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HOLGER ERDTMAN Kungl. Tekniska Hogskolan, lnstitutionen f o r Organisk Kemi, Stockholm 70, Sweden
Outstanding Problems in Lignin Chemistry Views of the structure of ligriins are vague and largely based on analogies and speculation. Much more degradative work is needed, especially to establish the nature of nonaromatic regions, and biochemical studies to shed light on biogenesis of lignins
THE
structure of lignins has been studied by a variety of methods, including analytical and degradative methods and the parallel examination of the reactions of lignins and model compounds. Owing to the unusual complexity of the problem,’ speculation has played a great role in lignin chemistry. Klason’s old hypothesis (1897) that lignins are chemically and genetically related to coniferin and some analogous compounds has been a guiding idea in lignin chemistry; he considered lignins to be macromolecular compounds in the modern sense of the word. If Klason’s hypothesis is limited to the statement that lignins are built up mainly from Ce-Ca components, it must be agreed that the hypothesis was raised to the state of a well-founded theory by the hard labor of the next generation of lignin chemists. Adkins and Harris as well as Hibbert and his coworkers succeeded in degrading lignins to products from which considerable amounts of definite C-Cs compounds were isolated. Freudenberg and his collaborators, especially Lautsch, made an invaluable contribution to lignin chemistry by the discovery of the nitrobenzene-alkali degradation of lignins to benzaldehyde derivatives in high yield. However, in spite of the overwhelming amount of evidence in favor of the aromatic nature of lignins, a small band of chemists tenaciously maintained that “lignins” are artefacts formed by aromatization of unknown wood constituents. In this situation the author suggested to P. Lange that he investigate the ultraviolet absorption properties of chemically untreated wood, and as the result of Lange’s studies this controversy came to an end. Another hypothesis which has given rise to extensive and successful investigations was advanced by B. Holmberg (1935, 1936). From his experiences he concluded that lignins must contain
large amounts of benzyl alcoholic groups -C&-C(0H)-C-Cand that they, free or etherified, are responsible for several of the most important reactions of the lignins. T o the further development of this idea several Swedish chemists have contributed, including Adler, Erdtman, Hagglund, Kullgren, Leopold, and Lindgren, and the results have been important for the understanding of technical pulping processes. Holmberg’s hypothesis is today a generally accepted theory, although the presence of benzyl alcoholic groups in lignins has not yet been directly demonstrated, linkage of CB-CB Units
How are the C-Cg units linked to each other? I t has long been assumed that phenol ether linkages are responsible for this and it has therefore been a matter of great concern to organic chemists to estimate the amount of free phenolic hydroxyl groups present in lignins and lignin derivatives. The results have long been very inconsistent, owing to inaccuracy of the analytical methods, and it was only after the recent introduction of optical methods, particularly the development of the “A€method” by G. Aulin-Erdtman, that one could begin to feel safe on this point. She found less free hydroxyl groups per methoxyl group than had generally been supposed and her results have since been largely confirmed by other workers employing new, purely chemical methods. During the important degradative studies of the Heidelberg school another lignin chemical key substance was found-namely, isohemipinic acid-the isolation of which indicated the presence of C-C linkages between the C6-Cs units. The yield of acid was small, however, and the greater part of it was obtained only after drastic treatment of the lignins with strong alkali, which decreases its value as an argument
in lignin chemical discussions. For various reasons, it is likely that carboncarbon linkages occur in position 5 of some propyl guaiacol elements, but further work is needed for safe estimation of their frequency. Freudenberg early attempted to summarize the available data in very useful structural schemes which, however, have sometimes been misunderstood as representing true structural formulas. Much work was initiated by the author’s suggestion (1932, 1933) that lignins may owe their formation to oxidative polymerization of coniferyl alcohol and related compounds. This hypothesis originated from considerations of the probable biogenesis of guaiaretic acid, in those days a C6-C3 compound of unique structure. I t is now a member of a large group of natural dimers of phenylpropanes, lignans, all of which possess a bond between the P-carbon atoms of the side chains postulated to have been formed by a novel type of phenol dehydrogenation. Investigation of the known dimeric dehydrogenation product of isoeugenol, dehydrodiisoeugenol, showed that it contains a bond between C 6 in one isoeugenol molecule and the P-carbon atom of the side chain of the other, which is in accordance with the theory of phenol dehydrogenations. In dehydrodiferulic acid another dimeric dehydrogenation product was found, possessing the lignan p-p bond. From the theoretically wellfounded rules for phenol dehydrogenations it tould be expected that the ether, I, should also be formed from isoeugenol. However, because of its quinone methide nature it cannot be a stable compound but should stabilize by polymerization to amorphous products (the main product of the reaction) or by addition of water (or its equivalents) whereby the benzyl alcoholic derivative, 11, would be formed: VOL. 49, NO. 9
SEPTEMBER 1957
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Unanswered Questions
1
OCHI
/ -O-CH-CH(OH)-~-OH _ .
