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Repeating Units in Spruce Lignin
KAJ FORSS
Downloaded by TUFTS UNIV on September 29, 2014 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0059.ch003
The Finnish Pulp and Paper Research Institute, Helsinki, Finland
The
prevailing opinion today is that lignin is a statistical polymer for which no structural formula in the true sense can be presented but only a structural scheme of the type proposed by Adler and Freudenberg. Careful fractionation of the lignosulfonates and other compounds that are dissolved in the liquor in a sulfite cook shows, however, that the lignosulfonic acids may not be derived from a statistical polymer but from an ordered polymer composed of repeating units, and that sprucewood contains appreciable amounts of aromatic compounds which are related to true lignin as hemicellulose is to cellulose. We believe that it is necessary in lignin studies to be sure that the lignin preparation studied does not contain any of these hemilignin components. We have studied the properties of spruce lignin by first dissolving the lignin and hemilignin from the wood by sulfite cooking liquors of varying composition under different experimental conditions and then fractionating the formed lignosulfonates by gel filtration. From the fractions resulting on gel filtration through a column of Sephadex G-25, 25 ft. long, we have isolated a series of nine low molecular weight lignosulfonates that form a homologous series. We have concluded from analytical data for these lignosulfonates that spruce lignin is composed of repeating units, each consisting of 16 guaiacylpropane units and two />-hydroxyphenylpropane units. The repeating unit has four carbon atoms which can be sulfonated without previous hydrolysis. Sulfonating these carbon atoms leads to the formation of undissolved lignosulfonic acids in the wood. The repeating units are bound to each other by ether bonds that are partially broken by hydrolysis during the cook. They are further bound to carbohydrates by four ether bonds that are hydrolyzed and at least partially sulfonated during the cook. The smallest complete lignosulfonic acid is composed of only one repeating unit. Further hydrolysis and sulfonation of this acid lead to the successive liberation of eight guaiacylpropane units and hence to the isolated series of homologous lignosulfonates, each of which comprises one incomplete repeating unit composed of 15 to eight guaiacylpropane units and two ^>-hydroxyphenylpropane units. 36 In Lignin Structure and Reactions; Marton, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
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FORSS
Repeating Units
37
The theoretical composition deduced for spruce lignin was confirmed by analytical data for several low sulfonated, high molecular weight lignosulfonate fractions.
The results of these studies are presented in detail in
Downloaded by TUFTS UNIV on September 29, 2014 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0059.ch003
Paperi Puu 47, 443 (1965).
In Lignin Structure and Reactions; Marton, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
