as
321
60 -
0
5
10
Number
15
of flashes
20
(45
25
30
J)
Fig. 5. Mole fractions (%) (by HPLC analysis) of reactant and products present in an acetonitrile solution (initially 6.3 x 1O-4 M) of I after being subjected to a number of flashes (45 J):. 4chlorophenol (I); p-benzoquinone (II); hydroquinone (III); Zhydroxy-p-benzoquinone (Iv); 1,2,4trihydrovbenzene (V). Smaller amounts of unidentified compounds were detected.
the only signticant photoproduct. Our results differ from the data reported in the literature [4-4], which were obtained using conventional methods, i.e. a steady state source of W irradiation and analytical methods that made the investigation of dilute solutions untenable. Photolysis of intermediate products, occurring during the steady state irradiation, makes the study of the mechanism very difficult. If initial concentrations are increased so as to be able to investigate the reaction at a lower conversion fraction, the possibility of bimolecular reaction steps may change the mechanism and lead to different products. The new method described here should be very attractive for investigations of photochemical reactions occurring in dilute solutions. The application of flash photolysis with HPLC analysis makes it possible to detect and identify moderately-lived (more than 15 min) intermediates and products. A significant conversion can be obtained with a single flash. In addition, by using multiple flashes, the sequence of intermediates of the reaction can be followed. Our new method is particularly useful, in combination with other methods, for studying the photodegradation reactions of organic pollutants in aquatic systems, where these compounds are usually found at very low concentrations.
322
References 1 2 3 45 6 7 8 9
W. T. G. Z. K. P. E. A. E.
Wolf and N. Kharasch, J. Org. Chem., 30 (1965) 2493. Matsuura and K. Omura, Bull. Chem. Sm. Jpn., 39 (1966) 944. E. Robinson and J. M. Vermon, Chem. Commun. (1969) 977. R. Grabowski, Z. Phys. Chem., 27 (1961) 239. Omura and T. Matsuura, Tetrahedron, 27 (1971) 3101. Boule, C. Guyon and J. Lemaire, Chemosphere, II (1982) 1179. Lipczynska-Kochany and .I. Kochany, in preparation. I. Ononye and J. R. Bolton, L Phys. Chem., 90 (1986) 6271. Lipczynska-Kochany, J. Kochany and J. R. Bolton, submitted for publication.
31.5
Chem., 58 (1991) 315-322
Flash photolysis/HPLC method for studying the sequence of photochemical reactions: applications to 4-chlorophenol in aerated aqueous solution Ewa Lipczynska-Kochany?
and James R, Bolton
Department of Chemistry, The Univetsity of Western Ontario, London, (Canada)
Ontario ASA 5B7
(Received August 14, 1990)
Abstract A new method involving the use of flash photolysis followed by rapid high performance liquid chromatography analysis is described and applied to the examination of the photodegradation of 4-chlorophenol (I) in aerated aqueous solution (6.3~ lop4 M). Analysis after a single flash indicates that p-benzoquinone (II) is the onIy observed photoproduct. Irradiation with a more intense flash or with multiple flashes yields other products (principally hydroxy-p-benzoquinone (III) and hydroquinone (IV)), which are known photoproducts of the photolysis of II. The photoreaction of I is relatively independent of pH, except that above pH 7 the product distribution changes in accord with the known dark chemistry of II at high pH. Oxygen is a reagent in the photoreaction of I, as bubbling with argon signiscantly decreases the yield of II. This new method should be particularly useful for studying photochemical reactions in dilute solutions, since photolysis of intermediate products can be avoided.
1. Introduction in spite of its universal availability, water is seldom used as a solvent in organic photochemistry. However, aqueous photochemistry has recently received considerable attention, because of the growing interest in processes involving photochemical transformations of organic pollutants in the natural environment. Mechanistic investigations of these complex reactions are of great importance, since there are cases where intermediates and/or photoproducts are more toxic than the starting compounds. It is usually very difficult to observe the progress of these photochemical reactions using conventional steady state photolysis, since their primary products often undergo secondary photochemical transformations, which can occu even at a relatively low degree of photoconversion of the starting materials. The usu: solution to this problem is to work at very low conversion fractions and low concentmtion however, the sensitivity of detection methods available in the past has been too low under these conditions. In order to circumvent this problem, we have developed 1 new technique involving the use of flash photolysis with the analysis of intermediate products by high performance liquid chromatography (HPLC). *On leave from Warsaw Technical University, Department of Chemistry, uL Koszykowa 75, 00-662 Warsaw, Poland.
