The lignin Problem M O R , than 100 years ago, the major noncellulosic material comprising around 25% of the woody tissue in plants was given the name “lignin.” A great many investigations of lignin have now been completed and much has been learned about where lignin is situated in plant tissue, how it can be extracted, and what its properties are. Chemists have established that lignin is a polymer made u p of structural units which are a t least largely phenolic propane types and are somewhat different in softwoods and hardwoods. However, the complete chemical structure of lignin has still not been elucidated and many important theoretical and practical problems remain to be solved. Thus the 1956 Lignin Symposium of the AMERICAN CHEMICAL SOCIETY was held at Atlantic City as part of the program of the Cellulose Division. Its objective was to present and discuss results of research obtained since the 1951 Lignin Symposium in New York. Twenty-five research papers were contributed to the 1956 symposium. Research results concerning a wide range of lignin topics were reported : biochemical mechanisms of lignin formation (Freudenberg, Nord, Schubert) ; extraction of lignins with neutral solvents (Bjorkman), with enzymes (Pew), by kraft pulping (Gierer, Alfredsson), and by alkaline hydrogenations (Schuerch) ; isolation of water-soluble lignins (Lyness, Schenker) and of hardwood lignins (Browning, Harris, Hogan, Pearl, Beyer) ; characterizations of lignins by color reactions (Kitaura, Nakamura), by methoxyl group stability (von Wacek), by ultraviolet absorption spectra (Aulin-Erdtman, Goldschmid, Maranville), by thermal plas-
I
ticity (Kratzl), by molecular weights (Doughty, Lawson, Gordon, Mason, Felicetta, McCarthy), by solution viscosity (Luner), and by reactions of oxidation (Kratzl, Pearl, Smith, Purves), of alcoholysis (Sarkanen, Schuerch), and in the presence of mineral acids (Adler, Pepper) ; the nature of the structural units of lignins (Adler) ; and the remaining research questions (Erdtman). This topical diversity manifests the great scope of the lignin problem. Symposium papers thought to be of interest to the general readers of INDUSTRIAL AND ENGINEERING CHEMISTRY are published here; the other papers appear elsewhere. The results reported demonstrate that substantial progress is being made toward elucidation of the genesis, extraction, physical properties, and chemical structure and reactions of lignin. They also seem to foreshadow the development of important and profitable industrial silvaproducts and silvachemicals from this interesting phenolic polymer, which can be had in such huge amounts from almost universally available and perpetually renewable natural sources. These products and chemicals can come directly from woods, or from the spent liquors of wood cellulose pulp production-for example, some 1,000,000,000 pounds of lignin sulfonates and 250,000,000 pounds of sugars are now available each year in sulfite spent liquors in the Pacific Northwest alone. The economic utilization of these materials is a continuing challenge to chemists and chemical engineers and their business colleagues throughout the world. JOSEPHL. MCCARTHY Chairman
ERICH ADLER Chalmers University of Technology, Gb’teborg, Sweden
Structural Elements of lignin A detailed knowledge of lignin structures will furnish a solid base for closer understanding of processes involved in pulp manufacture, and open new possibilities in commercial utilization of lignin
E L U C I D A T I O N of the structure of lignin is of twofold importance. Primarily, it may be looked upon as a purely scientific problem, lignin being a natural product synthesized in tremendous amounts in plant tissues. Secondarily, it is of considerable practical importance in giving a closer understanding of the processes involved in pulp manufacture, and opening new possibilities in the commercial utilization of lignin. Although it is still impossible to present a correct structural picture of the lignin macromolecule or macrornolecuIes,
recent views of the structural elements occurring in lignin are summarized here, For the sake of simplicity, the discussion is confined to the lignin of coniferous wood, or “guaiacyl lignin” according to the nomenclature suggested by Nord (25). Because the lignin isolated by BjBrkman (7475) from finely divided wood, in its structural features, is reasonably close to protolignin, examination of the analytical composition of lignin may be based on Bjorkman lignin. Elemental analysis and methoxyl analysis of a Bjorkman spruce lignin prep-
aration which had been extracted with dioxane-water (25 to 1) from wood meal disintegrated in toluene suspension under nitrogen by means of a vibrational ball mill, gave the following formula for a phenylpropane or CQunit :-CQH8.8302. 81(OCHs)n.s,. ’ The ’ox$gen belongs to a variety of functions and groupings. Of the 2.37 atoms of oxygen present per CQ (except in the methoxyl group), 1.0 must be the phenolic oxygen of the guaiacyl nucleus; only 0.29 oxygen atom, however, occurs in the form of free phenolic hydroxyl VOL. 49, NO. 9
SEPTEMBER 1957
1377
I
I
CHOH
a
I
CHO-C
b
.
