Condensation reactions, lignins

The most likely lignin condensation reaction during the kraft cook is the formation of diphenylmethane structures 29, 30). These structures are also proposed 14) as being already present in native lignin. Phenolic diphenylmethanes do not exhibit any color, but they can easily be dehydrogenated to quinonemethides 20, 23) or quinonemethide radicals (P). 

Search for Lignin Condensation Reactions with Modern NMR Techniques... 

Lignin condensation reactions have been studied for many years, mainly because they possibly interfere with efficient delignification of wood. Numerous model compound studies established the various types of condensation reactions that may occur between lignin fragments (1-5) or between lignin and carbohydrates (1,2,6,7) during alkaline pulping. An important class of condensation reactions involves formaldehyde. 

Condensation reactions. The nature of phenyl units and reaction conditions has been shown to influence lignin condensation reaction [311-313]. Syringyl nuclei condensed more readily than guaiacyl nuclei with vanillyl alcohol [311]. Yasuda et al. [312] observed the formation of benzyl chloride on treatments of (3-0-4 dimers in hydrochloric acid this would reduce condensation at the benzyl position. They also observed an intramolecular condensation product of a phenylcoumaran type [313]. This intramolecular condensation was shown to be dominant in an 85% formic acid solution [314,315] and was insignificant in 50% aqueous ethanol containing 0.2 M HCl [306]. 

Lignin. On acidic treatments of lignin in aqueous media, lignin condensation reactions dominate and lead to the formation of acid-insoluble residues. This principle serves as the basis for quantitative determination of lignin content in plant materials [26]. Lignin condensation reactions, however, can be minimized by using mild conditions, organic solvents, or nucleophiles. 

Lignin condensation reactions include the formation of diphenylmethanes (57) and (52) and a-carbohydrate ether linkage (55) that may be derived from the quinonemethide (42) [32,277,336-340] or the epoxide (49) [342,343] intermediate. Coniferyl alcohol may be involved in the formation of p-7-linked condensed unit (54) [338,339]. 

Landucci, L. L. (1989) Search for lignin condensation reactions with modern NMR techniques. In. Adhesives from renewable resources, Amer. Chem. Soc., Washington, DC. 

Unionized hydrogen peroxide (HOOH) is a much weaker nucleophile than hydroperoxide ion (HOO ) but, from model compound studies [65], it may be able to react with benzyl carbonium ions in competition with lignin condensation reactions. 

Acid hydrolysis of the polysaccharide portion of wood will release lignin but also causes major condensation reactions in the product(2l). These reactions can be minimized by using 41 wt. percent hydrochloric acid in place of other mineral acids but some condensation reactions still occur(22). This is not an effective method by which to obtain unaltered lignin. On the other hand, lignin can be solvent extracted from wood at temperature of 175°C using solvent mixtures such as 50/50 by volume water/1,4-dioxacyclohexane(23) Changes in lignin under these conditions appear to be minor. 

Naturally lignins that have been modified by chemical reactions, e.g. condensed or sulfonated by pulping, are far less susceptible to biological decay than natural lignins, because the weak, readily hydrolyzable benzyl aryl ether bonds or the readily oxidisable benzyl alcohols of guaiacyl-glycerol units have been sulfonated or transformed by condensation reactions into strong carbon-carbon bonds. 

The acid condensation reaction of the aromatic and phenolic units is a typical reaction of lignin. The presence of acids results in resonance stabilized carbonium ion structures formed in the lignin macromolecule. These car-bonium ion structures react further, e.g., with unsubstituted positions in the lignin macromolecule. Thus, thermal treatment of powdered wood in acidic conditions causes condensation, the coniferyl aldehyde and coniferyl alcohol groups being especially reactive. In addition, other inter- and/or intramolecular condensations may occur. 

One of the most widely used condensation reactions is between lignin and phenol using formaldehyde as a coupling agent. This reaction depends... 

Condensation reactions of lignin have been mentioned 22) in connection with sulfite pulping as being responsible for color formation. However, most of the likely condensation reactions occurring during a kraft cook do not lead directly to chromophore formation. One potential condensation reaction which would give a chromophore is the formation of a chalcone (XIX) from vanillin and acetoguaiacone, both of which have been isolated by Enkvist 12) from black liquor. 

On the basis of our present concept of lignin structure, generally two types of condensation reaction are expected to occur A and B. 

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