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nullify any benefit caused by another, especially as Schulte’s claim is on the basis of scavenging effect on the nitrogen rather than on the basis of true alloying action. The aluminium used in the Bureau of Mines experiments was of good quality and probably not contaminated with aluminium nitride. If we accept the unproved assumption that reclaimed boring ingot, for example, does contain aluminium nibride, the possibility remains that a very small “dose” of cerium might help, but it seems certain that in the usual run of good metal used for casting alloys, the use of cerium would be an expense not attended by any benefit. The test pieces of the Bureau of Mines experiments have not been analyzed. The charges were carefully weighed and
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there was no sign of loss of cerium. Only cursory microscopic examination of the pieces has been made. I n view of the discouraging results of the physical tests it was felt that-analytical and micrographic work would be time wasted.
ACKNOWLEDGMENT Grateful acknowledgment is due to H. S. Miner, of the Welsbach Company, for his cooperation in this work which was done at the joint expense of the Welsbach Company and the Bureau of Mines, under cooperative agreement, to E. Blough of the Aluminum Company of America, for reading the manuscript, and to Cornel1 University for use of its laboratories and testing facilities.
Contributions t o Chemistry of Wood Cellulose I I-Nature of Wood Cellulose’ By Louis E. Wise NEWYORE STATE COLLEGE OF FORESTRY, SYRACUSE, N. Y.
‘
During the World W a r the dearth of cotton in Germany and Austria stimulated research which had for its object the industrial substitution of wood cellulose for cotton cellulose. I t also encouraged comparative studies on the chemical constitution of cotfon cellulose on the one hand, and of wood cellulose on the other. The value of such studies i s naturally of more than passing academic interest. Their ,industrial importance m a y be gaged from a number of articles that have appeared within the past eight years in American and foreign journals of applied chemistry,2 and the ultimate results of these investigations must have a decided bearing on those industries which require the use of a highly purified cellulose in theirmanufacfuringproc-
esses. The accumulation of data. howeuer, gives the industrialist little more than a labyrinth of isolated facts, unless these data are subjected to a thorough critical analysis. Not infrequently such data may be interpreted in a variety of ways, and the danger then arises of accepting an interpretation which will limit or cripplefurther advance in this par t icular branch of cellulose chemistry. This article has for its object the brief recapitulation of the experimental work of a number of inuestigators, the results of which may perhaps be welded into a constructive working hypothesis on the nature of wood cellulose.
\
presence of the cellobiose linkage or residue, or more correctly the anhydrocellobiose linkage, which may be represented by either Formulas I3or II:4 CH20H
I
HC-
CH--O-C-
H
I
0
H !\ 0
\.CH
$\
OHH
I
I
t
CH-OH
C-C-OH
-0-CHZ
FORMULA I1
1
\
H ‘\ I \ H--C -C--C.-
I
I
t
OH OH
I
C-H -0-C-H
I
H
I I
CHzOH
FORMULA I 1 Presented. before t h e Division of Cellulose Chemistry a t the 64th Meeting of the American Chemical Society, Pittsburgh, Pa., September 4 t o 8, 1922,in connection with t h e symposium on “The Nature of Wood Cellulose.” 2 Articles on comparison of wood and cotton cellulose include those by Heuser a n d Boedeker, 2 angew. Chem., 34 (1921),461; Lenze, Pleus, a n d . 101 (1920),213; Wise and Russell, THISJOURNAL, Mueller, J. p ~ a k t Chem., 14 (1922),285; Mahood a n d Cable, Jbid., 14 (1922), 727. Articles on nitration of wood cellulose include those b y Schwalbe, Z. angew. Chem., 27 (1914),662; Wells and Edwards, Papev, 23 (1919),180; Woodbridge, THIS J O U R N A L , 12 (1920),380. A modern research on acetylation of wood cellulose is t h a t by Hagglund, Lbfman, and Farber, Cellulosechemie, 3 (1922),13. 3 Hibbert, THIS JOURNAL, 13 (1921),256; Irvine, J . Chem. Soc., 123 (1923),525. 4 Karrer, Helvclica Chim. Acta, 6 (1922),187.