lOlO-6030/91/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
316
4-Chlorophenol (I) was chosen as a model compound for testing the method, since the aqueous photoreaction of this fungicide has been studied previously using conventional methods [l-6]. Photolysis of halogenols in non-polar organic solvents is well known to lead to the homoiytic fission of a carbon-halogen bond. For example, irradiation of monohalogenophenols in benzene gives phenylphenols [l-3]. In an early report, Grabowski [4] described the results of the polarographic analysis of mixtures of chlorophenols after photolysis in aqueous alkali and suggested that photolysis of these compounds resulted in the replacement of chlorine atoms with hydroxyl groups. Omura and Matsuura [5] examined the preparative scale photochemistry of chlorophenols in alkaline aqueous solutions. They found that after 5 h of irradiation (at 40 “C), a concentrated (approximately 0.1 M) solution of I yielded a mixture of products including hydroquinone and 2,4’-dihydroxydiphenyl. The photochemical reaction of I in water was re-examined by Boule er al. [6]. It was found that the photochemical reaction of this compound in aqueous solution (2 x 10e4 to 1 x lop2 M) was non-specificgiving hydroquinone,p-benzoquinone, biphenyls They estimated that the quantum yield for and polyphenolylic oligomers as products. disappearance of I was 0.4 +O.l. 2. Experimental
details
Materials 4-Chlorophenol, hydroquinone and Aldrich Chemical Inc. (U.S.A.); methanol were obtained from Omnisolv (U.S.A.); Chemicals (Toronto). 2.1.
1,2,3_trihydroxybenzene were obtained from and acetonitrile (for liquid chromatography) other chemicals were obtained from BDH
2.2. Apparatus Samples were irradiated using a PRA International model FP 1000 flash photolysis system. The light was provided by two 100 J model FX 14lC-3.5 xenon lamps @G&G Inc., Electra-Optics Division, U.S.A.) located on either side of the sample chamber. The flash duration was approximately 10 ps (full width at half-maximum (FWHM)) at 49 J discharge (7 kV and 2 PF capacitor), but the lamp profile showed a long tail extending to approximately 30 p.s. HPLC was performed with a Waters model 501 solvent delivery system, equipped cl8 column, a Waters model 441 absorbance detector set at with a p BondapakTM A = 254 nm, a Waters Lambda-Max model 481 LC spectrophotometer set at A= 280 nm and a Fisher Recordall Series 5000 dual channel recorder. The flow rate was 2.0 ml min-I. Preparative liquid chromatography was performed using a LiChroprep Si 60 (40-63 pm) Lobar column (E. Merck, Darmstadt) with a basic outfit for liquid chromatography (E. Merck, Darmstadt) equipped with a Masterfex pump (Cole Parmer, U.S.A.) and a Hewlett-Packard model 8450A diode array UV-visible spectrometer as a detector. IR spectra were recorded with a model IFS 32 Bruker Fourier transform IR (FTIR) spectrometer with an IBM system 9000 computer; proton nuclear magnetic resonance (lH NMR) spectra were run on a Varian XL-200 NMR spectrometer and WV-visible spectra were measured on a Hewlett-Packard model 8450A diode array spectrometer. pH values were adjusted using a model E520 pH meter (Metrohm Herisau, Switzerland).
