Environ. Sci. Technol. 2001, 35, 4145-4148
Dehalogenation of Aromatic Halides Using Metallic Calcium in Ethanol YOSHIHARU MITOMA,‡ SATOKO NAGASHIMA,‡ CRISTIAN SIMION,‡ ALINA M. SIMION,‡ TOMOKO YAMADA,‡ KEISUKE MIMURA,‡ KEIKO ISHIMOTO,† AND M A S A S H I T A S H I R O * ,‡ Tohwa Institute for Science, and Department of Industrial Chemistry, Faculty of Engineering, Tohwa University, 1-1-1 Chikushigaoka, Minami-ku, Fukuoka, 815-8510, Japan
FIGURE 1. The reaction of 4-chlorobiphenyl 1 with Raney Ni-Al alloy in an alkaline aqueous solution.
FIGURE 2. The reaction of aromatic halides with metallic calcium in ethanol at room temperature.
The scope and limitations of the dehalogenation of aromatic halides 1 and 4a-p using metallic calcium in ethanol at room temperature were revealed. The cleavage of the carbon-chlorine bond on the aromatic ring bearing electron-donating group was difficult compared to the one bearing electron-withdrawing group. Moreover, we applied this method to the dechlorination of polychlorinated biphenyls (PCBs) in transformer oil. It was also found that the dechlorination took place easily under mild conditions. The existence of PCBs residue in the reaction at room temperature was less than 0.04% according to the GC-MS analysis. The chlorine was identified as calcium chloride.
Introduction It is widely recognized that polychlorinated biphenyls (PCBs) are one of mankind’s most dangerous pollutants and widespread environmental contaminants (1, 2). In the past decades, investigations focused especially on the removal and destruction of PCBs. Among the methods proposed (3, 4), incineration seemed to be preferred (5). Nevertheless, the interest in the recovery of reusable materials (e.g., PCBs are present mostly in transformer oils) and the necessity to treat contaminated products with low concentration of PCBs have renewed the interest in the dechlorination methods. The usual dechlorination methods of PCBs developed thus far (such as using sodium borohydride (6-8), photochemical dechlorination (9-13) or electroreduction (14-16)) present several disadvantages that are the prevention of moisture, the generation of undesired byproducts, but also the high costs of the required materials and energy. Now, a few selective decontamination methods are available, based on chemical (17, 18) or biological (19) dechlorination technologies. On the other hand, we have been investigating for some time the reduction of some functions in water as a hydrogen source. For examples, we have previously reported the reduction and reductive coupling of imines (20, 21) or carbonyl compounds (22-25) and the dehalogenation of aromatic halides (26-29) using metals in an aqueous solution. Moreover, we also found that the reaction of 4-chlorobiphenyl * Corresponding author phone: +81-92-541-1512; fax: +81-92925-2737; e-mail:
[email protected]. † Department of Industrial Chemistry, Faculty of Engineering, Tohwa University. ‡ Tohwa Institute for Science, Tohwa University. 10.1021/es010716+ CCC: $20.00 Published on Web 09/12/2001
2001 American Chemical Society
1 with Raney Ni-Al alloy, in a diluted aqueous alkaline solution at 90 °C was very effective for the dehalogenation process, yielding biphenyl 2 (in 10% aqueous NaOH solution) or cyclohexylbenzene 3 (in 0.5% aqueous KOH solution), respectively (30) (Figure 1). Recently, we discovered that the reaction of aromatic halides 4a,b,d-k with commercially available metallic calcium in ethanol at room temperature formed the corresponding dehalogenated compounds in good yields (Figure 2). Many dehalogenation methods using low-valent metal such as alkali metal in alcohol (31, 32), Mg (33), and Zn/acidic (34) or basic (35) solution, using metal hydrides (36), or a catalyst such as Raney Ni (37) or Pdcarbon (38) under hydrogen gas were developed up to now. In the dehalogenation process using metallic calcium, to the best of our knowledge, only one method, treatment of Arochlor with metallic calcium in alcohols under a nitrogen atmosphere, was reported by J. A. Hawari and co-workers as a patent (39). The authors state that GC analysis indicated disappearance of about 50% of the PCBs in the first treatment in methanol, but there was not enough information in the patent to estimate the scope and limitations of their method. We wish to report here an efficient and environmentalfriendly method for dehalogenation of aromatic halides using metallic calcium in ethanol with emphasis on PCBs in transformer oil (where recovery of unreacted PCBs is under 0.04%).