There is at present, however, no definite agreement regarding thelimiting yield of c e l l o b i o ~ ethat ~ , ~ may be obtained from cott,on cellulose. Investigators are by no means in accord when called upon to decide whether or not the cellulose molecule is made u p entirely of cellobiose residues. Different hypotheses regarding the size of the cellulose molecule have also appeared in the cellulose literature. Views on this subject may be briefly summarized as follows: (a) a relatively enormous molecule made up of a great number of anhydrodextrose residues,as6many of which go to make up the anhydrocellobiose linkages’ referred to above; (b) a large 8 Ost, A n n . , 398 (1913), 337; Madsen, Dissertation, Hannover, 1917. Hess and Wittelsbach, 2.Elektvochem., 26 (1920),232; Freudenberg, B e y . . 64 (1921). 767, Irrine, Zoc. c i t . e von Euler, Chem. Z t g . , 46 (1921),977,998. T h e older hypotheses, which did not take t h e cellobiose grouping into consideration, include those of Tollens, “Handbuch der Kohlenhydrate,” p. 252; Green a n d Perkin, J. Chem. SOL.,8 1 (1906),811; Cross and Bevan “Cellulose,” p 75; Vignon, Bull SOL. chim , a 1 (1899),599.
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Vol. 15, KO.7
aggregate made up of comparatively small units, each one of gate (the wall). It is then that the primary valences within which contains the anhydrocellobiose linkage a relatively the cellulose units are disturbed. Hydrolysis, acetolysis, and small number of times. The units m e held together in the oxidation of cellulose are examples of such attack. Partial larger aggregate by means of secondary v a l e n ~ e * ~or~ in ~ ’ ~ hydrolysis-usually referred t o under the obscure term some similar way which is not yet fully determined. From “hydrocellu1ose”-would mean the hydrolysis of a fraction of the work of Herzog and his eo-workersll it is probable that the number of units originally present in the aggregate. A the cellulose aggregate is crystalline. similar picture would obtain in the case of partially oxidized cellulose (oxycellulose). I n either case the hydrolyzed or oxidized material may be adsorbed on the surface of unchanged cellulose units. I n practice these two types of changes cannot be readily distinguished from each other. It is almost impossible to drastically alter a cellulose aggregate, by meddling with secondary valence, without causing some chemical change in a t least a few of the cellulose units. When the surface area is increased, the chemical resistance of the unit is FIG.1 weakened. This must be borne in mind in seeking to interpret any analytical data in connection with cellulose. The viewpoint outlined under ( a ) , although still held by It should also be taken into account when we indulge in the individual investigators (like von Euler), has been largely common but rather questionable practice of bolstering our displaced by that referred to under (b). Just what the con- ideas on the constitution of some particular cellulose with a stitution of the individual cellulose unit really is, still forms mass of analytical data. the subject of extended debates which require no space in this WOODCELLULOSE paper. Most investigators are agreed that the cotton-cellulose aggregate is composed of a relatively large number of I n general, wood cellulose is a residue remaining after more small units held together by forces which, for the lack of a or less drastic treatment of wood to remove lignin and carbobetter name, may be referred to as secondary or auxiliary hydrates other than cellulose. Such treatment may involve valences. A crude but rather useful pictureof such an hypothe- the preparation of chemical pulp by any one of the three sis is given in Fig. 1. The cellulose units may be likened t o well-known processes, or, if the isolation is analytical, may bricks in a thick wall, the cellulose aggregate. The mortar cause the removal of lignin by alternate treatment of the finely between these bricks would represent the secondary valence. divided wood with chlorine and sodium sulfite. I n the If the units (bricks) remain undisturbed, but the secondary United States, a t least, the term “wood cellulose” has genvalence (mortar) is tampered with, changes in state which are erally referred to the residue isolated by a chlorination so characteristic of cellulose are noted. The individual method.13 units or blocks of these units become more or less separated from each other, and a greater surface area is exposed (Fig. 2). Adsorption, which invariably precedes swelling of cellulose, affects the secondary valence. Swelling does not chemically change the little cellulose units; it disturbs the aggregate as a whole.8 For example, in the swelling caused by the mercerization with alkali, after washing with water, dilute acids (and water), and drying, cellulose shows increased hygroscopicity and an enhanced power of adsorbing substantive