Environ. Sci. Technol. 1984, 18, 235-241
Acetone-Sensitized and Nonsensitized Photolyses of Tetra-, Penta-, and Hexachlorobenzenes in Acetonitrile-Water Mixtures: Photoisomerization and Formation of Several Products Including Polychlorobiphenylst Ghulam Ghaus Choudhrys and Otto Hutzlnger'
Laboratory of Environmental and Toxicological Chemistry, University of Amsterdai
rn Photochemical reactions of tetra-, penta-, and hexachlorobenzenes in the presence and absence of acetone as sensitizer at wavelengths 2 285 nm have been studied. The reductive dechlorination is the main photochemical pathway in both sensitized and nonsensitized photolyses. 1,2,4-Trichlorobenzeneappears as the main photoproduct of the nonsensitized photolyses of 1,2,3,5-tetrachlorobenzene, while the major photoproduct of the sensitized irradiation of this substrate is 1,3,5-trichlorobenzene. The nonsensitized photoreactions of tetrachlorobenzeses yield photoisomerized chlorobenzenes and also give product chlorobenzenes containing more chlorine atoms than the starting material. Yields of up to several percent of polychlorobiphenyls (PCBs) are obtained in the case of sensitized irradiation of tetra- and pentachlorobenzenes, but this reaction is less significant in the case of direct photolysis. Introduction Polychlorobenzenes (PCBzs) are utilized on industrial scale (1-5). PCBzs have also been identified in the emisions from some municipal and industrial incinerators, e.g. (6-8),as well as in various water bodies (9). Moreover, acetone occurs in natural waters (10). Therefore, it is likely that in our environment aquatic systems contaminated with PCBzs also contain acetone. Several reports on the studies of monochlorobenzene photolysis have been seen in the literature (11,12). Parlar, Korte, and their co-workers have reported the solution phase photochemical (A < 254 nm) reductive dechlorination of o-, m-, and p-dichlorobenzenes (13,14) and their derivatives (13-15) containing one substituent such as CH3, CHzOH, OH, OCH3, CHO, NHz, NOz, CN, and Ph. Choudhry et al. (16) have demonstrated the reductive dechlorination, isomerization, and formation of polychlorobiphenyls (PCBs) by exposing solutions of trichlorobenzenes in methanol and acetonitrilewater mixture with radiation of X 2 285 nm. The photoreduction of pentachlorobenzene in organic solvents like hexane, cyclohexane, acetone, and 95% ethanol by utilizing ultraviolet (UV) light of X = 253.7 nm has been performed by Crosby and Hamadmad (5). Plimmer and Klingebiel(4) reported the reductive dechlorination of hexachlorobenzene in methanol and hexane carried out by light of wavelengths greater than 260 and 220 nm, respectively. Furthermore, Uyeta and his co-workers (17)have shown the photoformation of PCBs by irradiating neat mono-, di-, tri-, tetra- (except the 1,2,3,5-isomer), and hexachlorobenzenes by sunlight. *Address correspondence to this author at thechair of Ecological Chemistry and Geochemistry, University of Bayreuth, D-8580 Bayreuth, West Germany. This article is part 3 of the series 'Photochemistry of Halogenated Benzene Derivatives". For part 2, see ref 18. Present address: Pesticide Research Laboratory, Department of Soil Science, The University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2.
