Lignin partial structure

In general, the aliphatic hydroxyl content of a lignin preparation is determined by subtraction of the phenolic hydroxyl content (see Chap. 7.2) from the total hydroxyl content of the lignin. The total hydroxyl content of lignin (partial structure H) can be determined by acetylation with acetic anhydride-pyridine reagent (10) followed by saponification of the acetate (partial structure 12) and titration of the resulting acetic acid (14) with a standard 0.05 N sodium hydroxide solution (Roth 1953). 

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

Fig. 3 Partial structure of lignin. After Killops and Killops (1993).

FIGURE 5 Partial structure of lignin, which is a polyphenol built up from units of phenylpropane derivatives (Fig. 4) by condensation and dehydrogenation within the plant. Condensation points, lignin precursors via ether, and C-C bonds are indicated. Modihed after Killops and Killops (1993). 

T ignin is one of the most abundant natural products constituting about one-fourth of the woody tissue in plants. Nature has chosen a unique synthetic technique to prepare this cross-linked polymeric material from coniferyl alcohol and related substances. The mechanism of lignin formation is not completely known yet, and the structural characterization of lignin has been only partially successful despite considerable research. 

Schultz TP, Glasser WG (1986) Quantitative structural analysis of lignin by diffuse reflectance Fourier transform infrared spectromety Holzforschung 40 (Suppl) 37-44 Schultz TP, Nicholas DD (1987) Fourier transform infrared spectrometry Detection of incipient brown rot decay in wood Int Analyst 41 35-39 Sjostrom M, Wold, S, Lindberg W, Persson J-A, Martens H (1983) A multivariate calibration problem in analytical chemistry solved by partial least-squares models in latent variables Anal Chim Acta 150 61-70... 

Another factor which may influence the yield of acids obtained from lignin in wood is the presence of easily hydrolyzable aryl ether linkages such as those represented by phenylpropane-a-aryl ether structures In phenolic units, such structures are easily cleaved even under very mild alkaline conditions (Adler et al 1968) Therefore, it can be assumed that, in the alkylation step of the oxidative degradation procedure, a partial hydrolysis of such structures may occur, thereby creating new phenolic end groups that may affect the analysis... 

The adhesive properties of lignin, its reactivity with formaldehyde, and its structural similarity with phenolic adhesives invited investigation of the applicability of lignin in adhesive resin systems. Therefore, during the past several years, numerous attempts have been made to replace the expensive petrochemical resins totally or partially with the renewable raw material lignin (i). 

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