dyes, etc. These properties are accounted for in our hypothesis by the increased surface area when the units become partially separated from each other. Hydration due FIG.2 to mechanical beating of cellulose with water, and the first stages in the disintegration of cellulose by means of acids may The residue when subjected to further purification, such as be similarly explained. Complete peptization of cotton in digestion with cold alkali and careful washing with water cuprammonium, zinc chloride, and other so-called cellulose and acid, has been termed “normal” or “alpha” cellulose. solvents, involves a complete disintegration of the cellulose The limitations of these terms have been commented upon in aggregate, with more or less complete separation of the cel- a previous communication. l 4 Normal wood cellulose, as lulose units from one another. When cellulose is precipitated defined, possesses the following properties in common with in “hydrated” gelatinous form from its sols, the bricks of the those of cotton cellulose: original wall are probably in large measure intact. They 1-It may be hydrolyzed almost quantitatively t o d-glucose. approach each other, but they no longer present the same orderly arrangement which was manifested in the original The rate of hydrolysis after peptization of the cellulose is nearly cellulose aggregate (wall). This is indicated by the change identical with that of cotton cellulose.I6 2-On acetolysis it yields appreciable quantities of cellobiose in crystalline structure in such substances as viscose.I2 octacetate.l6 A different picture obtains when the little units themselves 3-It shows a crystalline structure practically identical with become subject to chemical attack. Such attack often follows that of cotton cellulose.12 the partial or complete disintegration of the cellulose aggre8
Karrer, Helvetzca Chim. Acta, 4 (1921),811; Karrer, Cellulosechemie,
2 (1921),127.
* Esselen, THISJOURNAL, 12 (1920),801; see also Minor, Pager, 2s (19191,700. 10 Heas and Wittelsbach, lac. cit. 11 Cellulosechemie, 2 (1921),101 : Naturwissenschaflen, 8 (1920),34. 12 Herzog and Jancke, Bey., 88 (1920).2126.
Schorger, THIS JOURNAL, 9 (1917), 567; Dore, I b i d . , 12 (1920),266. Wise and Russell, lac. cit. 1s Heuser and Boedeker, loc. c i t . 18 Russell, a t the Pittsburgh meeting of the American Chemical Society, reported the formation of cellobiose octacetate on acetolysis of the normal cellulose isolated from each of ten commercial species of wood. Details are reserved for a later publication. 13
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4-On nitration for industrial purposes, it yields nitrates very similar t o those of cotton cellulose.17
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These facts taken by themselves might argue for the identity of cotton and wood cellulose, were it not for the following disconcerting data. Wood cellulose from coniferous woods, even when carefully purified, has been shown by Sherrard and Blancols t o yield small but appreciable quantities of mannose on hydrolysis. Wood cellulose isolated from the angiosperms when carefully purified still contains small amounts of furfural-yielding substances.lg Another argument against the identity of wood and cotton cellulose has been advanced on the basis of the absence of so-called 7-cellulose from purified cotton, and its presence in appreciable quantities in the cellulose obtained from wood.*O In the opinion of the writer, however, analytical data regarding 7-cellulose have no vital bearing on the true nature of wood cellulose. The term “y-cellulose” refers to a fraction, of very uncertain identity, obtained in a convenient but arbitrary analytical procedure, which has never been correlated with problems on the constitution of cellulose. It has been shown that conditions of chlorination influence the amount of a-cellulose very considerably in the same sample of cotton or wood cellulose. Successive chlorinations, while they do not appreciably affect the total cellulose content, serve to decrease the percentage of a-cellulose.21 This would mean an increase in the P- or y-cellulose content or in both. It would appear, therefore, that p- and ycellulose may actually be considered, in part at least, derivatives of the original cellulose. They may be produced during the chlorination, and are not necessarily part of the original cellulose aggregate. Needless to say, the formation of alkalisoluble cellulose CP- and y-cellulose) would depend in no small measure on the physical condition (state of aggregation) of the original cellulose. I n the case of celluloses which have undergone drastic treatment prior t o the cellulose analysis, whether this treatment is due to the action of molds or to a chemical process, lessened resistance to oxidation might be expected, and consequently less a-cellulose and correspondingly more p- and y-cellulose in the residue after