Experimental Section General. Distillated water was used in all reactions. Commercially available (Wako Pure Chemicals Industry, Ltd.) ethyl alcohol was used. Granular particles of metallic calcium (Kishida Chemical, Co. Ltd.) were directly put in the reaction without any treatment in advance. The GC-MS analysis was carried out on a HP 6890 series gas chromatograph (HewlettPackard) equipped with a 30 m DB-1 column (i.d. ) 0.25 µm) (J&W Scientific) and a JMS-AM II series (JEOL) which is a quadrupole mass spectrometer. Ionization was performed under 70 eV electron-impact (EI) conditions. The GC-17A (Shimadzu) gas chromatograph (30 m DB-1 and FID detection) was used for routine work and for the determination of the yield by using ethylbenzene as internal standard. Its recorder was a C-R6A Cromatopac (Shimazdu Ltd.). The programs of both GC were the same, as follows: the initial temperature of the column was 60 °C, the temperature was kept for 3 min, then the rate of temperature increase was 25 °C/min up to 250 °C, and the temperature was kept for 20 min. Typical Procedure for the Dehalogenation of Halogenated Benzenes (4b-e) and the Determination of Products VOL. 35, NO. 20, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Using the GC Analyses. A mixture of dichlorobenzene (0.796 g, 5.41 mmol), metallic calcium (2.4 g, 60 mmol), and ethanol (10 mL) was stirred at room temperature, in a round-bottom flask. After 24 h, the reaction mixture was poured into 100 mL of 1 N hydrochloric acid. The solution was extracted twice with ether and washed. The total volume of ether was exactly increased up to 40 mL by adding pure ether. An amount of 0.1 mL of the ether solution and another 0.1 mL of a 0.01 M solution of ethylbenzene in ether were mixed; 1 µL of the resulting mixture was injected by a microsyringe. The initial yields of products were obtained by estimating the ratio of the area of peaks and reporting to the area of the peak for ethylbenzene. It was found that the chlorobenzene was formed quantitatively. The structure and the yields of products were identified by GC-MS and GC analysis, respectively. Typical Procedure for the Dehalogenation of Halogenated Benzenes Having Nitrogen Atom in Its Chain (4m and 4p). A mixture of p-chloroaniline 4m (0.635 g, 4.98 mmol), metallic calcium (0.80 g, 20 mmol), and ethanol (10 mL) was stirred at room temperature. After 24 h, the solution was extracted with 3 × 20 mL ether. Combined organic layers were washed, dried on MgSO4, and evaporated. The residue (0.621 g) was dried in vacuo. The crude reaction product was analyzed by GC-MS analysis. The recovery of 4m was more than 97%. Typical Procedure for the Dechlorination of the 4-Chlorobiphenyl (1). A mixture of 4-chlorobiphenyl (1.0537 g, 5.59 mmol), metallic calcium (0.8 g, 20 mmol), and ethanol (10 mL) was stirred at room temperature. After 24 h, the reaction mixture was added to 100 mL of 1 N hydrochloric acid. The solution was extracted with 3 × 20 mL ether. Combined organic layers were washed, dried on MgSO4, and evaporated. The residue (0.840 g) was dried in vacuo. The crude reaction product was analyzed by GC-MS analysis. It was found that the ratio of products 2, 12, and 13 was 35%, 40%, and 15%, respectively. Typical Procedure for the Dehalogenation of Polychlorobiphenyls (PCBs) in Transformer Oil. Transformer oil containing a mixture of PCBs (1.0271 g), metallic calcium (0.8 g, 20 mmol), and ethanol (10 mL) were stirred at room temperature. After 24 h, the reaction mixture was added to 100 mL of 1 N hydrochloric acid. The solution was extracted with 3 × 20 mL ether. Combined organic layers were washed, dried on MgSO4, and evaporated. The residue (1.007 g) was dried in vacuo. The crude reaction product was analyzed by GC-MS analysis. The results were summarized in Table 2. Measurement of the Concentration of PCBs by the GCMS Analysis. In the measurement of PCBs, the conditions of the GC-MS analysis of chlorinated biphenyls were the follwing: the GC-MS analysis was carried out on a HP 6890 series gas chromatogragh (Hewlett-Packard) equipped with a 60 m DB-5 (i.d. ) 0.25 mm, 0.25 µm film thickness) (J&W Scientific) and JMS-700 series (JEOL). The program of GC was as follows: the initial temperature of the column was 105 °C, the temperature was kept for 3 min, then the rate of the temperature increase was 20 °C/min up to 210 °C, and the temperature was kept for 15 min. The temperature of the column was then increased by 1.5 °C/min up to 243 °C and then by 40 °C/min up to 280 °C, where it was maintained for 10 min. The injection was performed by a splitless mode. The carrier gas was He and its column flow was 2.0 mL/min. The temperature of the injection port, interface, and ion source were 295, 295, and 200 °C, respectively. The ionization energy and current were 45 eV and 600 µA. The resolution was 10 000. Monitor ion was as follows: 188.0393, 190.0363 (monosubstituted chlorinated biphenyls), 222.0003, 223.9974 (disubstituted chlorinated biphenyls), 255.9613, 257.9584 (trisubstituted chlorinated biphenyls), 289.9224, 291.9194 (tetrasubstituted chlorinated biphenyls), 301.9627 (M+), 4146
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TABLE 1. Reduction of Aromatic Halides Using Calcium in Ethanola
a Substrate: 5 mmol, calcium: 20 mmol, reaction temperature: room temperature, reaction time: 24 h. b Ratios and structures were determined by the GC-MS analyses. c The yield of the products was determined by the GC analyses using ethylbenzene as a internal standard. d The amount of calcium was 60 mmol.