Sulfonation neutral and acid (rapid) Number per OCH,
0.15
I
CHOH I
CHO-c I
C
d
neutral (slow) acid (rapid)
acid only
0.15
0.30
I
I
CH-SO sH
CH-SOIH
I
I
Guaiacyl carbinol structures
(27), and consequently 0.71 must be presplicated, fundamentally different ways ent in blocked phenolic groups, probably have had to be used to study it. Oxidaas aryl alkyl ether oxygen. The diftive breakdown, with permanganate or ference between total and aromatic with nitrobenzene in alkali, investigated oxygen-1.37 oxygen atoms-must beespecially by Freudenberg, disclosed the long to aliphatic hydroxyl, carbonyl, presence, in coniferous lignin, of two and ether groupings. Acetylation retypes of substituted guaiacol nuclei-4vealed the presence of 1.I 5 hydroxyl substituted and 4,G-substituted. Further valuable information was obtained by groups (75); of these, 0.29 is phenolic, thus, by difference, 0.86 is aliphatic hyattempts to characterize the so-called droxyl group. Primary, secondary, or “reactive groups” of lignin-Le., groups tertiary hydroxyl groups have not yet responsible for its most typical reactions, been differentiated. According to hysulfonation with sulfite solutions of vardroxylamine consumption, Bjorkman ligious pH, alkylation with alcohols, and nin possesses a comparatively high carreaction with thioglycolic acid in the bonyl group content-0.18 (70). Fipresence of mineral acids. nally, the remainder of the aliphatic oxyThe extensive investigations of the gen-0.33-is assumed to belong to mechanism of the sulfite pulping process, alkyl-alkyl ether bridges. carried out mainly in Swedish laboratoAs a consequence of this tentative oxyries by Hagglund, Kullgren, Holmberg, gen balance, the formula of Bjorkman Erdtman, Lindgren, and others, relignin can be dissolved into: C9H,.68- vealed that lignin contains several types (phenol OH),,,~(aliphatic OH)0.86(car- of reactive groups which differ in their O ) O . ~ ~ -reactivity towards sulfonation, espebony1 O),,,,(arylalkyl ether (dialkyl ether O)o.33. cially sulfonation a t various p H values. Because the structure of lignin is so comThe key to the understanding of the sul-
fonation reactions was given by B. Holmberg when he found, in the thirties, that secondary benzyl alcohols and their ethers could be sulfonated. On the basis of his idea that the sulfonation of lignin might be due to aryl carbinol and ether groupings, and with the support of numerous model investigations, a uniform and plausible view of the nature of the sulfonatable groups was developed, especially by Lindgren (27). According to this view, the initial rapid sulfonation of lignin, which takes place even with neutral sulfite solutions, is due to phenolic benzyl alcohols, a, and their ethers, b, resulting in the replacement of the aliphatic hydroxyl or ether group by a sulfonic acid group. A further grouping, reacting slowly at neutral reaction but readily with acidic sulfite solutions, is supposed to be a guaiacylcarbinol, etherified at the phenolic hydroxyl group, c. Finally, a structural element which can be sulfonated with acid sulfite solutions only, is assumed to be a guaiacylcarbinol, the reactivity of which is decreased by the etherification of the phenolic as well as the alcoholic hydroxyl group, d. Strong evidence for the presence of benzyl alcohol and benzyl ether groups in lignin was provided by investigation of the reaction of lignin with alcohols in the presence of small amounts of mineral acid. This reaction became of special interest, ?when it was found that Brauns lignin ( 4 ) or Bjorkman lignin (70) could be alkylated under very mild conditions-for instance, with methanol containing 0.5% hydrogen chloride, at 20’. A total of 0.7 methoxyl group per OCH, originally present was introduced, for instance. into Bjorkman lignin during 72 hours (Figure 1). The view held by some previous authors that the alkylation was due to acetalization of carbonyl groups or reacetalization of acetal groups could be shown to be incorrect or, at least, highly improbable. For instance, removing the carbonyl
>c=o 1 ,>C/OCH, \OR:!