*
0013-936X/84/0918-0235$01.50/0
218 WV Amstc-
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In view of our interest in the photochemical fate of halogenated pollutants (16,181 we decided to further investigate the photochemistry of PCBzs possessing four, five, and six chlorine substituents. The present paper deals with acetone-sensitized and nonsensitized photochemistry of the three tetrachlorobenzenes, pentachlorobenzene, and hexachlorobenzenein different acetonitrilewater mixtures utilizing radiation of wavelengths 2 285 nm. Experimental Section Substrates and Standards. The substrate (5) and standard trichlorobenzenes (7) were supplied by Chemical Service, Inc., West Chester, and Koninklijke/Shell-Laboratorium, Amsterdam, respectively. All other compounds were purchased from Aldrich. Solvents. Sources are described elsewhere (18). Preparation of Solutions. Stock solution of a substrate was prepared with acetonitrile as solvent. To a portioh of the stock solution was added water drop by drop with continuous manual shaking, until the required concentration of the substrate, i.e., ca. 1mM/L, was achieved. In this manner, sample solutions of all substrates (compounds 1-5) in water-acetonitrile using the maximum volume of water were made. For both types of photolysis, the ratios of acetonitrile-water used for the preparation of solutions were as follows: 1:l (v/v) for PCBzs 1 and 2; 6 4 (v/v) for PCBm 3 and 4; 9 1 for hexachlorobenzene (5). In the case of acetone sensitized irradiations (Table I), concentrations of substrates 1-5 were 1.204, 1.065, 1.112, 1:119, and 1.194 mM/L, respectively, while each solution contained 0.553 M/L acetone. For the direct photolyses (Table 11),concentrations of tetrachlorobenzenes 1-3 were 1.112 mM/L, whereas those of penta- and hexachlorobenzene (4 and 5) were 1.119 and 1.194 mM/L, respectively. Irradiation Equipment and Experiments. Detailed descriptions of irradiation equipment and experiments are given in ref 18. Irradiation times in the case of acetonesensitized photoreactions of substrates 1,3, and 4 were 4 h, while such sample solutions of substrates 2 and 5 were irradiated for 1.5 and 16 h, respectively. In the case of nonsensitized reactions, sample solutions of substrates 1, 2, and 4 were irradiated for 40,36, and 24 h, respectively, while irradiation times for substrates 3 and 5 were 8 h. Extraction Procedures. One milliliter of hexane each was added to photolyzed samples. After the samples were well shaken, the hexane layer was separated from the water-acetonitrile layer. A few drops of n-nonane were added to the hexane solution which was then reduced in volume to approximately 0.2 mL by using a rotary evaporator. The same procedure was also followed for unphotolyzed samples. n-Nonane was added to prevent loss of volatile compounds. Gas Chromatography. Two types of GCs were used. Qualitative and quantitative analyses of the photolysates were carried out on a Hewlett-Packard 5830A instrument with a flame ionization detector equipped with an 18850 GC terminal. The following three types of columns and GC conditions on this gas chromatograph were used:
0 1984 American Chemical Society
Environ. Sci. Technol., Vol. 18,
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column A, glass column (2 m X 0.2 cm) containing 5% Carbowax 20 M on 160-180 mesh Chromosorb WAW, temperature 1 (T,) = 70 "C, time 1 ( t l )= 2 min, rate 8 "C/min, temperature 2 (T,) = 240 "C, time 2 (tz)= 30 min, carrier gas (N,) flow = 24 mL/min, injection temperature = 260 "C, and FID temperature = 300 "C; column B, glass column (3 m X 0.2 cm) containing 10% Carbowax 20 M on 160-180 mesh Chromosorb WAW, Tl = 80 "C, tl = 20 min, rate 8 "C/min, T2= 240 "C, t 2 = 40 min, carrier gas (N2)flow = 18 mL/min, injection temperature = 260 "C, and FID temperature = 300 "C; column C, glass column (1.90 m X 0.2 cm) containing 3% Silicone OV 101 on 150-160 mesh Chromosorb WAW, Tl= 80 "C, tl = 2 min, rate 8 "C/min, T2= 240 "C, t 2 = 40 min, carrier gas (N,) flow = 25 