I
Stability of Methoxyl Groups in Wood and lignin
A micromethod has been worked out whereby these groups can be characterized by extent of hydrolysis to methanol. Quantitative determinations can be made for this alcohol in amounts of 50 to I O O Y to If:I Y at concentrations of 100 p.p.m.
F O R FUNCTIONAL GROUPS occirring in organic compounds, relatively good methods have been devised for determining the type of group and also character-e.g., primary us. secondary hydroxyl or aldehyde us. ketone carbonyl groups. However, no procedures have been developed for characterizing methoxyl groups which occur widely in lignin and wood. By the Zeisel-type procedure using hydriodic acid, total methoxyl content is readily determined as methyl iodide. This method, however, does not differentiate among methoxyl groups of differing stability such as those bound as esters, acetals, or aliphatic and aromatic ethers. Moreover, some polyhydroxyl substances such as glycol and glycerol with hydriodic acid yield volatile iodide compounds even if no methoxyl groups are present. Gran (7) has recently reaffirmed this result for sugars. Freudenberg (5) made one of the few reported investigations of methoxyl cleav-
Table I.
age by halogen hydracids. Using kinetic studies, he was able to distinguish glycoside from ether-bound groups and to reach certain conclusions about the status of methoxyl groups in lignin (4). Methoxyl groups can be characterized by the extent of hydrolysis to methanol in a given time under various conditions. To do this, however, methanol must be determined precisely in very dilute aqueous solutions. A suitable analytical method, worked out by Fisher and Schmidt (7 7) and substantially refined as to apparatus by Skrabal (72), has been developed in these laboratories into a micromethod (75). Thus, methanol can now be determined, in amounts of 50 to 100 y with a precision of f l y a t concentrations of about 100 p.p.m.
Known Substances Certain known substances were subjected to alkaline hydrolysis under two conditions-with 5% sodium hydroxide a t about 23' C. and 20% sodium hy-
droxide a t 100' C. (Table I). Under both conditions, the methoxyls present as aromatic ethers in vanillic acid and other acids were stable, but methoxyl present as an aliphatic ether in 3-methylglucose was about half hydrolyzed in the 5% solution at room temperature and quantitatively hydrolyzed in the 20% solution. Practically no methoxyl was split when 2-, 3-, and 4-methoxybenzoic acid were treated with 20% sodium hydroxide a t room tempeyature. Similar results were obtained with acid hydrolysis. When hydrolyzed with 44y0 hydrochloric acid for 22 hours a t room temperature, practically no cleavage of the aromatic methoxyl ethers occurred, but hydrolysis of the glucose methoxyl ether was extensive or nearly complete. This treatment for vanillic acid and 3methylglucose caused about 0.12 and 14.270, respectively, of the methoxyl groups to split. For these substances and vanillic coumaranone, when treated with 44% hydrochloric acid according to Willstatter, the methoxyl split
Splitting of Methoxyl Groups from Various Substances Methoxy',
Methoxy Split Off, % 20% NaOH, 1000 c. 0.04 0.09b 6.67 16.06 0.18 0.37 0.14 0.32 0.45 1.31 0.45 0.57
5% NaOH, 23' C.
Substance % Vanillic acid 18.46 3-Methylglucose 15.94 Pine wood 5.10 Beech wood 6.21 Freudenberg DHP lignin 16.57 Braun's native lignin 15.02 Determined by Viebock-Schwappach method. a Per cent of original substance-e.g., if all methoxyl were split off,value would be 18.40%.
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SEPTEMBER 1957
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Table II. Splitting of Methoxyl Groups from Three Lignin Preparations Lignin Tvoe. % OCHi
Characteristic
Acetone
Lignin methoxyl After Klason treatment Methanol in mother liquors Methoxyl in residue After KOH treatment Boiling, 3%, and acid pptn. Methanol in mother liquor 3% for 125 hr., about 23’ C. 3% for 125 hr., about 100° C. 20% for 1 hr., about 100’ C. a On same basis as described in Table I, footnote
amounted to 0.03, 0.04, and 7.4Y0, respectively.
Wood Alkaline hydrolysis was used to characterize methoxyl groups in pine and beech woods. At room temperature, the hydrolysis conditions were similar to those used for “degumming” or removal of hemicelluloses from wood-Le., 65 grams of wood was treated four times for 48 hours each time, \rith 1850 mi. of 5% sodium hydroxide solution. After each treatment, methanol in the mother liquors was determined. Hydrolyses at elevated temperature were conducted by boiling 0.7 gram of wood in 10 ml. of 20% sodium hydroxide solution for 1 hour. At room temperature, only a small part of the methoxyl was split from either pine or beech wood; this reaction was nearly complete in the first two treatments (Table I). iSot much additional methoxyl was split off even by treatment a t boiling temperature. 4-Methylglucuronic acid is present in hemicelluloses from various woods (7,2,4, 7, 70, 74) and its presence in pine wood was suggested by Hagglund, Richtzenhain, and Dryselius (8). Jones and Wise ( 9 ) showed that uronic acids are largely dissolved early in the hydrolysis. The hemicelluloses (substances soluble in 57, sodium hydroxide) in wood used for these experiments amounted to 9% for pine and 28% for beech (3, 6, 73, 74). Therefore, because of this higher hemicellulose content in beech wood, a much greater percentage of methanol in mother liquors from the beech wood was expected. This was surprising-the percentage of methox)l cleaved in cold 5% sodium hydroxide solution compared with that cleaved in a boiling 20% solution is about the same-for 3-methylglucose, 41.53%; for pine, 47.5570; and for beech wood, 44.06Yo. If the total alkali-hydrolyzable methoxyl is calculated as methylated uronic acids assumed to contain 14.9% of methoxyl, then the percentages of uronic acids in pine and beech woods are 2.4 and 2.15%, respectively. If some
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Methanol
15.1
17.4
23.4
0.1Q 15.0
2 . 4Za 16.18
16.8
11.68
15.35
19.32
0.34a I . 93a 1. b.
