VOL.7, NO. 8
RELATION OF COTTON TO CHEMISTRY
1803
THE CONSTITUTION AND PROPERTIES OF CELLULOSE*
The tremendous growth in recent years of the industries using cellulose as a raw material has made i t imperative that as full a knowledge as possible of the structure and chemistry of this body be obtained. In the early days of paper and textile manufacture, when "rule of thumb" prevailed and competition was not keen, the incentive to study cellulose was only that of pure research. But, as the artificial silk, film, and lacquer industries, which use cellulose derivatives, grew, it became of the utmost importance on the conlmercial side to understand the chemistry of this body as fully as possible, that processes might be cheapened and better products made. This has resulted in making the chemistry of cellulose
* Contribution to the symposium on "The Relation of Cotton to Chemistry," held jointly by the Division of Cellulose Chemistry and the Division of Chemical Education, at the 79thMeeting of the American Chemical Society, at Atlanta. Georgia, April 9, 1930.
1804
JOURNAL OF CHEMICAL EDUCATION
AUGUST,1930
a very important branch of this subject both from the standpoint of pure research and that of industry. Cellulose associated more or less with lignin is found in most plants. As to whether the cellulose is actually combined with the lignin (the socalled ligno-cellulose), is loosely held as by secondary valences, or the two are merely associated, is an open question. In attempting to decide this, the means employed are so drastic that one cannot tell whether bonds have been ruptured or not. Lignin is a very complicated body supposed to be four condensed molecules of coniferyl alcohol.' HC-CH
II I/ I I
HOC C-CH=CH-CHnOH
FZCH
OCHJ
Dr. Ritter2 of the Forest Products Laboratory has isolated from wood two distinct ligninsa The fibers forming the seed hairs of the cotton plant contain a higher percentage of cellulose than any other known source. The structure of these fibers is very interesting. They grow from the seed and are of varying lengths. When the long fibers have been removed by a process called ginning, shorter fibers are left which are removed by cutting and come into the market as linters. It is this form that is generally used in making the derivatives. A long fiber under the microscope appears as a flattened tube, twisted in places, with a broad flattened end, where i t has been torn from the seed and from this end gradually tapering to a point a t the other. Under high power and with polarized light a structure may be seen. Stomata or small pores appear a t the edge and running spirally from these are lines. Some believe that the fiber is a single cell, but personally I am inclined to believe that it consists of a number of cells. At various times I have tried to force liquid into the hollow portion by vacuum and by capillary action, but have never been successful. Later I found that de Mosentha14bad made the same attempts with the same results. If a fiher is treated with Schweitzer's reagent-copper hydroxide dissolved in ammonia-the fiber swells, not continuously, but in sections so that the result has the appearance of a string of sausages. As the treatment proceeds the constrictions burst as well as the sausages and the cellulose passes into solution. If a single fiber under these conditions is carefully observed, it will be seen that when a sausage bursts a very thin skin does not dissolve but floats away. This would point to a difference in the outer layer, a t least physical if not chemical. In one observation one of the spiral lines broke loose and unwound the length of the fiber.
VOL.7, No. 8
RELATION OF COTTON TO CHEMISTRY
1805
Balls,= by swelling a cotton fiber in alkali and carbon bisulfide, has been able to see the daily growth rings. According to him there are never less than 20 normore than 25; on the fuzz never less than 16. I bave discussed the structure of the fiber briefly that some of the difficulties in making the derivatives may be better understood. Cellulose is very resistant chemically. There are no true solvents in the sense that apparently the cellulose cannot be recovered from its solutions in the same condition as before dissolving. When recovered, it is more reactive and changes bave taken place, the exact nature of which are unknown. Any attempt to treat cellulose causes some difference. An example is standard ~ellulose.~This is made by extracting Wannamaker Cleveland sliver with alcohol and then ether. It is then boiled with 1% NaOH solution, care being taken to exclude air, till the solution comes free from the yellow color. It is then washed with boiling water until free from alkali, cooled without access to air and then treated with 1% acetic acid, washed, and dried. A 2% solution of the original sliver, the sliver after extraction, and the standard in cuprammonium gave the following viscosities: Original After extraction Standard
178,000 centipoises 230,000 37,700
Ultimate analysis shows pure cellulose to consist of carbon, hydrogen, and oxygen in the ratio of GH1005. The molecular weight has not been determined because it is not volatile and does not form true so1utions.t Therefore, a t present we will write the formula (GH1OOS)X, where x may be one or some other whole number. The empirical formula indicates that cellulose may be a carbohydrate. In 1883 Flechsig7 claimed that cellulose could be entirely converted into glucose. Ost and Wilkenings hydrolyzed cotton with sulfuric acid and obtained an almost theoretical yield of glucose, as estimated polarimetrically and by means of Fehling solution. Subsequently, Willstatter and Zechmeister9 showed that cellulose was soluble in 41% hydrochloric acid and that such a solution upon standing resulted in the hydrolysis of cellulose to glucose. The polarimeter and Fehling solution both indicated a 95% yield of glucose. That cellulose is capable of hydrolysis quantitatively to glucose was firmly established through the work of Monier-William~,'~ who hydrolyzed cellulose with sulfuric acid and actually isolated pure crystalline glucose to the extent of 91%. The work of Irvine and Soutar," done a t approximately the same time, confirmed the result. The latter workers subjected
7 After this paper was written, A. J. Stamm in a paper givenat theAtlantameeting of the American Chemical Society, April 7-11, 1930. gave the molecular weight as CaHn05X 200 to 240. These results were obtained hy dissolving the cellulose in cupramrnonium solvent and using the ultra centrifuge method devised by The Svedherg.