317
2.3. Methods 2.3.1. Irradiations
Irradiations were performed using a cylindrical quartz cell (length, 10.0 cm; diameter, 1.0 cm). Progress of the photochemical degradation of I in aerated water, methanol and acetonitrile was observed with the concentration maintained at 6.3 X lob4 M, while the number of flashes (45 J) was varied. The effect of concentration was determined by varying the concentration between 6 X lo-’ M and 1.2~ lo-’ M and keeping the energy of the flash irradiation constant (2X 50 J). The influence of the flash energy on the photodegradation of aqueous aerated solutions of I was studied with the concentration maintained constant (6 X 10m4 M) and the flash energy varied from 16 to 100 J. In order to observe any effect of oxygen, the solutions of I in water and methanol (approximately 6 x 10d4 M) were bubbled before flashing for approximately 1 h with oxygen or argon. Bubbling was also maintained during irradiation. Any pH effect (in the pH range 3.6-12.2) on the photochemical reaction of aqueous I (6.3X 10e4 M) was studied using a 1.0 mM potassium phosphate monobasic buffer, adjusted to the required pH value with sodium hydroxide (10%) or concentrated orthophosphoric acid. The pH of unbuffered aqueous solutions of I (6.3X 10e4 M) was 4.7. All flashed solutions were analysed by HPLC immediately after the flash irradiation sequence. In order to identify the main primary product of the photochemical degradation of I, the reaction was performed on a preparative scale. Thus samples (15 ml) of aerated solutions (approximately 8 x 10m4 M) of I in water were flashed (1 X lOO_J) in a quartz cell as described above. The procedure was repeated 100 times. Solutions were collected in the dark at 0 “C and extracted with ethyl ether. The ethyl ether was then removed on a rotary evaporator at room temperature; the main photoproduct was isolated on a Lobar column using chloroform as eluent and analysed immediately by FI’IR, W-visible and ‘H FTNMR spectroscopy. The spectral data of the photoproduct obtained, and the HPLC retention times, were identical with those of an authentic sample of p-benzoquinone (II). 2.3.2. HPLC analysis All the samples were analysed immediately after the reaction using the HPLC method. An acetonitrile-water mixture (Z&80), acidified to pH 4.5 with orthophosphoric acid, was used as eltrent. Details of the separations of chlorophenols will be reported elsewhere [7].
3. Results and discussion 3.1. Eflect of number of flashes and flash energy on the photochemicaI reaction of 4 chlorophenol in water
Aerated solutions of I (5 x 10v5 to 6.3 x 10m4 M) were repeatedly flashed (45 J) and analysed immediately by HPLC. Analysis of the solution following a single flash indicated the presence of only one primary photoproduct (Fig. l(a)), which had a retention time identical with that of II. The compound was isolated and identified by FTIR, ‘H FI’NMR, W and mass spectroscopy. After 5-10 flashes, new compounds begin to appear: hydroquinone (III) and 2-hydroxy-p-benzoquinone (IV), known products from the W irradiation of II [8]. After 20-30 flashes, traces of 1,2,4-trihydroxybenzene (V) and 4-chlorocatechol (VI) are also observed. The depletion of I and the formation of the photoproducts as a function of the number of flashes are shown in Fig. 2. The
318
(a)
I 0
C:5x
I
I
I
5
lo
15
TIME (min) Fig.
J):
K)-%A
I
20 -
1. High performance liquid chromatograms (a) 5 X 10e5 M; (b) 5 x lo-’ M. Detection
of aqueous solutions of I after wavelength: A=280 nm.
two flashes
(50
concentration of II rises sharply with a small number of flashes and reaches a maximum as the photochemistry of this compound becomes important. Thus the postulated sequence of steps in the photodegradation of I is shown in Scheme 1. The mechanism of the formation of II is being investigated using electron paramagnetic resonance (EPR) spectroscopy. The results of these studies will be reported in the near future [9]. p-Benzoquinone is formed initially at a low flash energy. As the energy of the flash increases to 100 J, the substrate peak shows a decrease indicating that the extent of the reaction increases with the flash energy. Other products (III, IV, V and VI) can be observed by HPLC. This can easily be explained since a higher energy flash has a longer lifetime and thus secondary photochemical reactions can take place. 3.2. Effect of concentration Our results are at some variance with those reported in refs. 4-6. However, these workers investigated more concentrated solutions using a steady state UV irradiation source. No significant effect of concentration on the yield of the reaction of I in water was observed during our experiments. However, when the concentration of I was increased above lo- 3 M, II was still the major photoproduct, but evidence for many coupling products was obtained (Fig. l(b)). 3.3. Effect of oxygen When a solution of I was bubbled with argon for 15 min, flash photolysis of I produced II in less than 50% of the yield observed with aerated solutions. When a solution of I was bubbled with pure oxygen for 15 min, the yield of II on flash photolysis of I increased by approximately 20% over that observed with an aerated solution. These results indicate that the photoreaction probably involves the reaction of oxygen with the excited state of I.
