Environ. sci. Technol. 1904, 28, 573-576
Reaction of a Lignin Model Dimer Sequentially with Chlorine and Sodium Hydroxide Bruce McKague' and Douglas W. Reeve Department of Chemical Englneering and Applied Chemistry and Pulp & Paper Centre, University of Toronto, 200 College Street, Toronto, Ontario, Canada M5S 1A4
Dimer 9, which is representative of diaryl methane structures reported present in residual lignin, was reacted with aqueous chlorine at 50 "C. The major product was identified as the tetrachloroquinone dimer 13 ( x = 2, y = 2). Similar reaction of the tetrachlorocatechol dimer 12 with aqueous chlorine gave a mixture of the tetrachloroquinones 15 and 16. When the mixture of the quinones was reacted with 2.5% sodium hydroxide, an intramolecular oxidation-reduction (Cannizzaro)reaction with the simultaneous loss of two chlorine atoms occurred as the main reaction. The products, 21, were characterized as having a chlorine-freemuconic acid lactone group attached to a dichlorocatechol. A third product 17 from the reaction with sodium hydroxide appeared to result from simple displacement of the two chlorine atoms in the quinone ring of 15 by hydroxyl groups. The results provide some clarification of the reactions occurring during classical chlorine bleaching. The results are also in agreement with literature reports concerning some of the structural characteristics of the high mass material.
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Introduction Classical chlorine bleaching is now being replaced by newer technologies which minimize or eliminate the use of chlorine. Since an extremely large quantity of material derived from chlorine bleaching has been discharged into the environment over the past 20-30 years, it remains of interest to know how chlorine reacts with lignin during bleaching, particularly with respect to formation of the high mass material, and research which provides information about the chemical structure of the high mass material will continue to be important. Recently, it was reported the simple model compound 1 reacted with chlorine to give a mixture of the quinone 2 and the dione 3 ( I ) as shown in Figure 1. Treatment of the dione 3 with sodium hydroxide resulted in a benzilic acid rearrangement to the cyclopentenecarboxylic acid 4 (I),while tetrachloro-1,2-benzoquinone5 underwent oxidationfreduction to the muconic acid lactone 6 and tetrachlorocatechol7 (2). The results showed that muconic acids may be terminal reaction products of lignin treated sequentially with chlorine and sodium hydroxide. Also, in a recent paper, the reaction of the lignin model dimer 8 (Figure 2) with various amounts of chlorine in water was reported (3). Ring A was chlorinated and oxidized to nonaromatic chlorinated quinones but ring B could not be dearomatized without cleavage of the @-arylether bond. On the basis of this result, it was concluded that fewp-aryl ether bonds were present in either residual lignin or the high mass material resulting from chlorine bleaching and that carbon-carbon bonding was the main form of bonding present. Condensation reactions resulting in the formation of new carbon-carbon bonds are thought to occur during 0013-938X/94/0928-0573$04.50/0
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0 1994 American Chemical Society
pulping (4,5). One suggested type of condensation results from the reaction of phenolic units in lignin with formaldehyde released during pulping to give diaryl methane structures linked in the ortho position to the phenolic group. In this paper, reactions of the model condensationtype dimer 9, representing of this type of diaryl methane, successively with chlorine and sodium hydroxide are reported.