chlorination. I n other words, there is at present little or no evidence that p- or y-cellulose are actually components of the original material from which cellulose has been isolated. The question now arises-can these other data be brought into harmony in the formulation of a n hypothesis regarding the constitution of wood cellulose? It must be assumed a t the outset that the wood-cellulose aggregate is a very variable one. It is doubtful whether the same investigator can succeed in isolating the same aggregate from the same sample of wood in two successive experiments. This does not mean that the great majority of units in such an aggregate (in purified wood cellulose) may not be identical with the units in cotton cellulose. If this is true, the minority units-i. e., those not identical with the cotton-cellulose unit-which perhaps make up a part of the wood-cellulose aggregate, might reasonably be considered the units of those carbohydrates which are adsorbed on the crllulose during the growth of the cell. Wislicenus and Kleinstu ck22have shown experimentally that woody tissue is probably built up by adsorption on the cellulose gel of substances derived from the cell sap. This essentially is the process of lignification. The fact that some of these adsorbed substances remain closely associated with the celSchwalbe, loc c i t . Preliminary reports at the Birmingham and Pittsburgh meetings of the American Chemical Society. 19 Wise and Russell, unpublished data; also Schorger, Zoc. cit. 20 Mahood and Cable, Zoc. czt. 2 1 Wise and Russell, lac cit. 2 2 Chem. I n d . Kollozde, 6 (1910), 17,87. 17 18
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lulose, even after drastic treatment when so-called typical pentosans, mannans, etc., are dissolved or destroyed by such treatment, is not out of harmony with our general knowledge regarding changes in the properties of substances following adsorption. Purification of wood cellulose always involves the attempted removal of extraneous material with the minimum change in the cellulose. On the other hand, it does not permit the true solution and reprecipitation of cellulose which might facilitate the removal of extraneous material, and it is common knowledge that solids adsorbed by gelatinous precipitates may be exceedingly difficult to remove.
f
FIG.3
Taking these facts into consideration, and realizing that cotton and wood cellulose have many chemical properties in common, it appears reasonable to formulate a n hypothesis for the structure of wood cellulose which is very similar to that advanced for cotton cellulose. I n the case of the woodcellulose aggregate (the wall), most of the units (bricks) appear t o be identical with those in the cotton-cellulose aggregate. Varying amounts, and varying types of other units, however, may also be present in the wood-cellulose aggregate, such units being adsorbed on the surface of the cellulose units during growth. Fig. 3 gives a crude picture of wood cellulose. Again we have the typical cellulose bricks in the cellulose wall held together by secondary valence mortar, but besides we have a smaller number of other units-mannans, pentosans, methyl pentosans, galactans, or any other type of polysaccharide-that can conceivably be adsorbed during the growth of the wall. The number and type of these foreign units depend on the conditions of cell growth and lignification, and also on the purification to which the wood cellulose has been subjected. The hypothesis naturally accounts for any mannose that may be found in the hydrolysis mixtures obtained from the cellulose of coniferous woods. It also explains the presence of furfural and methyl furfural-yielding substances in the a-cellulose isolated from hardwoods. From such an hypothesis we would expect the puyified wood-cellulose aggregate to manifest a behavior similar to that of cotton cellulose in cases where the secondary valences are disturbed. Furthermore, we would expect reactions akin to those of cotton cellulose when wood cellulose is subjected to hydrolysis, oxidation, esterification, and acetolysis. As a matter of fact, the properties of wood cellulose closely approximate those of cotton cellulose, and appear to be in harmony with the hypothesis. Whereas the foregoing should be taken only as a working hypothesis (and the brick-wall analogy should not be taken too literally), it appears more readily acceptable to the writer than one which admits the possibility of a different cellulose unit for each species of wood. Furthermore, it agrees with the more modern viewpoint on cotton cellulose. Its flexibility is apparent, since it accounts satisfactorily for any percentage of pentosans or hexosans that may be found in the cellulose aggregate. Finally, it seems to harmonize with the present-day concepts of lignification and with the views of plant physiologists and phytochemist^.^^ 28
Wislicenus, KoZloid-Z., 5 (1920),209.