1806
JOURNAL OF CHEMICAL EDUCATION
AUGUST,1930
cellulose to acetolysis with sulfuric acid in the presence of acetic anhydride and acetic acid and hydrolyzed the resulting glucose acetate with a methyl alcohol solution of hydrochloric acid, thereby obtaining a-methyl glucoside. This was, of course, readily converted to glucose. The yield obtained in this way was 85%. Subsequently in 1922, Irvine and Hirst,12 through modification of the acetolysis method and the use of slightly stronger alcoholic HCI, brought the yield of glucose to 95.1% of that demanded by the expression (GHIOOS)X -f CBH,~O~. By this time very strong evidence had been obtained that cellulose is a polysaccharide and that its sole product of ultimate hydrolysis is glucose. Glucose is an aldohexose and the structural formula was given as
HO~H
I
HCOH I
Later it was found that it had characteristics best represented by an oxide ring structure.
2
3 4
HC-
5
HCOH
0
I
The question now arises as to whether the linkage is a t the four or five position if we number the carbon atoms from 1 to 6, beginning with the aldehydic carbon. As to the exact location there is still a controversy. I have discussed glucose that you may understand some of the considerations which must be taken into account in formulating a structure for cellulose. Cellulose, while ordinarily very resistant chemically, under certain conditions becomes very reactive. I t is capable of forming esters and ethers; it is easily oxidized and hydrolyzed; in other words, its reactions parallel very closely those of an alcohol.
VOL. 7, No. 8
RELATION OF COTTON TO CHEMISTRY
1507
In 1832 BraconnotLJ treated cellulose with nitric acid and obtained a product which he called Xyloidine. Pelouze14and Dumas investigated this reaction further and Schonbein15 developed it technically. By treating cellulose with a mixture of nitric and sulfuric acids and water, he obtained a product diiering from the original material. It had a very harsh feel, was exceedingly inflammable, and contained nitrogen. This was called nitrocellulose, a misnomer, because subsequent investigation has shown this body to be a true ester, cellulose nitrate. The highest which may be obtained contains about 13.9% nitrogen and corresponds to a trinitrate (theoretical 14.16%). This would indicate that cellulose has three available hydroxyl groups for every C6 unit. Since the discovery of the nitrate the esters of many of the aliphatic acids and some of the aromatic have been made. But in no case, with one exception, has an ester been obtained higher than the tri. The exception is the tetra acetate obtained by Cross and Bevan.16 This is open to grave questioning as it was made from cellulose which had been regenerated from viscose and therefore probably very much degraded. It was discovered that if cellulose is treated with sodium hydroxide and methyl sulfate, an ether is obtained. The ethyl ether has been made and higher ethers have been produced. But here again in no case has an ether been made higher than a tri per C+group. Imine" and his co-workers applied the ether formation to a study of the structure of cellulose. By repeated methylation and subsequent hydrolysis they obtained a practically quantitative yield of crystalline 2,3,6-trimethyl glucose. Structural formulas for cellulose have been proposed by toll en^,'^ Vignon,'B Cross and Bevan,?"ernadou," N a s t y ~ k o v ,Green,%= ~~ Hess,%' Hibbert,25S c h ~ r g e rI, r~~~ i n eGray,28 , ~ ~ and odd^.^^ In constructing a iormula which correctly expresses the structure of cellulose, the following facts must be taken into consideration: 1. It must yield glucose as the sole ultimate product of hydrolysis. 2. It must have three available hydroxyl groups per Ce unit. 3. It must yield upon methylation and subsequent hydrolysis, 2,3,6trimethyl glucose only. There are other reactions such as the formation of bromomethyl furfural when cellulose is treated with hydrobromic acid in presence of ether or chloroform and the formation of isosaccharinic acid when oxidized cellulose is boiled with milk of lime, but personally I believe these reactions are too complicated to throw much light upon the subject a t present. Here I wish to correct an error which has crept into nearly all the literature. toll en^^^.^^ first found that oxidized cellulose when boiled with milk of lime gave isosaccharinic acid. Through an error, probably of translation, this was given as isosaccharic acid. Heuser3=notes this.
