Lignin aromatic units

FIGURE 12.8 Reaction of lignin aromatic units with peracetic acid after Lai and Sarkanen. (After Lai, Y-Z. and Sarkanen, K.V., Tappi, 51, 449-453, ... 

Figure 3.3 Partial structure (16 aromatic units) of a spruce lignin fragment.

Solubilized lignin solutions are easily oxidized and the presence of the aromatic units containing electron-withdrawing ether and alcohol moieties makes it available for electrophilic substitution reactions, such as nitration, halogenation, hydroxylation, etc. 

Lignin is a noncellulosic resinous component of wood. It is the second most abundant renewable natural resource. It has alcohol and ether units with many aromatic units. Much of lignin is sheetlike in structure. 

Additionally, formaldehyde may condense with two aromatic units to form another diphenylmethane-type condensed structure (59). These condensation reactions have been well established in lignin model studies (32,300,301]. 

Sodium periodate specifically oxidizes guaiacyl groups to quinones. Hydrogen and sodium peroxides in alkali are also somewhat selective in oxidizing lignin and destroy chromophoric groups such as quinones and carbonyl functions while also degrading only aromatic units with free phenolic hydroxyls to dibasic acids. 

Figure 2.1 is probably an oversimplified caricature of the complex lignin structure due to structural refinements of the outlined basic patterns and to the occurrence of other coupling modes between aromatic units, such as the p-6 one. There is a universal consensus for the predominance of the labile p-0-4 ether linkages (structure A, Figure 2.1) in native lignins, as well as the occurrence of the more resistant p-5,... 

FD Chan, KL Nguyen, AFA Wallis. Estimation of the aromatic units in lignin by nucleus exchange A reassessment of the method. J Wood Chem Technol 15 473 91, 1995. 

FIGURE 11.3 General reaction scheme for the oxidation of an aromatic unit in lignin and its alkaline hydrolysis. 

FIGURE 11.8 Initial reactions between an aromatic unit in lignin and ozone. In addition to oxidative cleavage of the aromatic ring (ozonolysis), a direct formation of radical intermediates takes place. 

Per 100 "Aromatic rings." As each lignin monomeric unit possesses an aromatic ring, these values are equivalent to mol percentage. Shows maximal and minimal value calculated by subtraction of the amount of benzyl ethers in a "sugar-free" preparation (PMWLp) [21]- Semiquantitative estimation nd, not detected. 

Per 100 "Aromatic rings." As each lignin monomeric unit possesses an aromatic ring, these values are equivalent to mol percentage. 

Sulfur lignin is a totally amorphous material Amorphous structure, particle sizes are between 1000-40000 nm Sulfur content in samples from different experiments varies between 10-26 mass-% Stable sulfur linkages between the aromatic units are formed... 

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. 

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