[CONTRIBUTION FROM
THE]:NSTITUTE
OF PAPER CHEXISTRY]
THE NATURE OF LIGNIN FROM WESTERN HEMLOCK (TSUGA HETEROPHYLLA) F. E. BRAUNS Received February 7, 1945
In connection with a study of the chemical composition of western woods, the lignin from western hemlock (Tsuya heterophylla) was studied and compared with that from black spruce (Picea mariuna). Two types of lignin were investigated : so-called native hemlock lignin and Willstatter or hydrochloric acid lignin. The former was isolated by continuous extraction of hemlock sawdust with 95% alcohol, the latter by treatment of unextracted, of benzene-alcoholextracted, and of sodium hydroxide-extracted hemlock woodmeal with supersaturated hydrochloric acid. All these lignin preparations were methylated with diazomethane to a constant methoxyl content and the products obtained were compared with the corresponding lignin derivatives from black spruce. Native hemlock lignin was obtained in a yield of about 1.2% of the wood, which is somewhat lower than that obtained from black spruce, probably because fresh black spruce was used, whereas the hemlock was seasoned. The methoxyl content of the purified native hemlock lignin was 14.7%, the same as that of native black spruce lignin (1). The lignins differ, however, in their color, the native black spruce lignin being a very light cream, whereas the hemlock lignin is light salmon pink because of the coprecipitation of a minute amount of red coloring matter. This coloring matter may also cause the slight difference in the ultraviolet absorption spectra, but both show absorption maxima a t 282 mmu as found by Glading (2)(Figure 1:1. The Willstatter lignins from the unextracted and extracted hemlock samples vary slightly in their yields as was to be expected, but show no difference in their methoxyl contents. On methylation of the native hemlock lignin with diazomethane, the methoxyl content increases to 21.4%) which is the same as that obtained with native spruce lignin but, although the color of the latter is almost white, that of methylated native hemlock lignin still has a reddish tinge. The methoxyl contents of the methylated Willstatter lignins are practically identical with that of black spruce. The methoxyl contents of these products are given in Table I. The results show that native lignin and Willstatter lignin from western hemlock have the same number of methoxyl groups per lignin building unit (1) as the corresponding lignin derivatives from black spruce and also the same number of hydroxyl groups capable of methylation with diazomethane. The ultraviolet absorption spectra of the two native lignins also show a close structural relationship. EXPERIMENTAL
Lignin determination on western hemlock. The lignin content was determined on wood which had been given a mild alkaline extraction. This was done by treating 30 g. of western hemlock sawdust in an atmosphere of nitrogen with a solution of 1.5 g. of sodium hydroxide 211
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F. E. BRAUNS
i n 750 cc. of water at room temperature for 3 hours. After this period the wood was filtered, washed with water, then with 1$%acetic acid, and finally with water. I n this treatment
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210
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FIG.1. ULTRAVIOLET ABSORPTION SPECTRA IN 95% ETHANOL
_-__-____, Native hemlock
lignin; - - -, Native black spruce lignin
TABLE I METHOXYL CONTENTSOF WESTERNHEMLOCK LIGNINSAND
TEE
CORRESPONDING Burn
SPRUCELIGNINS LIGNIN PEEPABATION
Native hemlock lignin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Native black spruce lignin. ............................. Willstatter hemlock lignin from (a) unextracted wood. ................................ (b) C&-alcohol-extracted wood. ..................... (c) NaOH-extracted wood.. .......................... Willstiitter black spruce lignin from C&-alcohol-extracted wood. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
OBIGINAL PPODUCT, %
HETEYWLTED WIl% DIAZOHETHANE, %
14.7 14.8
21.4 21.5
15.6 15.4 15.8
21.1 21.3 21.0
15.5-16
21.2
some of the soluble lignin was removed with the major part of the coloring matter and some other alkali-soluble products. The extracted sawdust was ground and screened through a