I
-c-oH(--o-c) I
OJ
\OCHa
I
--C-OCHx I
0.6 0.1
OH
I
-c-oH(-o-c) I
0.2
1
2
1
6
12
I
I8 24 30 36 REACTION TIME, HOURS
42
I
-C-OCHI I
‘2 48
54
Figure 1. Methanol-dioxane ( 1 to 1) containing 0.5y0 HCI, 20’ C.
1 378
I
OH
INDUSTRIAL AND ENGINEERING CHEMISTRY
c-0
I
c-0
I
Methylation of lignin with CH30H-HCI
LIGNIN groups by reducing the lignin with sodium borohydride did not prevent the uptake of new methoxyl groups on subsequent treatment with methanolic hydrochloric acid. O n the other hand, the treatment of numerous model substances of the benzyl alcohol and benzyl ether type with methanolic hydrochloric acid resulted in replacement of the alcoholic or ether group by methoxyl, the rate of reaction being highly dependent on the structural type examined ( 2 ) . The sulfonation and alkylation studies, although providing strong evidence for the occurrence of benzyl alcohol and benzyl ether groups, could not yield definite proof of these structural details, in the sense of classical organic chemistry. Such proof, however, has been recently delivered by Gierer (ZO), a t least for the phenolic benzyl alcohol structure. Gierer found that quinone monochloroimide, when acting upon wood or isolated native lignin, produces the blue dyestuff, guaiacyl indophenol, identical with the product obtained from free guaiacol and quinone monochloroimide. I t was clearly shown that this reaction must be due to the presence of phenolic guaiacylcarbinol groups in lignin, the alcoholic substituent being split off as an aldehyde under the conditions of the coupling reaction. The reaction does not take place unless both phenolic and alcoholic hydroxyl groups are free. Quantitative measurements indicated that in Brauns lignin about every seventh, in Bjorkman lignin every fourteenth, phenylpropane unit has the guaiacylcarbinol structure. Direct demonstration of these groups supports the conclusions drawn from sulfonation and alkylation studies and indicates the presence of etherified derivatives of this structure. The basic structural unit of coniferous lignin is the guaiacylpropane monomer, and, as in other polymeric substances, the mechanism of linkage between the monomers is important. In recent years, several methods have become available for determining the amount of free phenolic hydroxyl groups in lignin, previously one of the most disputed questions in lignin chemistry. Satisfactory agreement has now been obtained by different methods, such as the spectroHAOH
photometric Ae method of Aulin-Erdtman, used in a modified way by Goldschmid, the potentiometric titration in nonaqueous solvents, practiced by Freudenberg and Enkvist, the conductometric titration reported by Schuerch and Sarkanen, and the periodate demethylation method, developed in this laboratory (6, 27)' In Bjorkman lignin, only 0.3 guaiacyl nucleus per OCH,, or not more than one third of the total of aromatic nuclei, carries free phenolic hydroxyl groups. Consequently, 0.7, or a t least two thirds of the nuclei, must be involved in ether links between two monomers. As yet there are no indications as to the occurrence of diary1 ether groupings in lignin, and consequently the aryl oxygen may be assumed to be linked to one of the three carbon atoms of the propane side chain of the adjacent monomer. Which of the three carbon atoms then carries the aryl ether bridge? Although the occurrence of benzylor a-ether groupings is indicated by sulfonation studies, these are not benzyl aryl ethers. Neither sulfonation nor mild alkylation with alc,oholic mineral acid a t 20' liberates phenolic hydroxyl groups in significant amounts, whereas in corresponding model substances the aryl ether bridge is readily opened. Groupings of the @- and 7-aryl ether types should be much more resistant, and the fact that, according to Freudenberg, lignin contains a large amount of primary alcohol groups seemed to be in favor of a 8-conjunction. Further