Experimental Methods General. Proton and carbon magnetic resonance (1H NMR and 13C NMR) spectra were recorded in CDC13 solution, unless otherwise indicated, using Bruker AM 300 MHz or AM 500 MHz high-resolution spectrometers. Signal positions are given in ppm (6) relative to MedSi. Infrared (IR) spectra were recorded on a Perkin Elmer Model 598 spectrophotometer. A Hewlett Packard 5890 gas chromatograph equipped with a 25-m HP-1 capillary column was used for gas chromatography (GC). Gas chromatography/mass spectrometry (GC/MS) was done on the same gas chromatograph coupled to a VG analytical ZAB-SE high-mass high-resolution mass spectrometer. Silica gel (Si02) used for chromatography was 100-200 mesh. Thin-layer chromatography (TLC) was done on Si02 plates, 0.25 mm thickness, containing a fluorescent indicator. Methylations were done with diazomethane. Preparation of Model Dimer 9. Dimer 9 was prepared by refluxing a mixture of 1 N NaOH (0.1 mol), 4-methylguaiacol (0.1 mol), and 37% formaldehyde (0.25 mol) with stirring for 2.5 h. After being cooled, the product was acidified with dilute HCl and extracted with ether. Environ. Sci. Technol., Voi. 28,
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(35%), 267 (60%), 139 (80951, 75 (72%), 63 (66%). Evaporation of the ether and crystallization of the crude Reduction with NaBH4 in methanol give back 12. Simproduct (97% yield) from methanol gave dimer 9, mp 129ilarly, drying (MgS04) of the filtrate and evaporation gave 130 "C (reported 126 "C, ref 6). 'H NMR 6: 2.23 (6H, s, a red oil (185 mg), which gave back 12 on reduction with 2ArCH3),3.84 (6H,s,20CH3),3.91(2H,s,CH2),6.54(2H, NaBH4. s,ArH), 6.59 (2H, s, ArH). 13CNMR6: 21.1 (ArCHs),29.1 (CHz),55.8 (OCH3), 109.8,122.7,126.2,128.9,140.7,146.4 Successive Reaction of 12 with Clz and NaOH. (Ar). MS (rnle): 288 (M+,go%), 151 (40%),138 (100%). Chlorination was performed as described above. The total Preparation of 2,2'-Methylenebis(3,5-dichloro-6product (500 mg) was stirred in 2.5% NaOH (50 mL) at 60 "C for 20 min. After being cooled, NaCl(5 g) was added, hydroxy-4-methylphenol)12. Demethylation of dimer and the mixture acidified with concentrated HC1, and 9 by stirring with 1M BBr3 in dichloromethane (2.2 mole extracted with ether. The combined extracts were washed ratio) at room temperature for 1h gave 2,2'-methylenebiswith small portions of H20, dried (MgS04),and evaporated (6-hydroxy-4-methylphenol) 10 (99% yield), which had to give a brown product (350 mg) which showed three mp 185-186 "C after recrystallization from toluene. lH peaks on GC after methylation at retention times of 9.7, NMR (DMSO-ds) 6: 2.04 (6H, s, 2ArCH3), 3.68 (2H, s, CHz), 6.23 (2H, d, ArH), 6.41 (2H, d, ArH). MS (rnle): 11.9,and 12.9min when the temperature was programmed from 180 to 280 "C at 20 "Clmin. Fractionation of the 260 (M+,75%), 137 (loo%),136 (47%),124 (100%).The NaOH product on Si02 (20 g) and elution with ether and tetrachloro derivative 12 was prepared by reaction of 10 ethecmethanol, 9:1, gave a series of fractions (total 255 with a 4.4 mole ratio of SOzC12 in HOAc for 1h at 40 "C. After workup by being poured into H2O and extraction mg). Methylation and refractionation of the material eluted with ether gave the product with a retention time with ether, the product was purified by fractionation on of 9.7 min as a brown gum. Similar treatment of the ether: Si02. Elution with hexane:ethyl acetate 4:l containing 5 94 HOAc gave initially 2,7-dimethyl-4,5-dihydroxy- methanol 9:l eluate gave the products with retention times of 11.9 and 12.9 rnin as brown amorphous solids. The 1,3,6,8-tetrachloro-9H-xanthene 11 (16% yield) as a very three methylated products were about 90% pure by GC insoluble pale orange solid. Recrystallization of a small and had the following spectral data. For the 9.7-min amount from EtOAc gave small orange needles, mp >300 compound, lH NMR 6: 2.03 (3H, s, CH3), 2.39 (3H, s, "C. lH NMR (DMSO-d6) 6: 2.41 (6H, s, 2ArCH3), 3.97 CH3),3.85 (2 or 3H, s, CHz or OCH3),3.95 (2 or 3H, 8,CH2 (2H, s, CH2). MS (rnle): 382 (M+ + 4,48%), 380 (M+ + or OCH3),4.03 (2 or 3H, s, CH2 or OCH3). MS (rnle): 388 2, loo%), 378 (M+, 79%), 345 (98%), 343 (100%). (M+ 2,65%), 386 (M+,loo%), 358 (40%),351 (35%), Dimethyl ether derivative MS (rnle): 410 (M+ + 4,51%1, 327 (40%),301 (40%),299 (60%),284 (45%),283 (44%), 408 (M+ 2,100%),406 (M+,81%),377 (42%),375 (40%), 269 (60%),267 (80%1, 255 (40%). Exact mass calcd for 373 (25%),371 (28%). This was followed by the tetraC17H&1~06: 386.0323; found: 386.0325. For the 11.9chloro derivative 12 (63% yield) which was crystallized min compound, 1H NMR 6: 1.79 (3H, s, CH3), 2.43 (3H, from toluene to give pale yellow needles, mp 240-244 "C. s, CH3), 2.97 (approximately l H , s, CH2), 3.18 (approxIR (Nujol) vmax 3500,3350,1595 cm-'. lH NMR (DMSOimately lH, s, CHz),3.81 (3H,s,OCH3),3.98(3H,s, OCH3), de)6: 2.26 (6H, s,2ArCH3), 4.14 (2H, 8, CHd. MS W e ) : 4.19 (3H, s, OCH3). MS (rnle): 418 (M+ + 2,60%), 416 400 (M++ 4,16%),398 (M++ 2,3376) 396 (M+,26%)364 (M+,90%),386(20%),351 (25%),349 (65%),197 (loo%), (28%),362 (50%), 360 (37%),345 (18%),325 (27%),207 141 (35%). Exact mass calcd for CleH18C120.1: 416.0429; ~ (43%),171(35%),158 (64%),205(loo%),194 ( 2 7 %192 found: 416.0437. For the 12.9-min compound, 'H NMR (24%), 77 (23%). Exact mass calcd for C16HlzC1404: 6: 1.92 (3H, s, CH3),2.42 (3H, s, CH3),3.24 (approximately 395.9487; found: 395.9496. 2H, 9, CHz), 3.82 (3H, 9, OCH3), 4.06 (3H, 9, OCHd, 4.28 Reaction of Dimer 9 with Clz. Chlorine was bubbled (3H, s, OCH3). MS (rnle): 418 (M+ + 2,60%), 416 (M+, into a stirred suspension of the dimer (500 mg) in H2O loo%), 341 (60%),339 (loo%), 218 (40%), 167 (100%). (500 mL) at 50 "C for 15 min. The mixture was cooled, Exact mass calcd for C18H&120,: 416.0429; found: extracted with ether (3 X 50 mL), washed with H2O (3 X 416.0447. 10mL),dried (MgS04),and evaporated togive the product (800 mg). Half of the product was dissolved in MeOH (5 Results and Discussion mL) and reduced with NaBH4 at room temperature for 15 min. The product was diluted with HzO, acidified, and Preparation of the Tetrachloro Derivative 12. The after the usual workup fractionated on Si02 to give a major tetrachloro derivative 12 was prepared as shown in Figure fraction (151 mg, eluted with hexane:EtOAc, 1:l) con3. The product was consistently contaminated with a taining 5% HOAc as sticky gray crystals. MS (rnle): 400 byproduct, which was identified as the tetrachloroxanthene (M+ 4,20%), 398 (M+ + 2,40%), 396 (M+, 30%), 364 11by its characteristic mass spectrum (7)and methylation (IF,%), 362 (25%),360 (20%),345 (25%),325 (28%),207 t o give the dimethyl derivative. The formation of xan(68%),205(loo%),194 ( 2 7 %192 ~ (41%),185(20%),172 thenes by cyclodehydration of phenols normally occurs at (38%),77 (19%). Crystallization from toluene gave tan a much higher temperature (8). The xanthene 11 was crystals, mp 230-234 "C, which was identified as 12 by IR. very insoluble and had a melting point >300 "C. Reaction of 2,2'-Methylenebis(3,5-dichloro-6-h~Reaction of Dimer 9 with Clz. Dimer 9 was simuldroxy-4-methylphenol) 12 with Cl2. Chlorine was taneously chlorinated and oxidized to give a mixture of bubbled gently into a stirred suspension of 12 (500 mg) in products, which could be shown to contain mainly the H20(500mL) at 50 "C for 15min. The mixture was cooled, tetrachloroquinone 13 ( x = y = 2) by reduction of the extracted with ether (3 X 100 mL), washed with H2O (3 crude product with NaBH4 (Figure 4). The major product x 20 mL), and filtered after the last wash to give a mixture from reduction was the tetrachlorocatechol dimer 14 ( x = of the insoluble quinones 15 and 16 (330 mg). MS (rnle): 2, y = 2), which was identified by comparison with material 396(5%),394(9%),392(7%),362(34%),360(99%),358 prepared separately as shown in Figure 3. Analysis of the (loo%),331 (50%),329 ( 5 0 %297 ~ (25%1,295 (40%1,269
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Envlron. Sci. Technol., Vol. 28, No. 4, 1994
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NaBH4 reduction product by TLC also indicated other chlorinated catechol dimers with the general structure 14 ( x , y = 0-2) were present. Thus, dimer 9 undergoes chlorination and oxidative dealkylation in the same general manner as lignin (9) and various guaiacol monomers (IO, 11).