mL/min, injection temperature = 260 "C, and FID temperature = 300 "C. Photolyzed samples containing water were directly injected into column A for the determination of the disappearance of starting material and chlorobenzenes appearing as photoproducts. Column B was used for the quantitative analyses of the hexane extracts of photolysates, which contained 1,2,3,5- and 1,2,4,5-C14-Bz(2 and 3, respectively). The C14-Bz 2 and 3 were only partly resolvable on column B; the retention time of both compounds differed by 0.20 min. Quantitative analyses of the PCBs formed during the photolyses of substrates 1-4 were carried out on column C by injecting hexane extract of the corresponding photolysate. Chemical yields of photoproducts such as PCBzs and PCBs were calculated from the amount of photoconverted starting material. For the determination of yields of PCBzs, appropriate standard solutions of authentic chlorobenzenes were used, while for those of PCBs, the amount of undecomposed starting material present in a photolysate was utilized as internal standard. Polychlorobiphenyls from nonsensitized photolysates (Table 11)were detected on a Packard 428 gas chromatograph utilized with a @Nielectron detector (ECD) and a capillary injection system according to Grob and Grob (19). The fused silica capillary column (25 m X 0.25 mm i.d.) was coated with CP-SIL 7, film thickness being 0.4 pm (Chrompack). The pressure of the carrier gas (N,) at the inlet of the column was 0.9 atm, the gas flow in the column being ca. 1-2 mL f min. The other GC conditions were the following: Tl= 40 "C, tl = 4 min, rate = 39 "C/min, T2 = 240 "C, t2 = 40 min, injector temperature = 250 O C , and ECD temperature = 300 "C. Two minutes after injection the splitter was opened manually to flush the injector. The splitter was kept closed manually for 2 min before injecting each sample. Gas Chromatography-Mass Spectrometry. Qualitative analysis was conducted on a Hewlett-Packard 5894 A mass spectrometer operating in the electron-impact mode at 70 eV. For the identifications of chlorobenzenes, especially di- and trichlorobenzenes present in a photolyzed sample, previously described (18)glass column and operating conditions were utilized. Although some other photoproducts like PCBs could be identified on the above-mentioned column (0.2% Carbowax 20M (18), all photoproducts (except Cl,-Bzs and some Cl,-Bzs) (documented in Tables I and 11) were qualitatively recognized on the GC-MS equipped with a fused silica capillary column (25 m X 0.25 mm i.d.), which was coated with CP-SIL 5, the film thickness being 0.4 pm (provided by Chrompack). The procedure of Grob and Grob (19)was followed for operating the capillary injection system. The GC conditions for this capillary system were the following: Tl= 70 "C, tl = 0 min, and rates = 32 236 Environ. Sci. Technol., Vol. 18, No. 4, 1984
"C/min; after 2 min rate = 8 "C/min, T2= 300 "C, and t 2 = 30 min.
The mass (M) ranges were 100 Q M 6 500. For the search of chlorine-containingphotoproducts present in the total ion chromatogram (TIC) of a photolysate, the computer program for specific detection of organochlorine and organobromine compounds (20) was used (e.g., see Figure 1). Identification of Photoproducts. Photoproducts like PCBzs were identified as mentioned previously (18). Because of unavailability of authentic standards, structures to all other photoproduds (Table I and 11) were tentatively ascribed by interpreting their mass spectra.