1808
JOURNAL OF CHEMICAL EDUCATION
AUGUST, 1930
From the known facts we can now see that cellulose is a polysaccharide composed of one or more anhydro glucose units. It now remains t o ascertain how the oxygen bridges are connected and the value of x in the empirical formula. Before proceeding, however, I wish t o add another factor t o this formula. The molecule of cellulose forms aggregates. How, as yet, we do not know. So we may write the formula thus: (C6H,,Os),, where x represents the molecular unit and y the aggregate. Of the formulas proposed, I shall restrict myself t o a discussion of three. HibbertZ5suggested this formula.
I ~ i n e proposed ~' the following:
It is in accord with the facts which I have given and gives upon hydrolysis a trisaccharide and a disaccharide, cellobiose, both of which have been isolated. However, chemical evidence has accumulated ~ different from the other which indicates that one hydroxyl per C Z is eleven. When viscose is ripened it reaches a certain stage which upon analysis shows one xanthate group per Cw. Lilienfeld found that if vis-
yS is acted upon with the sodium salt of monochloroacetic YSNa acid, the following reaction takes place. cose R-O-
R-+C
'
s
~
+ NaCl
C-COOH H ~
VOL.7. NO.8
RELATION OF COTTON TO CHEMISTRY
1809
If this is then treated with an aromatic amine another change takes place. R-SC
//s
C-COOH
+ NH,X
+
By etherification
Analyses of the xanthoanilides have shown that there is one xanthoanilide group for each Cx. A study of the action of concentrated HN03 acid on cellulose in the cold has pointed to this difference also. H. T. Clarkeaafound that linters in boiling acetic acid upon washing and drying show an acetyl content equivalent to one acetyl per Czr. With this evidence in mind I have suggested a formula in which one hydroxyl group should have different properties from the other eleven.
1
I
H H H C-O-C-C-C-C-C-(tCH
I
H
H
You will notice that in Irvine's formula the value of x is 3 while in that of Gray it is 4. H e r ~ o gby ,~~ studying the X-ray diagram given by cellulose, came to the conclusion that the molecular unit of cellulose must be one, two, or four, but never three anhydro-glucose groups. Sponsler and D ~ r e , ~ ~ by the same means applied to ramie, arrived at 8 as the unit cell of cellulose. They consider this substance to be made up of anhydro-glucose units in the form of amylene oxide rings apparently united by primary valences in chains of indefinite length. These chains are parallel to the
JOURNAL OF CHEMICAL EDUCATION
1810
AUGUST, 1930
longitudinal axis of the fiber. The ramie fiber is a hollow cylinder in which the crystal units are so placed that one of the diagonals of the unit cell always occupies a tangential position. They state that a group of eight anhydro-glucose units is the simplest unit that can represent the structure of cellulose. The authors consider that the continuous primary valences account for the tensile strength of the fibers in a longitudinal direction while they are stabilized laterally by the secondary valences between the oxygen atoms of adjacent chains. Ester formation is shown to be possible. This may decrease the secondary valence force with a consequent separation of the longitudinal chains and a resultant weakening of the fibrous structure. Recently G. L. Clark36has come to the conclusion that the unit cell contains four &HloOs groups. An authoritative review of this field has been recently presented by Sir William Bragg.37 Hessas and his co-workers, by dissolving cellulose in cuprammonium, studying the optical rotations of the solutions, and calculating the ratios of copper to cellulose, have arrived a t the conclusion that cellulose is monomolecular; in other words, anhydro-glucose. He explains the formation of cellobiose octa acetate on the grounds that two units, while changing from anhydro-glucose to glucose, unite to form this substance. When one considers that the acetolyzing mixture is a very powerful hydrolytic agent and that cellobiose is hydrolyzed by it, one doubts very much if it is formed in this way. The formula suggested by Gray is in accord with the chain theory. It is believed that its contribution is in making one oxygen bridge in each group of four differentfrom the other three, thus accounting for the seeming differenceof one hydroxyl per four CaHloOsgroups. I have endeavored to give briefly the salient points of the chemistry of cellulose which have led to a better structural understanding of this material. But until we are able to show absolutely the linkage of the oxygen bridge in glucose and the value of x in the empirical formula, any structural formula is purely speculative as far as these factors are concerned. Bibliography Charnbovet Translation-Paper (February 9, 1921). R I ~ RG,. J., Ind. Eng. Chem., 20,941 (1928). EDUC., 6,840--2 ( M a y . 1929). Cf. J. CHEM. ' DE MOSENTHAL, H. J., J. SOC.C h m . Ind., 23, 292 (1904). ' BALLS,Phi2. Trans., 191SB (1915). COREY, A. B., and GRAY,H. LBB., Ind. Eng. Chem., 16, 853 (Aug., 1924); 16, 1130 (Nov., 1924).