Reaction of the Tetrachlorocatechol Dimer 12 with Aqueous Clz. Reaction of 12 with chlorine in HzO gave a mixture of the quinones 15 and 16 (Figure 5). Weak molecular ions corresponding to both quinones could be seen in the mass spectrum of the product, and the base peak was m / e 358, which corresponded to loss of HC1 from the monoquinone 15. Reduction of the product with NaBH4 gave back the starting material 12. In view of this result, oxidation products which contained one quinone ring were also likely present in the reaction product from dimer 9. Successive Reaction of 12 with Clz and NaOH. The overall objective of this work is to use model oligomers with known structures to obtain information about the structure and behavior of the high mass material produced
during pulp bleaching. In order to simplify the analysis of products resulting from this two-step reaction sequence, the tetrachlorocatechol dimer 12 was used as the starting material since it would initially be oxidized to the mixture of quinones 15 and 16, which are the major products of the reaction of dimer 9. After reaction of the chlorination product with NaOH, fractionation, methylation, and refractionation eventually gave three products that were about 90% pure by GC. The product that had a retention time of 9.7 min under the conditions reported in the Experimental Section had a molecular mass of 386 and a molecular formula of C17H16ClzO6. Both the other products had a molecular mass of 416 and a molecular formula of C18H18C1~07.Thus, each product had two chlorine atoms less than the starting material, and methylation had resulted in the addition of two and three methyl groups, respectively. In a previous study (9), it was shown that tetrachloro1,2-benzoquinone 5 underwent an Cannizzaro-type reaction with NaOH to give tetrachloromuconic acid, which cyclized with the loss of the HCl to give the lactone 6 and tetrachlorocatechol 7 (Figure 1). In the present study, absence of the dimer 12 in the product meant that the quinones 15 and 16 had not undergone a similar intermolecular Cannizzaro reaction. A structure that is consistent with the lH NMR and MS can be derived for the product having molecular formula C17H&1206 simply by replacing the chlorine atoms of 15 with hydroxyl groups via a Michael-type addition-elimination, followed by methylation of two of the hydroxyl groups to give 18 as shown in Figure 6. Diazomethane, used for methylation, is a relatively weak methylating agent and unlikely to methylate all the hydroxyl groups. The mass spectra of the two products having molecular mass 416 are shown in Figure 7. Each spectrum had a large fragment that did not contain chlorine ( m / e197 and 167) and a smaller fragment at m/e 218 or m/e 219 that contained the two chlorine atoms. This suggested that both the chlorine atoms were located in the same part of the molecule, which then fragmented into two, approximately equal parts. The NMR spectra of these two compounds were very similar and contained two methyl groups in the range 1.8-2.5 ppm and three singlets in the range 3.8-4.3 ppm. One of the products had a quartet at 3.1 ppm and the other had a singlet at 3.24 ppm. On the basis of the above data, the dimeric quinone 16 had undergone an intramolecular Cannizzaro reaction as shown in Figure 8. The resultant muconic acid 19 would Environ. Sci. Technol., Vol. 28, No. 4, 1994 575
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muconic acid portion of the molecule are much more susceptible to displacement by hydroxide ion than those on the aromatic ring. The results of the study are in general agreement with some of the reported characteristics of the high mass material in alkaline-spent bleach liquors (12). After studying the spectroscopic characteristics of the material, Lindstrom and Osterberg concluded "the high molecular mass matter in all the alkaline SBL's contains a large number of carboxylic groups, to a high degree conjugated with double bonds". 21 (which could be in equilibrium with the open muconic acid in HzO) is consistent with the statement. These authors also reported the carbon/ chlorine ratio for the high mass material as a whole was 14,but the aromatic carbon/chlorine ratio was 5 for alkaline liquor from chlorine bleaching. In other words, the chlorine content per carbon atom was much higher in the aromatic material. 17 and 21 (Figures 5 and 7) represent the simplified extreme case of this distribution where all the chlorine is located in the aromatic portion and none in the remainder of the molecule.
