Studies on the Color of Lignin Part.X

Effects of Chromophoric Groups on the Color of Various Lignins

Abstract

The effects of alcoholic hydroxyl group, phenolic hydroxyl group, carboxyl group and carbonyl group on the color of softwood thiolignin were discussed in the previous paper.The objects of this paper are to discuss the effects of quinoid structure on the lignin color in addition to the above functional groups, and further the color difference among thiolignin, Iignosulfonate and Björkman lignin.
The results are summarized as follows :
(1) The electronic absorption spectra of softwood thoiolignin and lignosulfonate have the clear-cut peaks around 6, 000cm^-1 (1, 667mμ).This peak seems to be the first overtone of the hydroxyl stretching vibration.
(2) The Δε_i and Δε_r spectra of lignin reduced with sodium hydrosulfite are similar to those of original lignin. Quinoid structure is reduced with sodium hydrosulfite, but carbonyl group is not. The brightening of lignin with sodium hydrosulfite is due to the reduction of quinoid structures.
(3) When the o-quinone structure in lignin is formed by the Fremy's salt oxidation, a shoulder is observed around 475 mμ and the color of lignin becomes dark. The difference spectrum between original and oxidized lignins has a clear-cut peak, at 475 mμ. This shows that the electronic absorption spectrum of o-quinone structure formed in lignin has a peak at 475mμ.
(4) The amounts of o-quinone structure in softwood thiolignin, hardwood thiolignin, softwood lignosulfonate and softwood Björkman lignin are estimated to be 0.030.04, 0.0450.06, 0.010.013 and 0.00150.002 per C6-C3, respectively. These values are in excellent agreement with the results which were estimated by the spectrum analysis.
(5) Softwood and hardwood thiolignins, softwood and hardwood lignosulfonates and softwood Bjorkman lignin in the visible region decrease the molar extinction coefficients by 45, 55, 78, 80 and 100% with sodium borohydride, respectively, and by 40, 47, 72, 75 and 100% with sodium hydrosulfite, respectively.These results show that most of the color of softwood Bjorkman lignin is due to the quinoid structure.
(6) By the diazomethane methylation of lignin, the molar extinction coefficient decreases in the visible region, and a peak around 280mμ is shifted to the shorter wavelength. In order to explain this result, the secular equations were set up for the simple lignin model compounds, guaiacol and veretrol, on the basis of molecular orbital theory (Hückel MO), and then the eigen values were calculated.In has become apparent that the electronic transition of methylated lignin is in a state of higher excite energy than that of original lignin.This explains the brightening by the methylation of lignin.