FLRCRSIG, E.. Z. physid. Chem., 7 , 5 2 3 (1883). OST, H., and WILKENING, H., Chem.-Ztg., 34,461 (1910). WILLSTATTER, R.. and Z B C ~ I SL., ~ Ber., R , 46,2401 (1913).
VOL.7. NO. 8
RELATION OF CQTTON TO CHEMISTRY
MONIER-WILLIAMS, G. W., J. Chem. Soc., 119,803 (1921). IRVINE,J. C., and SOUTAR, C. W., Ibid., 117, 1489 (1920). IRVINE, J. C., and HmST, E. L., Ibid., 121, 1585 (1922). 'QRACONNOT, Ann. chim. phys., (1833); Ann., 8,245 (1833). l4 PELOUZE, Ann. chim. phys., 29,38 (1838); Ann., 64,391 (1847). Is SCA~NBEIN. Phil. Mag., 31 [3], 7 (1847). ' 6 CROSS and BEVAN,J. Chem. Sac., 79,366 (1901). IRVINE,J. C., and H m s ~E. , L., Ihid., 123,518 (1923). ' 8 T ~ "Handbuch ~ ~ ~ der ~ ~ Kohlehydrate," . Vol. 11, E. Trewendt; Breslau, 1895,2nd edition, 1914. In VIGNON, LEO.B d l . SOC.Chim., 21, 599 (1899). so Cnoss, C. F., a n d B ~ v aE. ~ ,J., J. Chem. Soc., 79,366 (1901). 2 1 B ~J. B., ~ ''Srnokele~~ ~ ~ ~ Powder, ~ ~ Nitrocellulose, , and the Theory of the Cellulose Molecule," London, 1901, Chapman and Hall. e z N ~ s m u n o v J., , Russ. Phys.-Chem. Sac., 34,231,235, 505, 508 (1902). Pa GREENand PERKIN,J. Chem. Soc., 89,81 (1906); Ibid , 89,811 (1906). 24 HESS,K., Z. Elektrochem.. 26,234 (1920). HIBBERT.H., Ind. Eng. Chew., 13, 256, 334 (1921). SCHORGER, A. W.. Ibid., 16, 1274 (1924). IRVINK,J. C., and H m s ~E. , L., J. Chem. Soc., 123,521 (1923). GRAY,H. LBB., Ind. Eng. Chem., 18,811 (Aug., 1926). a ODDO,G., Gozeetla Chemice IfnCiana. LVIII, May 6, 1928. 3' TOLLKNS, B.. and FABBR. V.. Ber... 32.. 2594 11899). .~ . . " TOLLESS. B., hlr.nuhruw, and SACK.J.. ILid .34, 1432 (14JI). Ileusan. E.. "Lchrbucll drr C e l l u h , ~Chrrnie." ~ 32nd edition. I'rrss of Urorlwri Borntriger, 1927, p. 238. CLARKE,H. T., and MALM,C. I., J. Am. Chem. Soc., 51.274 (1929) . . 5%Enzo~, R. 0.. Celldosechemie, 2, 102 (1921). SPONSLER, 0 . L., and DORK,W.H., C01loidSymposiumMonorna~h.Vol. IV. D 174. * CLARK,G. L., and PICKETT,LUCYW., Science, 71, 294 (1930): a' BRAGG. Sm WILLIAM,Nature (Supplement) (Mar. 1, 1930). p. 315 HEss, K., WELTZIEN, W., and MEssawn, E., Ann., 435,l (1923). 'O
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