Results Acetone-Sensitized Photolysis. Results of irradiation of 1.1-1.2 mM/L solutions of 1,2,3,4-tetrachlorobenzene (1,2,3,4-ClA-B~)(I), 1,2,3,5-C14-Bz (2), 1,2,4,5-Cl4-Bz(3), pentachlorobenzene (Cl,-Bz) (41, and hexachlorobenzene (Cb-Bz) (5) in the presence of 0.553 M/L acetone as sensitizer at wavelengths (A) > 285 nm are recorded in Table I. Typical total ion chromatograms of the acetone-sensitized photolysates of the tetrachlorobenzenes (C14-Bzs) (1-3) are shown in Figure 1. The peaks labeled with numbers represent the compounds which have been identified and are documented in Table I. It is obvious from Table I that, as expected, the acetone-sensitized photolysis of the investigated chlorobenzenes leads mainly to reductive dechlorination. The usual reductively dechlorinated photoproducts of these substrates are chlorobenzenes containing one and two C1 atoms less than those of starting materials, the latter chlorobenzenesappearing as minor products. For example, tetrachlorobenzene 1 gives two trichlorobenzenes, 1,2,3C13-Bz (6) (9.2%) and 1,2,4-cl3-Bz (7)(32.6%) and two dichlorobenzenes, namely, 1,3-C12-Bz (10) (5.2 % ) and 1,4-C12-Bz(11) (1.5%), where the percentages in the parentheses represent the chemical yields of the corresponding product calculated on the basis of the amount of decomposed starting material. It is noteworthy that 49.3% of disappeared substrate 2 is photoconverted into 1,3,5-trichlorobenzene (B), the amount of which is approximately 10 times greater than those of the other two C1,-Bzs (6 and 7). Likewise, during the sensitized photoreaction of chlorobenzene 3, in addition to trichlorobenzene 7 as principal product (25.3%), 1,3-C12-Bz (10) appears in amounts about 2 times larger than that of 1,4-C12-Bz(ll), the yield of the latter dichlorobenzene amounting to 3.6%. In case of such photolysis of pentachlorobenzene (4), among the three Cl,-Bzs, 1,2,3,5-C14-Bz(2) is produced as chief product (52.8%),while the yield of the photoproduct 1,2,4-C13-Bz(7)is about 3 times smaller than that of 1,3,5-Cl8-Bz(8). Finally, 71% of the degraded hexachlorobenzene (5) undergoes photoconversion to pentachlorobenzene (4). Table I and also Figure 1 clearly indicate that the photoreactions of these polychlorobenzenes (except CJ-Bz, 5), sensitized by acetone provide considerable chemical yields of polychlorobiphenyls (PCBs). Compound 1 gives 2 heptachlorobiphenyls, most probably 2,2',3,3',4,4',5heptachlorobiphenyl (2,2',3,3',4,4',5-C17-BP) and 2,2',3,3',4,5,6'-C17-BP (12a and 12b; in Figure la) with a total chemical yield (C)of 3.74%, 10 hexachlorobiphenyls (10 Cl,-BPs) (13a-j) (E = -3.50%), and 5 pentachlorobiphenyls (14a-e) (E = -0.87%), the yields being determined, as mentioned above, from the amount of the decomposed substrate. Similarly, tetrachlorobenzene 2 yields three heptachlorobiphenyls including 2,3,3',4,4',5',6-C17-BP, 2,2',3,4,4',6,6'-C17-BP, and
2
23
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Figure 1. Plots of the total Ion chromatogram of hexane extract of acetone-sensttized photolysates of (a) 1,2,3,4-tetrachiorobenzene(l), (b) 1,2,3,5-tetrachIorobenzene(2), and (c) 1,2,4,5-tetrachlorobenzene (3) subJected to the halogen test computer program (20)for searching chlorine-containing compounds. GC-MS equipped with a capillary column was used (see Experimental Section for further detalls).
2,2’,3,3/,4,5/,6-Cl,-BP(24a-c) (C = 7.01% ), four C16-BPs (25a-d) (E = 4.69%), and a pentachlorobiphenyl (26) (0.64%). The sensitized irradiation of 1,2,4,5-C14-Bz(3) results in the formation of 2,2’,3,4’,5,5/,6-C1,-BP (33) (4.19%), four C&-BPs (34a-d)(E= 6.78%), four C15-BPs (35a-d) (E = ca. 2.33%), and a tetrachlorobiphenyl (36) (0.32%). Three nonachlorobiphenyls (Clg-BPs),namely, 2,2’,3,3’,4,4’,5,6,6’-Clg-BP,2,2’,3,3’,4,4’,5,5’,6-C19-BP,and 2,2’,3,3’,4,5,5’,6,6’-C&-BP(40a-c)of = 2.42%, along with a C18-BP (41) (0.53%) and a C17-BP (42) (0.49%) are formed in the case of sensitized photodegradation of pentachlorobenzene (4). During acetone-sensitized photolyses of tetrachlorobenzenes 1-3 in H20-CH3CN mixtures, no photoisomerized tetrachlorobenzenes are formed. However, in each case, photoisomerization leading to the production of chlorinated valence tautomers, possible valence-bond benzenes, has been noticed (see Table I and Figure 1). The