reasons for interest in the @-ether structure were related to biosynthetic considerations of Erdtman. T o study this structural hypothesis experimentally, model substances of the guaiacylglycerol type, with or without a guaiacyl ether group in the P-position, were synthesized. They were found to reflect typical lignin reactions in a very satisfactory manner. Thus, for instance, the veratrylglycerol-@-guaiacyl ether (7, 8, 77) yielded formaldehyde on heating with strong sulfuric acid, in agreement with the behavior of lignin, the primary carbinol being the source of the formaldehyde obtained. The secondary carbinol group in the benzyl position was
-
HA-0
bH Guaiacyl
indophenol Quinone tnonochloroimide reacrion on wood or isolated lignin
readily sulfonated by acidic sulfite solutions, and the sulfonation rate decreased with increasing p H of the reaction mixture. Finally, treatment with methanolic hydrochloric acid a t 20' resulted in methylation of the reactive carbinol group. These reactions, however, are not strictly specific for the total guaiacylglycerol-&aryl ether structure, but are given by other side-chain structures containing the benzyl alcoholic group. Very strong evidence, however, for the occurrence of this structural element in lignin was provided b y the finding (7, 8, 77) that these model substances, when heated with ethanolic hydrochloric acid for 48 hours or more, yielded the same keto1 ethers, ketones, and diketones as were obtained by Hibbert in the ethanolysis of lignin. Similar reactions were recently shown to take place on heating with hydrochloric acid in aqueous dioxane. I t is believed that the formation of Hibbert monomers is convincing evidence of the presence of the guaiacylglycerol-/3-aryl ether structure in lignin (9). About two years ago, Freudenberg and coworkers, in the course of their brilliant studies of the enzymatic dehydrogenation of coniferyl alcohol, also concluded that this structure was part of the lignin molecule. Coniferyl alcohol, when oxidized M. ith a comparatively crude mushroom enzyme preparation, was shown by Freudenberg (78) to yield a polymeric product (DHP, artificial lignin), containing less hydrogen and more oxygen than the starting material and reflecting some of the characteristic properties of lignin. At an intermediary stage of the oxidation reaction, a mixture of low-molecular products was obtained and shown to contain three dimeric substances in addition to a small amount of coniferyl aldehyde. By paper chromatography the same substances were shown to be present in the lignifying tissue of wood. The dimeric substances were dehydrodiconiferyl alcohol, D,L-pinoresinol, and guaiacylglycerol-P-coniferyl ether. Freudenberg has also shown that further enzymatic oxidation of the dimeric substances leads to amorphous, ligninlike products, and studies with labeled coniferyl alcohol in vitro and in vivo strongly support the view that coniferyl alcohol is utilized in the biosynthesis of lignin (78,22,23). Therefore, the formation of the dimeric products on enzymatic dehydrogenation of coniferyl alcohol in vitro, as well as their demonstration in the cambial tissue, represents extremely important information regarding possible elements of linkage between the phenylpropane monomers in lignin. However, care is needed in drawing quantitative conclusions from the results obtained in the in vitro experiments as to the strucVOL. 49, NO. 9
SEPTEMBER 1957
1379
CHzOH
C
I
7 CY
8
AI
\OCHa
0
\OCH3
*/-CHz-
I C I
I
H-
0
OH I \OCH3
V\OCH, AH
A \OCH3
C
CHOH
AH-&)
0
hAH O C H 3
Aryl oxygen may be linked to one of three carbon atoms of propane side chain
of adjacent monomer
/
28% HzS04 reflux
4
HCHO (0.19 mole)
CHzOH
CHiOH Sulfite pH 1.5-6
I I
HC-0-
'
HC-S03H I
O(,H,
h C P H 3
\
ii Synthesis of model substances of guaiacylglycerol type, reflecting typical lignin reactions e , guaiacylglycerol-@-arylether type
.
