Lignin ether groups

Lignin contains phenolic and a few carboxylic acid groups it therefore dissolves in sodium hydroxide. Soda pulping is based on this fact. Here, at the high temperatures used, ester bonds arc hydrolyzed, and some of the carbohydrate-lignin ether bonds are split. Cold caustic soda is used for semichemical processes. 

Since lignin is not a uniform entity, chemical criteria for its characterization have centred around analytical detenninations of its functional groups, e.g. total hydroxyl content, phenolic hydroxyls (56), methoxyl and other ether groups, benzyl alcohol groups (7a), carbonyl groups (6), etc., and estimations of its content of special structural features, e.g. phenylcoumaran units (5), biphenylyl linkages (123), etc. 

A typical lignin C6C3 unit see reaction scheme below) contains a side chain at position 1, a methoxyl at position 3, and a phenolic hydroxyl or phenol ether group at position 4. Another methoxyl group or a condensed... 

Neutral and Alkaline Sulfite Pulping In neutral sulfite pulping the most important reactions of lignin are restricted to phenolic lignin units only. The first stage always proceeds via the formation of a quinone methide with simultaneous cleavage of an a-hydroxyl or an a-ether group (Fig. 7-12). At least in noncyclic structures, the quinone methide is readily attacked by a... 

Figure 9.17. Summary of lignin structure. This natural thermoset plastic has a very irregular structure with various ether groups acting as crosslinks in many cases. (Kollman and Cote, 1968, p. 69 Adler, 1961).

The above considerations are not limited to concepts of chemical modification because even when lignins are considered as additives in the preparation of polymer blends or composites, their hydroxyl functions represent a key structural element in terms of polar contributions and sources of hydrogen bonds which will affect the quality of the interfacial interactions of the ensuing materials, just as the less-polar (ether groups, aromatic rings, etc.) and non-polar (aliphatic sequences) moieties will, in terms of hydrophobic interactions. 

The use of methionine doubly labeled in the methyl group was subsequently extended to experiments with higher plants by Byerrum, Ball, and their co-workers. It was shown that the methyl group of methionine was a precursor of the N-methyl of nicotine in tobacco (Dewey et al., 1954), the methyl ether groups of lignin in barley and tobacco (Byerrum et al., 1954), and the methyl esters of pectinic acid in radish (Sato et ah, 1957). In none of these methylations was the deuterium ratio in... 

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