status of the data does not permit us to decide whether these tautomers are benzvalene, Dewar, prismane, or (and) some other possible isomers. Thus, Table I indicates that a tetrachloro valence-bond benzene could be detected in the photolysates of 1 and 2 (see peaks 16 and 28 in Figure 1). Likewise, such irradiation of compounds 1-3 generates two, one, and one trichloro valence tautomers, respectively (peaks 15a,b, 27, and 37 in Figure 1). In addition to the photoproducts described in the preceding paragraphs of this section, products like trichloroacetophenones and trichlorophenylacetonitriles are formed, when acetonitrile-water solutions of tetrachlorobenzenes 1-3 containing acetone as sensitizer are exposed to UV light. Moreover, in the case of substrates 1 and 2, 1(trichlorophenyl)-2-propanes(23and 31,respectively) and unidentified compounds, viz,, C8H5ClzN0and C8H6C1N0 (for 1 only) together with several other products are apparent (see Table I and Figure 1). Finally, in order to detect polychlorophenols among the photoproducts, the sensitized photolysates of substrates 1-5 (given in Table I) were treated with the ethereal solution of diazomethane (CHzNz)and the ether plus acetonitrile layers were separated from the water layers. The concentrated organic extracts were analyzed on column A (see Experimental Section). Comparison of the retention times of both authentic standard and photoproducts gave evidence that either polychlorophenols are not formed during the acetone-sensitized irradiation of substrates 1-5 under the present circumstances or these phenols undergo further photoconversions. Nonsensitized Photolysis. Again, reductively dechlorinated compounds appear as main products of nonsensitized irradiation of PCBzs (see Table 11). For some substrates, distribution of such products is altered compared to that of acetone-sensitized photolysis. For instance, under these conditions, 1,2,4-trichlorobenzene (7) is produced as a major product of 1,2,3,5-CI4-Bz (2), showing a chemical yield (determined from the amount of disappeared starting material) of 24.3%. Furthermore, direct photolysis of chlorobenzenes 1-5 proceeds to yield reductively dechlorinated benzenes containing one and two chlorine atoms less than the starting materials. Such photoreactions of C15-Bz (4)and CI6-Bz (5)produce this sort of products bearing even three C1 atoms less than the corresponding starting chlorobenzene. It is important to note that during direct photolyses of some substrates, especially 1,2,3,4-C14-Bz(1) and C16-Bz (4),photoproducts, e.g., 1,4-CI2-Bz(11) (from 1) and 1,2,4-C13-Bz(7)(from 4) formed via losses of two C1 atoms are observed in large yields (30.4% and 12.7%, respectively) (see Table 11). Finally, nonsensitized exposure of Ce-Bz (5)to UV light results in the generation of pentachlorobenzene (4) (76.8%) as principal product accompanied with the formation of 1,2,3,5-CI4-Bz(2)(1.2%), 1,2,4,5-C14-Bz(3) (1.7%), and trichlorobenzene 7 (0.2%) in minor yields. Contrary to acetone-sensitized photoreactions, in the case of nonsensitized irradiation of each isomer of tetrachlorobenzenes 1-3,the process of photoisomerization with the eventual formation of isomerized C14-Bzsalso takes place. It can be seen from Table I1 that chlorobenzene 1 photoisomerizes to 1,2,3,5- and 1,2,4,5-C14-Bz(2,3), the chemical yields of which are 2.26 and 0.72%~~ respectively. Similarly, substrate 3 gives rise to the formation of two other isomers, namely, 1,2,3,4-C14-Bz(1) (0.45%) and 1,2,3,5-C4-Bz(2)(1.119%). However, during nonsensitized photolysis of the 1,2,3,5-tetrachlorobenzene(2),only one tetrachloro isomer, Le., 1, with a yield of 5.99%, could be detected as photoisomerized chlorobenzene. In addition Environ. Sci. Technoi., Vol. 18,No. 4, 1984 237
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238 Environ. Sci. Technot., Vol. 18, No. 4. 1984
