Uncondensed lignin

Thioacidolysis allows the distinction between products derived from lignin and products derived from />coumaric and ferulic acids, and the distinction between products derived from cinnamaldehydes and cinnamyl alcohols. Recent improvements have made it possible to estimate the fraction of free phenolic groups in uncondensed lignin (see Section 1.3.1), and to depolymerize the dimers, so that they can be included in the analysis of the lignin composition. 

The relative amounts of uncondensed guaiacyl, condensed guaiacyl, and syringyl nuclei in hydrol lignin preparations have been estimated at 43%, 23%, and 34% respectively for an acid-catalyzed product and 30%, 22%, and 48% for a base-catalyzed product. 

Uncondensed phenolic units in lignin have been estimated by a color reaction based on oxidation with potassium nitrosodisulfonate (Fremy s salt) (Adler and Lundquist 1961) as illustrated below ... 

To eliminate interfering side reactions caused by units having carbonyl and hydroxyl groups at the a-position, the lignin is reduced with borohydride and alkylated with methanolic hydrochloric acid prior to oxidation to convert these groups to o-quinones. Following a procedure based on the above reaction, spruce milled-wood lignin was found to contain 15-18% of uncondensed phenolic units (Adler and Lundquist 1961). 

Adler E, Lundquist K (1961) Estimation of uncondensed phenolic units in spruce lignin Acta Chem Scand 15 223-224... 

The extent to which lignin becomes a component of a polymer network may be constrained either by reactivity or by solubility. The contribution of lignins to phenolic resins is known to be limited by the number of available uncondensed... 

Abe I, Funaoka M, Kodama M (1987) Phenolic nuclei of condensed tannins. Approaches by the phenyl nucleus-exchange method. Mokuzai Gakkaishi 33 582-588 Adler E, Lundquist K (1961) Estimation of uncondensed phenolic units in spruce lignin. Acta Chem Scand 15 223-228... 

E Adler and K Lundquist. Estimation of Uncondensed Phenolic Units in Spruce Lignin. Acta Chem. Scand. 15 223-224, 1961. 

Smit et al. combined thioacidolysis and phosphitylation with reagent II, followed by NMR spectroscopy, to measure the total amount of condensed and uncondensed units in wood and lignin [41], Application of the method to Picea radiata wood and milled wood lignin showed that approximately 77% of the C9 units in the milled wood lignin and 71% of the C9 units in wood could be quantified. The amount of condensed structures/C9 units in MWL was determined to be as high as 8% P-5, 5% 4-0-5, and 16% 5-5 structures. 

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