319
80
I \
I remaining
60 II formed
IV formed
_
1
0
10
5
15
20
30
25
Number of flashes (45 J) Fig. 2. Mole fractions (5%) (by HPLC analysis) of reactant and products present in an aqueous solution (initially 6.3x.1Dm4 M) of I after being subjected to a number of flashes (45 J): 4-
chlorophenol (I);p-benmuinone (II); hydroquinone (III); 2-hydroxy-p-benzoquinone(IV). Smaller amounts of unidentified compounds were detected.
Cl
0
(1)
OH
0
Scheme 1.
3.4. Eflect of pH As in the photoreactions performed by conventional methods [6], no significant effect of pH on the rate of photodegradation of I was observed in the pH range 3.6-12.2. However, an increase in pH to greater than pH 7 influences the product distribution. As can Be seen from Fig. 3, the primary photoproduct begins to change from II to IV above piH 7. This is consistent with the known instability of II in alkaline solutions.
3.5. Eflect of solvent The
of I (6.3 x fOB4 M) was also studied using methanol or as solvent. The decrease in the concentration of I and the formation of
photodegradation
acetonitrile
320
0 Time (min)
5
10
Number
-
15
of flashes
20
(45
25
30
J)
Fig. 3. High performance liquid chromatograms of aqueous solutions (initially 6.3 x 10e4 M) of I after two flashes (50 J) at pH 6.9, 8.3, 11.3 and 12.8. Detection wavelength: A=254 nm. Fig. 4. Mole fractions (%) (by HPLC analysis) of reactant and products present in a methanol solution (initially 6.3 X lop4 m) of I after being subjected to a number of flashes (45 J): 4chlorophenol (I): p-benzoquinone (II); hydroquinone (III); 2-hydroxyp-benzoquinone (IV). products are shown in Figs. 4 and 5 as a function of the number of flashes (45 J). II is still the main photoproduct, but the reactions are more “messy” and the yields of the photodegradation of I are lower (0.59 for methanol and 0.29 for acetonitrile of the yield for an aqueous solution). Comparison of the results presented in Figs. 2, 4 and 5 shows that the formation of II is favoured in protic solvents, especially in water. The effect of oxygen on the formation of II in methanol and acetonitrile was similar but less pronounced than that observed in water. 3.6.
Photochemical
reaction
of p-benzoquinone
in waier
Aerated aqueous solutions (3 x 10S4 M) of II were flashed and analysed by HPLC in a similar way to that described for solutions of I. III and IV were the primary products of the reaction caused by two flashes (45 J). When the solution was flashed ten times (45 J), V was also detected. Thus the extra products observed after multiple flashes of solutions of I can be attributed to the photolysis of II.
4. Conclusions The results of our experiments show aerated dilute aqueous solutions proceeds
that the photoreaction of 4-chlorophenol very cleanly to produce p-benzoquinone
in as
321
60 -
0
5
10
Number
15
of flashes
20
(45
25
30
J)
Fig. 5. Mole fractions (%) (by HPLC analysis) of reactant and products present in an acetonitrile solution (initially 6.3 x 1O-4 M) of I after being subjected to a number of flashes (45 J):. 4chlorophenol (I); p-benzoquinone (II); hydroquinone (III); Zhydroxy-p-benzoquinone (Iv); 1,2,4trihydrovbenzene (V). Smaller amounts of unidentified compounds were detected.
the only signticant photoproduct. Our results differ from the data reported in the literature [4-4], which were obtained using conventional methods, i.e. a steady state source of W irradiation and analytical methods that made the investigation of dilute solutions untenable. Photolysis of intermediate products, occurring during the steady state irradiation, makes the study of the mechanism very difficult. If initial concentrations are increased so as to be able to investigate the reaction at a lower conversion fraction, the possibility of bimolecular reaction steps may change the mechanism and lead to different products. The new method described here should be very attractive for investigations of photochemical reactions occurring in dilute solutions. The application of flash photolysis with HPLC analysis makes it possible to detect and identify moderately-lived (more than 15 min) intermediates and products. A significant conversion can be obtained with a single flash. In addition, by using multiple flashes, the sequence of intermediates of the reaction can be followed. Our new method is particularly useful, in combination with other methods, for studying the photodegradation reactions of organic pollutants in aquatic systems, where these compounds are usually found at very low concentrations.
322
References 1 2 3 45 6 7 8 9
W. T. G. Z. K. P. E. A. E.
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