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to photoisomerized chlorobenzenes, compound 1 also provides a trichloro valence-bond benzene. During nonsensitized photolysis of these PCBzs, photoformation of PCBs is less significant as compared to the case of the acetone-sensitized photoreaction. In Table 11, we have recorded only those PCBs which were observable on GC with FID (column C) as well as on the capillary GC-MS system. It should be noted that several additional isomers of PCBs of trace levels are detected, when directly photolyzed samples of compounds 1-4 are checked on a capillary column with ECD utilizing acetone-sensitized photolysates (from Table I) as standards. Moreover, in preliminary photolysis of 1,2,3,5-C14-Bz(2) for 24 h in H20-CH3CN (1:l v/v) (5.61 mM/L), we have been able to identify one C17-BP,four C&-BPs,two C14-BPs,and one C13-BP among the photoproducts. Likewise, in another preliminary photoreaction of 4 for 24 h in H20-CH3CN (4:6 v/v) (1.20 mM/L) containing benzene (6.0 mM/L), the generation of four C14-BPsand one CIS-BPhas been observed. Identifications of the PCBs produced during such preliminary studies were carried out on the previously described GC-MS (18). In addition to the above cited photoproducts, on exposing C14-BZ (1) and C16-Bz (4) to UV radiation without sensitizer, unidentified compounds like C8H,C12N0and C8H4C13N0,two isomers of each, are also obtained, the products of the latter type being of trace levels (see Table 11). It is unexpected that direct photolysis of tetrachlorobenzenes 1-3 proceeds also to provide more highly chlorinated photoproducts. Thus, substrate 1 gives hexachloro valence-bond benzenes, whereas chlorobenzenes 2 and 3 yield pentachlorobenzene (1.43% ' amd trace levels (tr), respectively). Polychlorinated phenols were not among the photoproducts as shown by the method discussed above.
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Discussion Reductive Dechlorination. In general, the process of reductive dechlorination during both acetone-sensitized and nonsensitized photoreactions of the polychlorobenzenes (C1,C6H6-,, n = 4-6) in aqueous acetonitrile mixtures described above appears to proceed through the intermediacy of polychlorophenyl radicals (reaction 1) ClnC6H;-,
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generated by the homolysis of C-C1 bonds. In reaction 1, RH represents a hydrogen-donating solvent. The tentative detection of products such as chlorophenylacetonitriles, l-(chlorophenyl)-2-propanones,and chloroacetophenones (documented in Table I) by us as well as that of pentachlorobenzyl alcohol and tetrachlorobis(hydroxymethy1)benzene from 5 in methanol by Plimmer and Klingebiel (4) strongly supports the concept of involvement of the polychlorophenyl radical as an intermediate species. Furthermore, our identifications of heptachlorobiphenyls in the case of Cl,-Bzs 1-3 and nonachlorobiphenyls in the case of Cl,-Bz (4) is more evidence that chlorophenyl radicals appear in these irradiations. It is interesting to mention that the isotopic composition of biphenyl formed by photodechlorination of 2-C11-BP in equimolar solvent mixtures like CH3CN-H20, CH3CN-D20, and CD,CNH20 led Bunce (24) to conclude that only about 10-209'0 of the hydrogen abstraction originates from water, whereas the rest comes from acetonitrile. According to him, among aqueous organic mixtures, acetonitrile appears to be a promising cosolvent. Acetone has been detected in many natural waters (10). Since acetone is a well-known triplet sensitizer (10,25-27, and references cited therein), the product composition Environ. Sci. Technot., Vol. 18, No. 4, 1984
239
from both types of photolyses of 1,2,3,5-tetrachlorobenzene in the photolyzed sample of 1,3,5-C13-Bz(8) in solution (2) in an acetonitrile-water (1:lv/v) mixture (see Tables phase. In the case of photolyses of chlorobenzenes 1 and I and 11) is possibly understandable in terms of the fol2, two and three isomers of trichlorophenyl radicals are lowing concept. When molecules of substrate 2 are in the produced, respectively. Then chlorophenylation of 1%and first excited singlet state (SI), homolysis of a C-C1 bond 2%by these chlorophenyl radicals takes place, whereby two on position 1 (or 3) of compound 2 takes place, whereby and three isomers of C17-BP are formed, respectively. 