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60-mesh sieve. It was light salmon pink in color and gave a very strong purple color reaction with phloroglucinol-hydrochloric acid. The lignin determination was carried out with 72%)sulfuric acid and gave a lignin content of 29.7% (oven-dry basis) ;the Klason lignin thus obtained had a methoxyl content of l6.O%. Eastern black spruce contained the same amount of lignin and its Klason lignin, obtained under the same conditions, had a methoxyl content of 15.8%. Isolatzon of native lignin from western hemlock. About 40 pounds of western hemlock sawdust was placed in a large percolator and continuously extracted with 95% alcohol a t room temperature until the extract was alinost colorless, which required several weeks. The first extract was milky because of the presence of water in the wood but later the extract became clear and dark reddish-brown in color. I n order t o remove the slight amount of suspended solid, the solution was centrifuged and the clear extract concentrated in the presence of a small amount of calcium carbonate under reduced pressure at a bath temperature not above 40". As the alcohol and the water were removed, the solution became more and more cloudy and finally a precipitate settled out i n the form of a dark reddish-brown resin which was permeated with a crystalline: material [later identified as conidendrin (4)1. I n order to remove the remainder of the alcohol, distilled water was added and the mixture was again subjected t o a vacuum distillation. The excess water was then poured off the resin and the latter was dissolved in dioxane, giving a clear, dark reddish-brown solution. This was dried by addition of a n equivalent mixture of anhydrous magnesium sulfate and sodium sulfate, during which two layers were formed. The lower, clear, colorless aqueous solution was separated from the upper dioxane solution. The latter still contained some water which was removed by distillation i n vacuo with dioxane as an azeotropic mixture. When the distillate in the receiver crystallized on cooling with water of about 4",the dioxane solution in the distilling flask was free of water. The solution was again centrifuged and concentrated under reduced pressure t o give about a 10% solution (ea. 3 1.). When the solution w a s allowed t o stand for several days, a crystalline product (conidendrin), consisting of fine needles, separated. Disregarding this material, the dioxane solution was added dropwise t o anhydrous ether with vigorous stirring, Khereby the crude native hemlock lignin was precipitated as a brownish-red powder. It was washed with ether and petroleum ether and dried. The yield was about 220 g. For further purification a part of the material was dissolved i n methanol to give a 10% solution and the lignin was again precipitated by dropping the methanol solution, with vigorous stirring, into 10 times its volume of distilled water. The colloidal precipitate was coagulated by addition of a few grams of anhydrous sodium sulfate and the native lignin was filtered onto a Biichner funnel. It was thoroughly washed with distilled water until the filtrate mas free of sulfate ions and dried in a desiccator over sodium hydroxide. The light straw-yellow filtrate n-as discarded. For final purification the lignin was redissolved i n dioxane, the solution centrifuged and filtered, and the lignin isolated by dropping the dioxane solution into anhydrous ether. It was washed twice with ether, once with benzene, and twice with petroleum ether, and dried in a n Abderhalden drier at 100" over phosphorus pentoxide. It had a methoxyl content of 14.7%, which did not change after another purific5,tion. S a t i v e hemlock lignin has solubility properties very similar t o those of native spruce lignin (1) but i t is light salmon pink i n color. I t s ultraviolet absorption spectrum in 95% alcohol was determined by means of the multistep sector photometer (3) with the Bausch and Lomb medium quartz spectrograph, using a solution thickness of 1 mm. and 3 concentrations-i.e., 0.1,0.05, and 0.01%; the absorption maximum is at 283 mmu as compared with 2S2 mmu for native spruce lignin (F'g ' 1 ure 1). The emall differences in the two absorption spectra may be caused by the small amount of eoloring matter insoluble in ether which is coprecipitated with the native hemlock lignin. The dioxane-ether mother liquors obtained in the lignin precipitations were distilled, first a t ordinary pressure and finally under reduced pressure, giving a resinous residue which, after being kept in a vacuum desiccator over sulfuric acid for several weeks, became partially solid. I t was triturated with anhydrous ether until a light pinkish colored powder was obtained .After further purification f r o n dioxane and ether as described above, the
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F. E. BRAUNS
residue had a methoxyl content of 16.2%. It was not investigated further and might be native hemlock lignin contaminated by some high methoxyl-containing material (conidendrin). The concentrated ether extracts, after standing for several days, formed a n almost solid mixture of a crystallized product and a dark reddish resin. This crystalline material was definitely identified as conidendrin (4). Diazomethane-methylated native hemlock lignzn. A solution of 1.0 g. of purified native hemlock lignin i n 25 cc. of anhydrous dioxane was treated with diazomethane prepared from 10 cc. of N-nitroso-5-methylurethan until a constant methoxyl content was obtained (2 methylations). The methylated lignin was isolated by dropping the centrifuged and filtered solution into anhydrous ether; the product was washed with ether, benzene, and petroleum ether, and dried. It had a methoxyl