RESEARCH ^
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Is this the structure !
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of Hgnin? Groups net
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attached are possible
I f t^^ HCQH
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variations
HCOH
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Terra Incognita? Not quite but — b o t h organic and physical chemists are still speculating about lignin's structure
B
Chemists, b o t h here and abroad, still don't h a v e the answer to the structure of lig-
there is more agreement among them, but t h e r e is still no conclusive proof for lignun's structure. Some of t h e most extensive research has been conducted by Swedish researchers. This includes one of the most distinguished lignin investigators who addressed t h e D i v i sion of Cellulose Chemistry-—Holger E r d t m a n of t h e Royal Institute of T e c h nology, Stockholm. "Very little is really k n o w n about lignin chemistry," he told the division. Erdtman says that one portion, of the lignin molecule has definitely b e e n confirmed b y K. F r e u d e n b e r g : HO—C—C—C—
OCH3 Also, scientists are almost sure that an OH radical is attached to the first carbon atom in the above formula. H o w ever, Erdtman says that very little is known about the middle portion of hgnin. Also, the biggest problem c o n fronting lignin chemists is how the "molecules in lignin are coupled together. Another Swedish researcher, E. Ad4776
C&EN
OCT.
I,
1956
ler of the Chalmers Technical University of Goteborg, says that "lignin is no longer t o be classified as terra incognita/' Rather, "it is t o b e looked upon as a kind of panorama of a scenery on a somewhat hazy day, when w e can distinguish the more prominent features of the total view whereas other details may be hidden/' Here's the lignin panorama given by Adler. It is a siimmary of information now available on lignin structure: In a section of the lignin molecule several monomers show the guaiacylglycerol-j3guaiacyl ether structure; others, the phenylcumaran or the corresponding open structure, or the pinoresinol structure. Here and there, the diphenyl link alternates with these modes of linkage. Approximately, every third of 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 alleviation. The chain is terrninated by a coniferyl alcohol or coniferyl aldehyde group. However, colorimetric and spectrophotometric data indicate that there i s only one such group for about 3 5 phenylpropane monomers in Bjorkman hgnin. Therefore, dimerized and polymerized coniferyl alcohol end" groups, according to Freudenberg, may replace the unsaturated end groups—thus, link-
ing together two or more polymeric chains of this type. Cross linking may also be brought about by arylt S-) alkyl or by dialkyl ether linkages. The presence of a total of approximately 0.2 carbonyl groups per methoxyl requires the occurrence of carbonyl groups in saturated side chains, possibly in the form of aldehyde groups replacing primary alcohol groups. • Physical Side of Lignin. To find out more about the makeup of Hgnin and its sulfonates, J. L. McCarthy and coworkers at the University of Washington treated hemlock and spruce wood with bisulfite—sulfurous acid solutions. After several periods of treatment at elevated temperatures, McCarthy and his coworkers found that lower molecular weight lignin sulfonates are obtained first, and as the time of treatment is extended, the average molecular weight of the dissolved hgnin sulfonates increases to a maximum, decreases to a minium, and finally begins to rise again. (Molecular weights ranged from about 3000 on u p to 100,000. About one quarter of the total lignins were distributed primarily in the molecular weight range of about 5 0 0 to 10,000.) McCarthy and his coworkers attribu t e these molecular weight changes to: • Hydrolysis of hydrolyzable bonds i n the lignin polymer. • Polycondensation of some hgnin molecules with others. • Diffusion of soluble lignin sulfonates from the wood tissue into solution. McCarthy and his coworkers use light scattering, diffusion, and fractionation to obtain their test results. Another conclusion suggested b y McCarthy and his groups from their test results and those previously obtained by other researchers is that lignins are branched-chain polymers existing in woody tissue primarily as large molecules. These molecules are probably combined with each other and/or with carbohydrates to make three-dimensional networks. The low intrinsic viscosity of the hgnin sulfonates (as determined by Staudinger and other hgnin researchers) coupled with the comparatively high molecular weights of the sulfonates suggests this branched chain hnkage. • Fractionation of Lignin Sulfonates. At the Pulp and Paper Research Institute of Canada, S. G. Mason and J. L . Gardon separated h'gnin sulfonates into eight fractions of osmotic molecular weights by ultrafiltration and fractional dialysis. The lowest fraction had a
RESEARCH molecular weight of 350CX, t h e highest 58,000, and 3 5 % of the material h a d a molecular weight betvcreen 10 and 25,000. Also, a distribution curve indicated sulfonates with* molecular weights as high as 1QQ,OQO. Although Mason and his coworker msed different tests than McCarthy's group at the University of Washington, they obtained similar distribution ourves. Since the average molecular weight o£ ligniii has been determined to b e about 1000, Mason attributes the comparatively high molecular weights of t h e lignin sulfonates to polymerization of lignin when forming lignin sulfonate. Results of viscosity, conductivity, a n d dyestuff measurements show that lignin sulfonates are flexible polyelectrolytes, according to Mason. I n aqueous solution t h e molecules a r e coiled a t sufficiently high lignin sulfonate concentration a n d in t h e presence of a suitable electrolyte. Also, indications are t h a t the lowest molecular weight fractions associate in solution, and t h e molecules of the highest molecular weight fraction are more branched than those of other sulfonate fractions. Mason says that the adhesive a n d dispersing properties of lignin sulfonates c a n probably be explained if they are considered flexible poly electrolytes. T h e adsorption of amy polymer on solid surfaces is greatly favored if the polymer is flexible because t h e number of anchorage points per molecule is increased. This is a functional property of adhesives a n d dispersants—their ability to b e adsorbed on solid surfaces.