303.9597 (M+ + 2) (13C of biphenyl in tetrasubstituted chlorinated biphenyls). Identification of the Chlorine Ion. In the titration of chlorine ion, a large excess of pure water was added to the reaction mixture in order to quench the metallic calcium without the use of hydrochloric acid. Then, the solvent was removed in vacuo. The obtained solid was used in Mohr’s titration.
Results and Discussion Dehalogenation of Aromatic Halides. Although several other alcohols (such as methanol, iso- or n-propanol) were tested as solvents in this process, ethanol was by far the most efficient for the dehalogenation. Results of the dehalogenative process in the metallic calcium-ethanol system are resumed in Table 1. Entries 1-3 showed that if debromination and deiodination of the corresponding halogenated benzene is easy to realize, the dechlorination process is difficult, even when increasing the quantity of metallic calcium up to 60 mmol. On the other hand, dichlorobenzene isomers 4d and 4e reacted with 60 mmol of calcium yielding quantitatively 4c (entries 4 and 5). The reactions of 4-iodochlorobenzene 4f and 4-bromochlorobenzene 4g also produced 4c in quantitative yield (entries 6 and 7). In the same manner, bromoalkylbenzene 4h suffered debromination (in this case with increased quantity of metallic calcium), while pchlorotoluene 4i was completely recovered (entries 8-10). In contrast with alkylbenzenes, in alkenylchlorobenzenes
TABLE 2. Treatment of PCBs in Transformer Oil with Calcium and Ethanol at Room Temperature FIGURE 4. The reaction of 4-chlorobiphenyl 1 with metallic calcium in ethanol at room temperature.
a The concentration and identification were determined by GC-MS. The concentration is the ratio between the weights of PCBs (in µg) and total substances (in g). The N.D. means nondetection of any PCBs.
Application to the Dechlorination of Polychlorinated Biphenyls (PCBs). The next step in our investigation was to extend the research to the dechlorination of PCBs in transformer oil. In a previous research, we revealed that the direct use of Raney Ni-Al alloy in a dilute alkaline solution was effective for the dechlorination of 4-chlorobiphenyl 1 to give the cyclohexylbenzene 3 in good yield (30). In contrast with this result, it turned out that the reaction of 4-chlorobiphenyl 1 with the metallic calcium-ethanol system yielded biphenyl 2 along with the side products 12 and 13 (Figure 4). In addition, a mixture of PCBs in transformer oil such as alkylbenzenes was used as starting material in the same process. As previously mentioned, high concentrations of PCBs are present in dielectric or lube-used oils (1, 2). All PCBs present in the oil submitted to dechlorination were reduced to hydrocarbons by the calcium-ethanol system, at room temperature and in mild conditions (Table 2). After a 24 h treatment, at room temperature, the residue of PCBs in the reaction mixture was less than 0.04%, according to GCMS analysis. The product in the dehalogenation could not be isolated because interference with alkylbenzenes present in the transformer oil. The formation of chlorine ion during this process was confirmed by Mohr’s titration method. Moreover, ethanol could be recycled as solvent for the next PCBs dechlorination process. We also found out that small quantities of iodine added to the reaction mixture can shorten the reaction time down to only 5 h instead of 24 h. We believe that this method is environmental-friendly and presents significant economical advantage.
Literature Cited
FIGURE 3. The reaction scheme of 4-chloroacetophenone 4k. such as p-chlorostyrene 4j the dechlorination process occurred easily, accompanied by the reduction of the sidechain double bond (entry 11). When p-chloroacetophenone 4k was submitted to dehalogenation, almost equivalent quantities of 4l and 9 were formed (entry 12). While submitting 4l to dechlorination in the same conditions (entry 13), the recovery of the starting material leads to the assumption that in the transformation of 4k the first step is the dechlorination, and the reduction of the carbonyl group occurs only after elimination of the halogen (Figure 3). This process is somehow comparable with the dechlorination and reduction of 4j. In the same time, dechlorination of 4-amino-, 4-methoxy- and 4-hydroxychlorobenzene derivatives (4m-o) produced no transformation of the starting material. We can therefore assume that in halogenated benzenes bearing electron-withdrawing groups (4d-g or 4j, 4k) the dehalogenation process is favored compared to halogenated benzenes with electron-donating groups (4l-o and less in 4h and 7). An interesting behavior presented 4-nitrochlorobenzene 4p (thus, a chlorobenzene bearing an electronwithdrawing group) which gave the azo- and azoxy-coupling products 10 and 11, respectively, without dehalogenation and without isolation of the possible p-chloroaniline intermediate.
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Received for review March 6, 2001. Revised manuscript received June 21, 2001. Accepted July 2, 2001. ES010716+