content of 21.3%. Isolation of U'illstatter lignin j r o m western hemlock. In order to study the effect of preextraction of the wood upon the Willstiitter lignin, three lignin samples were prepared. (a) Unextracted hemlock sawlust was ground in a hand mill and sifted through a 40-mesh screen. When 27 g . of this wood flour was treated with supersaturated hydrochloric acid (d. 1.23 a t 0") according t o the method of Iialb and Lieser (5), a lignin was obtained i n a yield of 28.5y0. It had a rnethoxyl content of 15.6%. This was kept in a moist condition i n the presence of a small amount of toluene for further study. ( b ) Hemlock sawdust of the same lot was extracted with bcnzene-alcohol (2:1), ground, sifted, and treated with supersaturated hydrochloric acid as before. The yield of hemlock lignin was 28.3% (methoxy1 content of 15 4%). (c) .I third Willstgtter lignin was prepared from alkali-extracted hemlock. For this purpose 25 g. of hemlock woodmeal (40-mesh) was extracted i n a n atmosphere of nitrogen with M o cc. of 0.2% aqueous sodium hydroxide solution. The extraction was repeated 15 times, the extracts ranging from a dark reddish color for the first extraction t o a very pale straw color for the last. The woodmeal passed through various colors ranging from dark reddish-brown t o an almost canary yellow after the last extraction. After t h e last, extraction, the wood flour was washed with distilled water until the filtrate was colorless, then with 1y0acetic acid, and finally with water, followed by acetone t o accelerate drying. Thc acetone filtrate was colorless. Willstiitter lignin was prepared in this manner in a yield of 27.6y0 (methoxyl content of 15.8%). It was noted t h a t when western hemlock mood came into contact with ihe hydrochloric acid, the bright emerald color persisted much longer than with either white or black spruce. There was very little difference in the color of the dried Willstiitter lignins, although in the moist condition the lignin from the unextracted wood is darkest and t h a t from the alkali-extracted wood the lightest. The latter has almost the same color 5s Willstatter lignin from benzene-alcohol-extracted spruce. Methylation of the Willstiitter lignins. The three Willstiitter lignins were subjected t o methylations with diazomethare. Because lignin is much more reactive when it has not been previously dried, and since i t is necessary t o carry out the methylation under anhydrous conditions, the water in the moist lignin was first replaced by cyclohexanol. This was done by suspending 3.5 g. (on the oven-dry basis) of moist lignin i n 150 cc. of cyclohexanol and removing the water under reduced pressure. Fresh cyclohevanol %-asadded and the distillation continued until all of the water had been removed (indicated by the cyclohexanol crystallizing in t t e receiver). Diazomethane, prepared from 15 cc. of N-nitroso-N-methylurethan, was passed into the suspensions. Methylation was indicated by t h e evolution of nitrogen and by the brightening of the colors of the lignins. The reaction was slow, and after about 10 days the yellow color of the diazomethane solution had almost disappeared. An aliquot portion of the suspension from each experiment was added t o methyl alcohol, stirred for 10 minutes, and filtered. The samples of the diazomethanemethylated lignins were successivelywashed with acetone, dioxane, ether, benzene, and petroleum ether. Finally the methylated lignins were suspended i n distilled water, boiled in an open beaker for 15 minutes, filtered, and dried in a desiccator; they were then ground in a mortar and dried at 100"in an Abderhaldendrier andanalyzed for their methoxyl contents. They were found t o be 21.1,20.3, and 20.2y0, respectively. The methylations were repeated with diazomethane prepared from 5 cc. of nitrosomethylurethan. After 5 days the solutions
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215
were still strongly yellow i n color and no nitrogen evolution was noted; the methylated lignins were isolated as described above. The methoxyl contents of these products are given in Table I. ACKNORZEDGMENTS
The author is indebted to Dr. J. A. Van den Akker for taking the ultraviolet absorption spectra, and to the Crown Zellerbach Corporation for permission to publish these results. SUMMARY
Native lignin and hydrochloric acid lignin have been isolated from unextracted and benzene-alcohol- and sodium hydroxide-extracted western hemlock and the lignin preparations have been methylated with diaaomethane. The methoxyl contents of the original and methylated compounds were identical 144th the corresponding lignin derivatives from black spruce. The ultraviolet absorption spectra of the native lignins were also identical. APPLETON,WIS.
REFERENCES BRAUKS,J. Am. Chem. Soc., 81,2120 (1939). GLADING, Paper Trade J.,111, No. 23: 32 (Dec. 5,1940). VAN DEN AKKER,U. S. patent 2,312,010 (Feb. 23,1943). BRAUNS,J . Org. Chem., 10, 216 (1945). (5) KALBAND LIESER,Be?., 61, 1007 (1928).
(1) (2) (3) (4)