I1 Compound I1 has been sought in the reaction mixture, because its isolation would have been of, particular interest in view of Holmberg‘s theory, but hitherto without success. I n those days coniferyl alcohol was a difficultly available substance. Lignin chemistry has profited much by the discovery of the lithium aluminum hydride reduction method lvhich made it readily available. Freudenberg and his coworkers now commenced studies on the dehydrogenation of coniferyl alcohol and they found, as expected, that it reacts very much like isoeugenol. Products were obtained analogous to dehydrodiisoeugenol and dehydrodiferulic acid. Moreover, they succeeded in isolating a compound analogous to substance 11. As in the case of isoeugenol, amorphous compounds were also obtained. These products, called dehydrogenation polymerizate: DHP, were considered by Freudenberg to be related to lignin and even to constitute “artificial lignin.’’ I n some respects they are similar to lignins, but in others arc distinctly different. The author has always considered these ill-defined products as mainly stabilization products of quinone methides analogous to I. Freudenberg, holyever, thinks that they are formed by further dehydrogenation of the dimeric, crystalline, dehydrogenation products of coniferyl alcohol (which they- may well partly be), but this has not been demonstrated. Freudenberg further has claimed that the crystalline dimers are present in cambial sap, but the sap had been drawn from the tree and the compounds may be nonphysiological dehydrogenation products. Formation of Lignins
It will, indeed, be difficult to prove definitely whether lignins are formed in vivo-e.g., from coniferyl alcohol via the known dimers or other theoretically possible dimers or oligomers-because there exists no exact method of identifying two lignins. Simple reference to similar ultraviolet and infrared absorption is not satisfactory. This scepticism can also be extended to lignins formed in feeding experiments with labeled “precursors” or “intermediates,” especially in experiments where the compounds have been injected into
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the cambium. Organic chemists, unfortunately, have a tendency to overlook the complexity of the biological systems and the metabolic reactions and to deal with biochemical processes without properly defining the biological systems. A wounded and disorganized cambium will-there are several indications of this-react in a different way from a normal cambium. No natural compound has been structurally elucidated by following its biosynthesis, nor is it likely that this will be accomplished, especially in so complex a group of ill-defined compounds as the lignins. Nevertheless, in the near future, we shall certainly witness a great development in this field and many interesting facts will be found, some of which may be of value for the understanding of the nature of lignin. Uncritically conducted experiments and conclusions drawn from them, however, will only cause confusion. Great emphasis must be laid upon the necessity of careful studies of the components and the metabolism of the cambium and of the living parts of the sapwood and their biochemical factors. The presence of coniferyl alcohol and its analogs has been demonstrated in the cambia of only relatively few plants and it is not a t all certain that these compounds migrate from the cambia to the young cell walls to be deposited there in the form of lignin. I t seems equally possible that the lignin precursors are formed within the living cells. Already the uneven distribution of lignin in the cell walls may serve as an indication in favor of the latter view. The question whether lignins are formed by random polymerization of phenylpropane derivatives or whether there is some degree of order in the lignin molecules cannot be answered at present. Much can be said in favor of the first view. but, even if we accept the dehydrogenation hypothesis, this does not mean that lignins must necessarily be completely irregular. The reactivity in various positions of the primary radicals is certainly different and this will cause some degree of order. It is probably a great oversimplification to assume that the biological formation of lignin can be imitated-e.g., by crude enzvme preparations or by ferric ions.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Very little is known about the molecular weights of lignins, particularly of lignins in wood. Are they complexes formed from smaller “lignin molecules” connected by means of benzyl ether linkages, and in such case are they simply “anhydrides” of benzyl alcohols or mixed ethers? In the latter case, what is the nature of the nonbenzyl alcoholic moiety? The time is ripe to widen our knowledge of the lignins by careful, comparative studies of lignins from a large number of plants, gymnosperms and angiosperms as well as cryptogams. Our present ideas have, mainly, been developed from studies on spruce lignin. Why are the lignins of the gymnosperm Tetradinis of angiosperm type? Is it really simply due to a difference in substrate for an enzyme, or does this peculiar phenomenon indicate that lignification is a more involved process than many organic chemists n o ~ vare inclined to believe? There is at present among lignin chemists a tendency to circumvent the central, admittedly formidable, problem-the structure of the lignins-in favor or more facile studies of model compounds or by trespassing into domains which ought to be reserved for experienced, professional biochemists. It is essential that organic chemists try to develop new degradative methods and to isolate definite dimeric or oligomeric breakdown products of ligninse.g., from the alcoholysis products. 4 problem which deserves much attention is the nature of the nonaromatic portions of the lignins. An effort has recently been made to contribute to the solution of this problem by studying the ozonoiysis of a lignan, (-)-dihydroguaiaretic acid dimethyl ether. By this method the aromatic nuclei are degraded, leaving Lhe central portion intact (7). A reinvestigation of the ozonolysis products of lignins, employing modern methods for the isolation and characterization of the degradation products, appears to be worth while. Present views on the structure of the nonaromatic regions of the lignin molecules are largely due to analogies, exclusions, and biogenetic speculations. I n these days when many lignin chemists appear to believe that the ultimate solution of the lignin problem is near, it may be useful to remember that our belief is greater than our exact knowledge. Literature Cifed
(1) Carnmalm, B., Chem. €8 Ind. 1956, p. 1093.
RECEIVED for review December 3, 1956 ACCEPTED June 22, 1957 Division of Cellulose Chemistry, Lignin Symposium, 130th Meeting, ,4CS, Atlantic City, N. J., September 1936.