CHiOH
OH
CHzOH
I
CII
LEI
/I
CH I
eH
l
0 HzCOH I
I
HC-0
HC-d
t
I
HCOH I
Dehydrodiconiferyl alcohol
D,L-Pinoresinol
Guaiacylglycerol-pconiferyl ether
Three dimeric substances found in lignifying tissue of wood
1380
INDUSTRIAL AND ENGINEERING CHEMISTRY
ture of lignin synthesized in the living cell. The relative amounts of the various dimeric structures may be different in the artificial process and in the wood. The in vivo synthesis may produce structures not reflected by the intermediate dimers -for instance, during further oxidative polymerization of the dimers primarily formed. Freudenberg found that the composition of the dimeric mixture varied considerably, as experimental conditions were changed. Thus, if the aqueous solutions of coniferyi alcohol and the mushroom enzyme were mixed at once, and aerated, dehydrodiconiferyl alcohol amounted to 6 0 7 , of the total mixture of dimers, whereas on dropwise addition of coniferyl alcohol, guaiacylglycerol-Pconiferyl ether became the major product, with a yield of 70% (18). The possibilities of the enzymaticsynthetic approach seem to be limited (Q), and the final answer must be given by examination of the native lignin itself. Efforts to estimate the frequency of the guaiac!;lglycerol-0-aryl ether structure in lignin ( 9 ) showed that at least one fourth to one third of all phenylpropane monomers are guaiacylglyerol elements, etherified in the @-position with a phenolic group. Little information is available regarding the actual amounts of pinoresinol structures in lignin. In studies on the methylation of lignin it was found that pinoresinol, on 48-hour treatment with methanolic hydrochloric acid at 20°, although not completely stable, did not take up methoxyl whereas pinoresinol dimethyl ether remained totally unchanged ( 2 ) . Thus pinoresinol structures cannot be responsible for the reaction of lignin under similar conditions, resulting in the introduction of 0.7 new OCH, group per phenylpropane unit. This does not exclude the occurrence of pinoresinol structures in lignin. According to Purves and collaborators (16, 37), acid-treated lignins or alkali lignins, on drastic permanganate oxidation, yield u p to 5 to 6% of benzene polycarboxylic acids, which may originate from isolignane structures formed 'secondarily from lignane structures such as pinoresinol. So far, it appears reasonable to assume that not more than 10 to 20% of all phenylpropane units are present as pinoresinol structures. Experimental observations permit drawing some conclusions as to the frequency of the dehydrodiconiferyl alcohol system in lignin. In studies on the action of methanolic hydrochloric acid (2) or of hydrochloric acid in aqueous dioxane (9) upon lignin and lignin models, it was found that, when dihydrodehydrodiconiferyl alcohol or its methyl ether was heated with the acid reagent, the saturated phenylcoumaran system was converted into the unsaturated phenylcoumarone system.