2,3,5-trichlorophenyl radicals are produced. These radical Likewise, the isomers of nonachlorobiphenyl (C1,-BP) in species subsequently abstract H atoms from the solvent the case of 4 are formed through the intermediacy of three to yield 1,2,4-C13-Bz(7). When molecules of the starting chlorophenyl radicals, namely, 2,3,4,5-, 2,3,4,6-, and chlorobenzene (2) have reached the first excited triplet 2,3,5,6-tetrachloro isomers. In this manner, the tentative structures of C1,-BPs generated from Cl,-Bzs 1-3 and of state (TI)through intersystem crossing from the S1state and by transfer of excitational triplet energy of molecules C19-BPs produced from C16-Bz 4 were given. Similar of acetone to the ground state molecules So of substance chlorophenylation of the reductively dechlorinated pho2 (in the case of acetone-sensitizedreaction), the C-Cl bond toproducts also seems to happen; thus, PCBs containing on position 2 of Cb-Bz 2 is preferably homolytically cleaved lower chlorine contents are formed. In addition, chloroleading to the production of 2,4,6-trichlorophenyl radicals, phenyl radicals arising as a consequence of primary phothereby 1,3,5-C13-Bz (8) being eventually produced. tochemical processes are also likely to attack a position However, acetone sensitization of 2 does also account for bearing a C1 substituent of the ground-state molecules of the formation of substantial amounts of 1,2,4-trichlorothe starting material, e.g., Cl,-Bzs, as well as reductively benzene (7) (see Table I) which clearly cannot be explained dechlorinated photoproducts, viz., Cl,,-Bzs and Cl,,-Bzs, thereby Cl,_,-BPs and Cl,+.-BPs being eventually formed by the presented concept. Direct photolysis of 2 could have led to C13-Bz7, but this has to be ruled out by the fact that where n represents the number of C1 atoms in a starting the sensitizer absorbed nearly all of the radiation available. chlorobenzene. Furthermore, hexa-, penta-, and tetrachlorobiphenyls found in the photolysates of 1-3 can be Our results on the direct photolysis of pentachlorobenzene (4) (1.1mM/L) in aqueous acetonitrile (Table the consequence of the reductive dechlorination of the primary Cl,-BPs. Similarly, photodechlorination of C1911)are different from the results of Crosby and Hamadmad BPs in the case of photolysis of 4 can be taken into account (5) on 4 in hexane (1g/L) at 254 nm. They obtained only two C14-Bzs,namely, 1,2,4,5-C4-Bz(3) (50%)and 1,2,3,5in order to explain the appearance of octa and hepta isomers. C14-Bz(2) (13%) at a ratio of ca. 41, while we gained these Photoisomerization. With the support of our own data two isomers at a ratio of 1:5 accompanied with C14-Bz 1, and data from existing literature, detailed mechanisms two C13-Bzs,viz., 7 and 8, and C12-Bz11. Likewise, Plimexplaining the process of photoisomerization in the case mer and Klingebiel(4) could identify among C14-Bzsone of tetrachlorobenzenes which leads to the production of tetrachloro isomer, probably compound 2, as the phototetrachloro valence-bond benzenes and eventually photoproduct of the suspension of hexachlorobenzene 5 in isomerized tetrachlorobenzene(s) will be discussed in anmethanol, while we have been able to detect 2 isomers, other paper (28). namely, 2 and 3, in the nonsensitized photolysates of 5 (see Miscellaneous Processes. During these photoconTable 11). versions, the tentative identifications of chlorobenzene Further discussion on the reductive dechlorination of derivatives containing substituents like CH2-CN (recorded polychlorobenzenes (PCBzs) giving rise to the production in the last columns of Tables I and 11) are possibly proof chlorobenzenes having one C1 and especially two C1 duced through the simple combination of chlorophenyl contents less than the starting PCBzs will be presented radicals with CH,CN, previously generated as a conseelsewhere (28). quence of abstraction of the H atom from solvent CH3CN Formation of Polychlorobiphenyls (PCBs). The by other free radicals. Likewise, the formation of tridetection of PCBs in our photochemical studies of tetrachloroacetophenones 20, 29, and 38 in the case of aceand