Chromophores, lignin

Complexes between quinones and phenols have also been proposed as important lignin chromophores in pulping liquors. Furman and Lonsky [75,76] attributed two-thirds of the light absorption of kraft lignin to quinone-phenol charge-transfer complexes. 

Reactions of Hydrogen Peroxide with Lignin Chromophores.444... 

Reactions of Peracetic Acid with Lignin Chromophores.451... 

FIGURE 12.13 Reaction of coniferaldehyde and its methyl ether with alkaline hydrogen peroxide or peracetate. (Redrawn from Pan, G.X., Spencer, L. and Leary, G. J., A comparative study on reactions of hydrogen peroxide and peracetic acid with lignin chromophores. Part 1. The reaction of coniferaldehyde model componnds. Hol forschung, 54, Berlin Walter de Gruyter, 2000, pp. 144-152, Scheme 2. With permission.)... 

GX Pan, L Spencer, and GJ Leary. A comparative study on reactions of hydrogen peroxide and peracetic acid with lignin chromophores. Part 2. The reaction of stilbene-type model compounds. Holzforschung 54 153-158, 2000. 

The above studies raise the question of the selectivity of the bleaching reaction toward other lignin chromophores and leads to the work on the bleaching chemistry of tert-butylamineborane complex with lignin model compounds (Figure 13.13) in aqueous solutions [73]. 

Formation and Elimination of Multiple Bond Functionalities. Reactions that involve the formation and elimination of multiple bond functional groups may significantly effect the color of residual lignin in bleached and unbleached pulps. The ethylenic and carbonyl groups conjugated with phenoHc or quinoid stmctures are possible components of chromophore or leucochromophore systems that contribute to the color of lignin. 

In lignin, there are attendant ketone (sometimes, qninone) chromophoric moieties that give it a yellow/brown color. Cellulose is a white substance, which provides the desired strength and brightness properties to the paper products. The strength and whiteness of a paper product depends on the extent of removal of lignin from the pnlp fibers. Row woods contain as mnch as 20-30% lignin. 

Determining chromophores and auxochromic groups in kraft lignin. 

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