O.Va 4.72a 0.84“
methoxyl arises from cleavage of aromatic methyl ethers, these percentages will be still smaller unless there is present a more stable carbohydrate methoxyl grouping of the type contained in 3methylglucose. Unfortunately, no pure 4-methylglucuronic acid was available for testing as a model substance. lignin
Methoxyl stability experiments were also conducted on several lignins, including an artificial lignin, DHP (dehydrogenation-polymerization product of coniferyl alcohol), and a native lignin kindly furnished by Freudenberg and Brauns, respectively. These preparations behaved similarly in alkaline solutions (76)-about 370 of the total methoxyl was split from both samples by 57, sodium hydroxide a t room temperature, and 87‘ from the D H P sample by boiling in 20% sodium hydroxide. Nothing definite can be said about the ether group in Brauns’ lignin but, considering its origin, only aromatic methoxyl should occur in DHP. Its methoxyl group is only slightly less stable than the methoxyl in vanillic acid. T o secure other lignin preparations for investigation, wood samples were heated for 2 hours a t 170” C. with water solutions of water-miscible organic solvents. A part of the lignin dissolved and was later recovered for examination. I n this manner, an acetone lignin containing 15.1yo methoxyl was prepared, and by a procedure similar to that of Kleinert and Tayenthol (70), a methanol lignin (17.4% methoxyl) was obtained. Also, an absolute methanol lignin (23.470 methoxyl) was prepared using absolute methanol containing hydrochloric acid. This was done according to the alcoholysis procedure of Hibbert ( 2 ) . Alkaline hydrolysis splits of a relatively small percentage of methoxyl from acetone lignin but considerably more from other lignins (Table 11). Acid hydrolysis using 72% sulfuric acid solutions according to the Klason procedure for lignin determination, caused a small decrease in the methoxyl of acetone lignin but a larger splitting from other
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Abs. niethanol
6.60‘
1. 14a 7.95a 2. 08a
preparations where the acquired methoxyl was also greater. Methanol is split off in each step of the Klason lignin determination and Klason lignins always yield some methanol when subjected repeatedly to Klason treatment or alkaline hydrolysis, This indicates that characteristics of Klason lignins depend on the procedure used for isolation. Whether lignin preparations are subjected to acidic or alkaline hydrolysis, part of the methoxyl remains in the residues (Table 11). Since these show different methoxyl contents, entrance of the methyl group, seeming to occur in different ways, causes a difference in kind rather than degree between methanol lignin and absolute methanol lignin. literature Cited ( I ) Aspinall. G. O., Hirst, E. L., Mahorned, 1954, p. 1734. R. S., J . Chem. Soc. ( 2 ) Cramer, A .
B., Hunter. M. J., Hibbert, H., J . Am. C h m . SGC.61,
509 (1939). ( 3 ) Dwyer, M. O., Btochem. J . 33, 713 (1939); 34, 149 (1940). (4) Freudenberg, K., Belz, UT.?Nieman, C.. Ber. 62. 1560 (1929’). Freudenberg,’K., Sdff, K.; Ann. Chem. 494, 68 (1932). Gorrod, A . R. N., Jones, J . K. N., J . Chem. SGC.1954, p. 2522. Gran, C., Svensk Papberstidn. 56, 179 (1953). Hagglund, E., Richtzenhain, H., Dryselius, E., Bey. 89, 375 (1956). Jones, J. K. N., Wise, L. E., J. Chem. SGC.1952, pp. 2750, 3389, 3598. Kleinert, T., Tayenthal, T., Z. angew. Chem. 44, 788 (1931). Schmidt, k . , Bey. 57, 693 (192, 59, 679 (1926). Skrabal, R., Z. anal. Chem. 119, 222 (1940). Stewart, C. M., Foster, D. H., Australian J . Chem. 6, 431 (1953). Stewart, C. M., Foster, D. H., A‘uture 171,792 (1953). Wacek, &4.,Zeisler, F., Mikrochirn. Acta.1955, p. 29. Wacek, A., Zeisler, F., Riegelmayer. P., Monatsh. Chem. 85, 499 (1954),
A. v. WACEK, W. LIMONTSCHEW, and CHR. AAS Technical University of Graz, Graz, Austria