LIGNIN The mechanism of this reaction may be pictured as a n acid-catalyzed ring opening, followed by release of a proton to yield a substituted coniferyl alcohol structure, which by allylic rearrangement, ring closure, and migration of the semicyclic double bond yields the coumarone ring. The formation of the phenylcoumarone structure with its prolonged conjugation produces a very distinct change in ultraviolet absorption (Figure 2). Whereas the phenylcoumaran shows the typical guaiacylpropane absorption, with a maximum at 282 mp, the phenylcoumarone shows a strongly increased absorption as well as a bathochromic shift of the maximum to 310 mp. The transformation phenylcoumaran --*. phenylcoumarone during acid heating can easily be followed by absorption measurements. Phenylcoumaran systems present in lignin would be expected to give rise to a similar change in ultraviolet absorption, if the lignin was subjected to acid heating. T h e increase in absorbance when Bjorkman lignin was heated for 24 hours with methanolic hydrochloric acid, if measured a t the phenylcoumarone maximum (310 mp), indicated not more than 5 to 770 of phenylcoumaran systems in the lignin. If the lignin had made up 25 or 50Y0 of phenylcoumaran structures, the extinctions at 310 mp marked in Figure 3 would have been expected. Using an acetolytic ring opening reaction, Freudenberg concluded that native lignin contains very little phenylcoumaran. This structure, which in earlier phases of lignin chemistry was held to be dominant, has thus been degraded to minor importance. The carbon-carbon bond of the phenylcoumaran system linking two monomers between positions ortho to the guaiacyl oxygen and the 0-side-chain carbon of the adjacent monomer, is also present in an additional structure, which does not involve ring closure. As was found by Richtzenhain (32), diazomethane methylation of wood or isolated lignin preparations and subsequent permanganate oxidation yielded 1 to 270 of isohemipinic acid, which indicates the presence of phenolic nuclei, condensed in the ortho position (I). About twice as much isohemipinic acid was obtained, when the lignin was first heated with strong alkali, subsequently methylated with dimethyl sulfate, and finally oxidized with permanganate, by the classical procedure of Freudenberg. The additional isohemipinic acid may originate from phenylcoumaran structures (11), which are opened by the hotalkali treatment. Recently, Freudenberg (79) applied these two breakdown procedures to an “artificial lignin,” produced by action of mushroom oxidase upon coniferyl alco-
CHzOH
CHzOH
I CHz I
! I CHz I CHs
CHz
-
HOHzC
A
0.5% HCI in
CH30H or dioxane-HzO
OCH3
,
24 hours, reflux
I
n I
Hdr--b
HCS OH
I\
I\
HzC
r.;7
A
A i i
1 6H HCOH
I/ I c-0
HC-0
I\
I /\
A
Attion of methanolic hydrochloric acid or hydrochloric acid in aqweoos dioxane on dehydrodiconiferyl alcohol
c:
bI
I
n
I1
6
COOH
I
I
KMn04
+CHA”
n
I
n
Isohemipinic acid
I
Carbon-carbon bond between side-chain and adjacent nucleus VOL. 49, NO. 9
SEPTEMBER 1957
1381
‘
Ic
C
CHO
hol labeled ~ i t hradioactive carbon in the /3-position (marked with an asterisk). I n both cases, The isohemipinic acid obtained was radioactive, which indicates that the p-carbon atom has been linked to the guaiacyl nucleus. A third type of nuclear condensation is represented by the diphenyl structure. Its occurrence in lignin has been discussed previously, but only recently has clear experimental evidence been presented by Pew ( 2 9 ) ,who found a few per cent of dehydrodivanillin after oxidation of lignin with nitrobenzene in alkali; no dehydrodivanillin was obtained from models lacking the diphenyl linkage. Spectrochemical investigations by AulinErdtman (73) also point to the presence of the diphenyl structure in native lignins. The occurrence of this structure, as well as of the C-C linkages mentioned above the phenylcoumaran ring and the corresponding open structure (I1 and 1)-is in harmony with the dehydrogenation-polymerization theory of lignin formation proposed by Erdtman and further developed by Freudenberg. The total amount of “condensed units,” as they often are called, relative to the “noncondensed units,” is still not clear. Leopold (%), in Erdtman’s laboratory, concluded, by comparing yields of vanillin and related products obtained on nitrobenzene oxidation of lignin and model substances, that only about 50y0 of all guaiacyl nuclei in lignin were unsubstituted in the ortho position (111), the rest being linked by an 0-C-C bond (IV). Further studies with new model substances, now being carried out, seem to indicate that the relative amount of condensed units is ess than 5070,possibly only 30 to 400jo.
