Nitration of lignin

The earliest expts devoted to the nitration of lignin, previously isolated from woodpulp by means of HC1, were reported by Hagglund (Ref 3). Lignin mixed with fuming nitric acid... 

The earliest experiments devoted to the nitration of lignin, previously isolated... 

Attention is drawn to the monograph by Brauns and Brauns (42j.. Althougli published in 1960 it contains a description of the main work on nitration of lignin. 

The nitration techniques used in the measurement of the DP of a-cellulose merit further discussion, particularly in relation to the effect of time of nitration on the cellulosic constituents of the fiber. Nitration of lignin results in products soluble either in the nitrating acids or in methanol, and, by a suitable extraction procedure, the lignin component of the fiber can be completely removed. 

The generation of nitrogen containing radicals is connected with oxidative electron-transfer reactions initiated by dimers of NO in phenol units and nonphenolic structures. The subsequent radical conversions include processes of degradation and modification (i.e., nitrosation and nitration) of lignin. 

The aromatic nature of lignin contrasts with the aliphatic stmcture of the carbohydrates and permits the selective use of electrophilic substitution reactions, eg, chlorination, sulfonation, or nitration. A portion of the phenoUc hydroxyl units, which are estimated to comprise 30 wt % of softwood lignin, are unsubstituted. In alkaline systems the ionized hydroxyl group is highly susceptible to oxidative reactions. 

The inclusion of nitrate may lead to various complications, which have been discussed in Chapter 2. In addition, the nitrogen status of the growth medium determines the levels of lignin peroxidases and manganese-dependent peroxidases that are synthesized in Phanerochaete chrysosporium. The role of Mn concentration is noted later and in Chapter 3, Part 5. 

In order to separate lignin from carbohydrates, stronger reagents were used in the expts of Friese et al ( Refs 9 12) for the nitration of wood pulp, namely normal mixed acids. In this way nitrolignin with a higher N content was obtained. When nitric-sulfuric acids (1 2 ratio) were used, a prod contg 8.45% N was obtained with a yield of 55%... 

This treatment does not remove the lignin. The fibers are dark in color and very stiff. They are not used in paper manuf nor for the nitration of NC, but can be pressed into a product of extreme durability hardness, known as "Masonite Presswood ... 

An appreciable part of the lignin molecule is aromatic in character. Hence it would be expected to take part readily in the reaction of nitration. It has been found, however, that the capacity of lignin to undergo oxidation predominates especially in the presence of dilute nitric acid. This can be explained by the fact that the aromatic part of the lignin molecule derives from pyrocatechol. The reaction of lignin with nitric acid forms the basis of a method of separating cellulose from wood pulp, that consists in treating this mass with dilute nitric acid (3-10%). 

A property of the nitration product of lignin which attracted attention was its failure to form detectable amino groups on reduction. The product of reduction... 

Other processes in which the aromatic ring is modified via electrophilic substitution reactions are nitration and ozonation. The electrophilic substitution of nitro groups on the aromatic moiety of lignin and accompanying reactions have been reviewed by Dence (1971). A nitration process consisting of the pretreatment of chemical pulp with nitrogen dioxide followed by treatment... 

Nitrates of various carbohydrates Polyvinyl nitrate Nitro-derivatives of lignin References... 

After removal of lignin, the nitrated cellulosic products can be separated into three fractions, which are designated A, B, and C, of which Fraction A is insoluble in acetone Fraction B is soluble in acetone, but insoluble in water and Fraction C is soluble in both acetone and water. Analysis shows the acetone-soluble fractions B and C to consist of nitrated a-cellulose of DP about 4450, and nitrated hemicellulose, respectively. Both these products are also found in the acetone-insoluble fraction, A. 

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