This work was part of a research consortium on Organochlorine in Pulp and Paper Products, Effluent Treatment and the Environment. Funding is from the Ontario Ministry of Universities and Colleges, University Research Incentive Fund, and the following companies: Aracruz Cellulose S.A.; Champion International Corp.; CRS Sirrine, Inc.; Fletcher Challenge Canada Ltd.; International Paper; ITT Rayonier Incorp.; James River Corp.; Kymmene Corp.;Nippon Paper Industries Co.,Ltd.; Noranda Inc.; The Procter & Gamble Cellulose Co.; Sterling Pulp ChemicalsLa.; and Weyerhaeuser Co. NMR and mass spectral services were provided by Dr. A. A. Grey and Dr.H. Pang, respectively, at the University of Toronto Carbohydrate Research Centre, which receives funding from the Medical Research Council of Canada. Literature Cited
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Figure 8. Reaction of diquinone dimer 16 with NaOH.
cyclize to 20, which would eliminate HC1 to give the final product 21. This reaction scheme would be consistent with that of tetrachloro-1,2-benzoquinone(Figure 1).The product 21 would be a mixture of geometric isomers and/ or could form different trimethyl derivatives when methylated. The methylated derivatives would fragment between the rings in the mass spectrometer to give two main fragments, one of which contained both the chlorine atoms and which had masses corresponding to those shown in Figure 7. The mechanism shown in Figure 8 explains how both of the remaining chlorine atoms end up on the same side of the molecule. Once the initial oxidationreduction to 19 has occurred, both chlorine atoms on the 576
Envlron. Scl. Technol., Vol. 28, No. 4, 1994
(1) McKague, A. B.; Reeve, D. W.; Rettig, S.J.;Trotter,J. Can. J . Chem. 1992, 70,1711-1716. (2) McKague,A.B.;Reeve,D. W.; Rettig, S.J.; Trotter,J. Can. J . Chem. 1992, 70, 1706-1710. (3) McKague, A. B.; Kang, G.; Reeve, D. W. Holzforschung 1993,47, 497-500. (4) Gierer, J. Wood Sci. Technol. 1985, 19, 289-312. (5) Lai, Y.-Z. In Wood and Cellulosic Chemistry; Hon, D. N.S., Shiraishi, N., Eds.; Marcel Dekker, Inc.: New York, 1991; pp 455-523. (6) Bailey, H. C. British Patent 845,608, 1961; Chem. Abstr. l961,55,7365d. (7) Kuehl, D. W.; Butterworth, B. C.; De Vita, W. M.; Sauer, C. P. Biomed. Environ. Mass Spectrom. 1987,14,443-447. (8) Sen,R. N.; Sarker, N. N. J.Am: Chem. SOC.1925,47,10791091. (9) Gierer, J. Wood Sci. Technol. 1986, 20, 1-33. (10) Sarkanen, K. V.; Strauss, R. W. Tappi 1961,44,459-464. (11) Gees, J. M.; Dence, C. W. Tappi 1971,54, 1114-1121. (12) Lindstrom, K. bsterberg, F. Holzforschung 1984,38,201212.
Received for review April 27, 1993. Revised manuscript received October 22, 1993. Accepted December 15, 1993.' Abstract published in Advance ACSAbstracts, February 1,1994.