CHO
Dehy drodivanillin C
C
IV
I11 CHyOH
CHyOH
I
CH ICH
H@ ---f
I
2 mol.
C
CH1
CH20H
CH-
CH
1
C
C
C
c- I
I
I
I
I
CH 3
I
I
I
i
CU(OH):NaOH
I CHz I C I
CH3
I
CHz ---C
I
I
VI Diphenyl structure, CY$-, and a,a-side chain C-C
linkages
4.4 4.2
4.0 3.8
3.6 3.4 u)
3 3.2
I
C“,&
h. miL
Figure 2. formation
1 382
Effect of phenylcoumarone on ultraviolet absorption
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 3.
Effect of treatment with
0.5% hydrochloric acid in methanol
24 hours, reflux
LIGNIN [CHzOH]
CHO
CHzOH
CH
CH2
I
II
CH I
I
I
HC I
CHzOH
I
-CH I
Hk-
hazy day, when the more prominent features of the view can be distinguished, whereas other details may be hidden from our eyes or appear unsharp, in a scattered light. As a whole, however, lignin need no longer be classified as terra incognita.
literature Cited (1) Adler, E., Bjorkqvist, K. J., Haggroth, s.,Acta Chem. & z n d . 2, 93 (1948). (2) Adler, E., Delin, S., unpublished results. (3) Adler, E., Ellmer, L., Acta Chem. Scand. 2,839 (1948). (4) Adler, E., Gierer, J., Zbid., 9, 84 \/ i1n7 cJ cJ \J .
(5) Adler,
0
HC--
I
t
H
HC-0
I
HCOH I
-
\
/v\ 1 OCH3
HC
I OH HCOH I
The structural elements of lignin
-
I n addition to the C-C linkages between nucleus and side chain or between two nuclei, lignin also contains C-C bridges from side chain to side chain. T h e pinoresinol structure represents one of these linkages, but one or two further linkages of this type have been discussed. Freudenberg (77) has presented experimental evidence for a possible spontaneous dimerization or polymerization of the cinnamic alcohol side chains of dehydrodiconiferyl alcohol and guaiacylglycerol-P-coniferyl ether groupings formed as intermediates in the biosynthesis of lignin, resulting in the production of a,P-C-C linkages (V). Pearl and Beyer (28) suggested the occurrence of a,a-C-C linkages in lignin
(VI), as oxidation of lignin sulfonic acids with alkaline copper hydroxide yielded small amounts of stilbene derivatives. Stilbene structures of this type were proposed some years ago as responsible for the oxidative reddening of sulfite pulp (5) and it appears possible that such diphenylethane or ethylene structures are not present in the original lignin but arise during the sulfite cook. The information on lignin structure which has been discussed might be fused into one picture as shown herewith. I n a section of the lignin molecule several monomers show the guaiacylglycerol-P-guaiacyl ether structure, others the phenylcoumaran or corresponding open structure, or the pinoresinol structure. Here and there, the diphenyl link alternates with these modes of linkage. Approximately every third guaiacyl nucleus is of the phenolic type, and benzyl alcoholic groups or benzyl alkyl ethers indicate the seats of typical lignin reactions such as sulfonation and alkylation. The chain is terminated by a coniferyl alcohol or coniferyl aldehyde group, the latter being responsible for