pentachlorobenzenes (1-4) can best be explained via tone-sensitized photolyses of C14-Bzsis probably the result free radical mechanisms. For instance, in the case of of an interaction between intermediate trichlorophenyl photolyses of 1,2,4,5-tetrachlorobenzene(3), light-excited radicals and acetyl radicals (CH&=O), the latter radicals molecules of substrate 3 yield 2,4,54richlorophenylradicals being produced through the photolysis of acetone. The (43), which then attack ground-state molecules of 3, Le., molecular ion (M+.), in the mass spectra of these aceto350,to produce a-complex intermediate 44. Intermediate phenones, exhibited the losses of masses of 15 (Le., CH3 species 44 in a subsequent step suffered loss of the H atom t o yield 2,2’,3,4’,5,5’,6-heptachlorobiphenyl group) and 43 (Le., acetyl group), thereby providing the clear evidence for the presence of acetyl substituents. (2,2’,3,4’,5,5’,6-C17BP,33) (as explained in reaction 2). Isopropyltrichlorobenzenes can be considered alternative structures for products 20,29, and 38. However, under the present circumstances, the photoformation of chlorinated isopropylbenzenes seems unlikely. Likewise, 14trichlorophenyl)-2-propanones23 and31 are formed by the 3 combination of corresponding trichlorophenyl radicals with CH2C(0)CH3radicals presumably generated via the abstraction of H atoms by the intermediate free radical species from acetone. L
s.3
Although the structure designated as PCB 33 is tentative, only one isomer, heptachlorobiphenyl (Cl,-BP), formed via reaction 2 is expected. In a previous study (16),we have detected and confirmed the presence of 2,3’,4,5’,6-C&-BP 240
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Environmentally Significant Conclusions In the environment, the photodegradation of tetra- and pentachlorobenzenes is possibly accelerated by the presence of acetone or similar sensitizers, while such a photoreaction of hexachlorobenzene is ca. 2 times slower than its direct photolysis. For the evaluation of photochemical
fate of tetrachlorobenzenes present in aquatic environments, the process of photoisomerization needs to be considered. For instance, nonsensitized photolysis of 1,2,3,4-tetrachlorobenzeneyields 1,2,3,5- and 1,2,4,5tetrachloro isomers, each photoisomerized Cl,-Bz being readily photodegradable and more effectively converted to polychlorobiphenyls than the starting tetrachlorobenzene. The identifications of PCBs and chlorinated phenylacetonitriles, acetophenones, l-phenyl-2propanones, etc., presumably formed through the intermediacy of polychlorophenyl radicals, show that relatively stable intermediate chlorophenyl radicals produced from PCBzs in the aquatic environment are likely to interact with the free radical centers of humic substances which are known to have 10’’ to lo1* spins/g (29) and aryl aromatic compounds, viz., substituted benzenes derived from humic substances and lignins (30) and aromatic hydrocarbons frequently found in natural water bodies. Although the photogeneration of PCBs from PCBzs seems to be dependent on the concentration of the chlorobenzenes, the appearance of PCBs in the acetone-sensitized photolyses of 1,2,3,4-, 1,2,3,5-, and 1,2,4,5-tetrachlorobenzene and pentachlorobenzene with total chemical yields (calculated from the photodecomposed chlorobenzene) of ca. 8.11%, 12.34%, 13.62%, and 3.44%, respectively, indicates that acetone which occurs in many natural waters may play an important role in the photoformation of PCBs from PCBzs present in lower concentrations.
Acknowledgments We thank H. Parlar of the Institute of Ecological Chemistry, Gesellschaft fur Strahlen- und Umweltforschung mbH Munchen, West Germany, and R. G. Zepp of the Environmental Research Laboratory, U.S. EPA, Athens, GA, for the critical comments on the manuscript.
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Received for review December 30, 1982. Revised manuscript received September 9, 1983. Accepted October 28, 1983.
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