sulfinic acids and t h e products that can be m a d e from them. Working a t the University of Maine, Irwin B. Douglass and Donald R. Poole prepared the organosulfur trichlorides by dissolving an organic disulfide or mercaptan in methylene chloride, cooling to —30° C , a n d passing an anhydrous chlorine until a white, solid organosulfur trichloride precipitated. Then, upon addition to the reaction mixture of equimolecular amounts of water, methanol, or acetic acid, warming to about —15° C , and allowing time for evolution of HC1 to cease, a clear solution resulted from which the sulfinyl chloride was fractionally distilled. With water, methanol, and acetic acid, t h e reaction proceeds: RSC1 3 + H 2 0 - » RSOC1 + 2 HC1 RSCI3 + C H 3 O H - » RSOC1 + CH 3 C1 + HC1 RSC1 3 +
CH3COOH -> RSOC1 + CH3COCI + HC1
• Yields Almost Q u a n t i t a t i v e . Until results of this work, reported at the Division of Organic Chemistry, only one lower alkanesulfinyl chloride h a d been studied. This was ethanesulfinyl chloride. Now, using methyl-, ethyl-, n-propyl-, isopropyl-, a n d n-butylsulfur trichlorides, the corresponding sulfinyl chlorides have been prepared in excellent yield. Phenylsulfur trichloride was found by Douglass and Poole to react with water and butyl alcohol to form a product showing the expected characteristics of benzenesulfinyl chloride. In themselves, the organosulfur triNew Route to Sulftnic Acids chlorides are unstable at room temperatures and have so far been investigated Organosulfur t r i c h l o r i d e s only as a stepping stone toward other eliminate tedious steps in products. The lower alkane sulfinyl chlorides prepared from them are preparation of sulfinyl faindy yellow, fuming liquids, readily chlorides purified by distillation a t reduced pressures. They do, however, show eviA previously dence of change on standing several little known class months, say the researchers. In addiof compounds— tion to their usefulness in preparation the organosulfur of the sulfinic acids (previously comt r i c h l o r i d e s , monly prepared b y reaction of Grignard ____ ESCI3—are n o w reagents with S O s or reduction of sulcutting a path to fonyl chlorides), the sulfinyl chlorides a better understanding of t h e chemis- should also prove interesting because they contain sulfur in an intermediate try of the sulfinyl chlorides. T h e new compounds, economically oxidation state, suggesting many reacprepared from either organic disulfides tions involving disproportionation. or mercaptans, obviate the heretofore Because the n e w preparative method tedious steps involved in preparation of starts with cheap materials and is so the sulfinyl chlorides w h i c h can b e simple in t h e steps involved, says Dougreadily hydrolyzed to tfcie correspond- lass, it should m a k e t h e sulfinyl chloring snlfinie acids. Thixs t h e organo- ic JS readily available a n d should stimusulfur trichlorides offer a simple a n d late much interest in their chemical economical route to the preparation of reactions.
Low Temperature Tars
"In says K. W. Wainwright of U . S. Bureau, of Mines. This revival, he indicates, can b e traced to successful application of fluidized-bed techniques. Today, fluidized carbonization offers a new market for coal, mainly in connection w i t h a n integrated electric power-tar industry—in. which carbonized char is used for toiler fuel. But in t h e future, low-temperature carbonization may also provide char for other purposes: • Smokeless fuel for domestic consumption. • Production of synthesis gas. • F o r blending with high volatile coals to produce metallurgical coke. "Regardless of how t h e char might be used, t h e most important economic factor is successful utilization of byproduct tar," Wainwright told the Division of Gas and Fuel Chemistry. • Three-Point Program. Now the Bureau of Mines has a research program u n d e r way at i t s new Low Temperature T a r Laboratory a t Morgantown, W . Va. The objectives, Wainwright says, are threefold: • To investigate yields as well as chemical a n d physical properties of tars produced from low-temperature carbonization processes. • T o investigate means of economically upgrading these tars into chemical, rather than fuel products. • Provide industry and the public with data essential for increasing coal utilization in a new chemical industry based on these tars.
Exhausting Lead In city-type driving automobile l e a d exhaust is only a fraction o f that present in t h e gasoline consumed
"Under on its surfaces," says D . A. Hirschler of Ethyl's research laboratories. "And a t low speeds the OCT.
I, 1956 C&EN
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