phloroglucinol or aniline color reactions of lignin (7, 3, 72, 24, 29). Colorimetric and spectrophotometric data, however, indicate that there is only one such group for about 35 phenylpropane monomers in Bjorkman lignin. Dimerized or polymerized coniferyl alcohol end groups described by Freudenberg (77) may replace the unsaturated end groups, linking together two or more polymeric chains of this type. Cross linking may also be brought about by aryl (P-)alkyl or dialkyl ether linkages. The presence of approximately 0.2 carbonyl group per methoxyl (70), compared with the small amount of coniferyl aldehyde groups,' requires the occurrence of carbonyl groups in saturated side chains; a t present, their actual positions cannot be stated with any certainty. This picture is not supposed to be a structural formula in the proper sense. Rather it is to be looked upon as a kind of panorama of scenery on a somewhat
(6) (7) (8)
(9)
E., Haggroth, S., Suensk, Papperstidn. 53, 321 (1950). Adler, E., Hernestam, S., Acta Chem. Scand. 9. 319 (1955). Adler, E.; LindgreG B. O., Suensk Papperstidn. 55, 563 (1952). Adler, E., Lindgren, B. O., Saeden, U., Zbid., 55, 245 (1952). Adler, E.,. Pepper, ' J. M.,Eriksoo, E., IND. ENG. CHEM. 49, 1391
(1957).
(10) Adler, E., WalldCn, I., unpublished
(19)
results. Adler, E., Yllner, S., Suensk Papperstidn. 57, 78 (1954). Aulin-Erdtman. G.. Svensk Pabberstidn. 55, 745 (1952); 56;' 91 (1953); 57, 745 (1954). Ibid., 59, 363 (1956). Bjorkman, A., Zbid., 59, 477 (1956). Bjorkman, A., Persson, B., Ibid., 60, 158 (1957). Cabott, I. M., Purves, C. B., Pulp @ PaperiMag. Can. 57,151 (1956). Freudenberg, K., Ahlhaus, O., Monatsh. Chem. 87, l(1956). Freudenberg, K., in Zechmeister, L., Progr. Chem. Org. Natl. Prods. 11, 43 (1954); Angew. Chem. 68, 84, 508 (1956). Freud&berg, K., Niedercorn, F., Chem. Ber. 89.2168 - (1956'1. Gierer, J., Aita Chem. Scand. 8, 1319 (1954); Chem. Ber. 80, 257 (1956). Hernestam, S., Suensk Kem. Tidskr. 67, 37 (1955). Kratzl. K.. Billek. G.. Monatsh. Chem. 7
(20) (21) (22) . .
-
\
Holzforschung 7,'66 Hkzforschkng 7, 66 (lb53). (1953). (23) Kratzl, K., Billek, G., Graf, A., Schweers, W., Monatsh. Chem. 87, 60 (1956). (24) . . Kratzl, K., Rettenbacher.~,F.. Zbid.. 80, 622 (1949). (25) Kudzin, S . F., Nord, F. F., J . A m . Chem. 5'06. 73, 4619 (1951). (26) Leopold, B., Suensk Kem. Tidskr. 64, 18 (1952). (27) Lindgren, B. O., Svensk Papperstidn. 55, 78 (1952). (28) Pearl, I. A., Beyer, D. L., Tappi 39. 171 (1956). Pew,' J. d, J.' Am. Chem. SOC.73, 1678 (1951); 74,2850 (1952). Zbid., 77, 2831 (1955). Read, D. E., Purves, C. B., Ibid., 74, 210 (1952). Richtzenhain, H., Suensk Papperstidn. 53. 644 (1950): Acta Chem. Scand. 4,'206, 589 (1950).
RECEIVED for review December 3, 1956 ACCEPTED June 2, 1957 Division of Cellulose Chemistry, Lignin Symposium, 130th Meeting, ACS, Atlantic City, N. J., September 1956. VOL. 49, NO. 9
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