I
Action of Mineral Acid on lignin and Model Substances of Guaiacylglycerol-beta-aryl Ether Type
-
4)
0.6 7)
-
- 0.16 0.2
- 0,r /
4
/
I
12
1
I
I
I
I
I
24
I
0.20
-
0.12
-
0.08
C - CH, OCH,
I
40
HEATING TIME, HOURS CHzOH 0.30’
’
’
I
I
,
I
,
l
0.04
After 48-hour heating
Ph/OHiiberated/OCHr C-CHaiormed/OCHa VOL. 49, NO. 9
= 0.26 = 0.16 SEPTEMBER 1957
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insoluble solid. Figure 2 shows that the solids obtained from the dioxanehydrochloric acid lignin after increasing periods of heating were characterized by increasing contents of free phenolic hydroxyl and C-CH, groups. Similar analytical changes were found in the solids obtained from Bjorkman lignin.
carbon-carbon linkage in the 5- position or by a nonhydrolyzable aryl ether bond (cf. formulas X I 1 to XIV), such elements will suffer similar transformations in their side chains without being released from the macromolecules, as indicated in X V and XVI. This will explain the increase of phenolic hydroxyl
A Hf
H+
I/
___j
XII
___)
XI11
/I
HC
XIV
+
IX, x, X I C
and
HO-@-C-C-CHI
C C-O---(IZ)-C-C-CH3
/
OCHa
OCH3
xv
XVI CHzOH
CHiOH
I CHz I
CH2
I
CH2
CH z I
I
contain appreciable amounts of other acid-sensitive phenol-ether linkages, this would indicate the presence, per OCH,, of 0.31 arylglycerol unit carrying an aryl ether linkage in the p- position of the side chain (XII). The total increase of C-CH, groups in the same experiment was 0.21 C--CH,/ OCH,. In evaluating this analytical result two corrections have to be made. About 0.05 C-CHB/OCH3 may arise from the conversion of hydroxymethylcoumaran systems into the methylcoumarone systems, as indicated by the model experiment X V I I -+ XVIJI, which would reduce the amount of C-CH3 groups originating from arylglycerol-@-aryl ether elements (XII) to 0.16. O n the other hand, the model acidolysis with substance I V revealed that only about 50% of the possible amount of C-CH, groups is formed during 48 hours' heating (cf. Figure 1). Therefore, it seems justifiable to multiply the latter C-CH, value by a factor of 2, to obtain the XI1 structures. This would yield a value of 0.32 such structures per OCH,, in agreement with the valueobtained on the basis of the phenolic hydroxyl groups liberated. Only slightly lower values for the frequency of structures X.11 (0.26 and 0.27 per OCH,) \vere derived from the data obtained on acidolysis of the dioxane-hydrochloric acid lignin mentioned above. These experiments seem to indicate that one fourth to one third of the phenylpropane units are guaiacylglycerol units carrying a @-aryl ether linkage (XII).
Literature Cited (1) Adler, E., Eriksoo, E., Acta Chem. Scand. 9, 34 (1955). ( 2 ) Adler, E., Lindgren, B. O., Suensk
Papperstidn. 55, 563 (1952). (3) ~, Adler, E., Lindpren. B. O., Saedkn, U.,'16id., 55, 245 (1952). (4) Adler, E., Yllner, S., Zbid., 57, 78
(1954).
These results seem to be in harmony with the following assumptions. Guaiacylglycerol-P-aryl ether structures (XII) in lignin are acid-hydrolyzable under the conditions used. Presumably, the primary step of the reaction is a dehydration, yielding an enol aryl ether (XIII) susceptible to acid hydrolysis. This will produce a new phenolic hydroxyl group in addition to @-hydroxyconiferyl alcohol (XIV). The latter will undergo Hibbert rearrangements to yield the monomeric ketones, I X to XI, which will appear in the ethersoluble fraction with dimeric and oligomeric products of similar structure. If the acidolysis reaction attacks arylglycerol-@-aryl ether elements in lignin which are joined to an adjacent unit by a
1392
and C--CH, contents in the etherinsoluble, solid reaction products. O n the basis of the model experiment (Figure 1) and the maximum increase of free phenolic hydroxyl and C-CHs groups obtained in the lignin acidolysis experiments, a preliminary estimate of the amount of guaiacylglycerol-@-aryl ether structures (XII) in lignin has been made. The Bjorkman lignin used contained 0.30 phenolic OH/OCH3; on 48 hours' acidolysis it yielded 4570 of etherinsoluble material with 0.57 phenolic OH/OCH, and 55% of ether-solubles with 0.64 phenolic OH/OCH,, corresponding to a total liberation of 0.31 phenolic OH/OCHB. As there are good reasons to believe that lignin does not
INDUSTRIAL AND ENGINEERING CHEMISTRY
( 5 ) Bjorkma'n, A., Zbid.,59, 477 (1956). ( 6 ) Bjorkman, A.. Persson, B., Ibid., 6 0 , 1 5 8 (1957). (7) Erdtman, H., Research (London) 3, 8 3 (1950). (8) Erdtman, H., Leopold, B., Acta Chem. S c a d 3, 1358 (1949), ( 9 ) Freudenberg, K., Anpew. Chem. 68, 84 (1956). (10) Freudenberg, K., Schliiter, H., Chem. Ber. 88, 617 (1955). (1 1) Freudenberg, K., Schliiter, H., Eisenhut, W.. n'uturwissenschaften 41,
576(1954).
ERICH ADLER
J. M. PEPPER and EDGAR ERIKSOO Chalmers University of Technology Goteborg, Sweden Division of Cellulose Chemistry, Lignin Symposium, 130th Meeting, ACS, Atlantic City, N. J., September 1956.
