Lignin biphenyl structure

Our studies were concerned with the reactions occurring during mild oxidation. This paper deals with model experiments on one of the possible reaction sequences—namely, the formation of biphenyl structures and oxidative degradation. Numerous oxidation experiments with suitable lignin models were performed, and during these studies we determined the rate of oxygen consumption by the compounds dissolved in 0.27V NaOH at 70°C. A detailed description of our method is given under Experimental. 

Carbon-0-carbon coupling might, of course, also be considered in the case of lignin oxidation. Because there is no accurate method for determining biphenyl structures in lignins, the extent of such couplings could not be estimated. 

The isolation of 2,2 -dihydroxy-3,3 -dimethoxy-5,5 -dialkylbiphenyls from both neutral (IS) and alkaline hydrogenation supports Pew s proposal that the biphenyl structure is of considerable importance in lignin... 

Biphenyl structures and a-carbonyl-p-aryl ether structures, which are both assumed to be present in native lignin with a higher abundance than coniferyl alcohol structures (22,23) and both considered to be important leucochromophores, were not observed among die products, presumably because they are not present in spruce lignin as end groups. Both these types of structures are very stable and unlikely to be structurally changed during mild acid hydrolysis (24). 

Remko, M. 1985. MO investigations on lignin model compounds. XIX. CNDO/S study of electronic spectra of biphenyl structure. Cell. Chem. Technol. 19(4) 423 29. 

Remko, M., and I. Sekerka. 1983. MO investigations on lignin model compounds. XVI. Conformational analysis of P-aryl ether, benzyl aryl ether, phenyl coumaran, diphenyl ether and biphenyl structures. Z. Phys. Chem. (Weisbaden), 134 135-148. 

For lignin substructures containing a carbon substituent in the aromatic C-5 position, or for stilbenes originating from phenylcoumaran structures, the oxidative degradation leads to the formation of isohemipinic acid methyl ester (4) (Fig. 6.3.2, R=CH3). However, Gellerstedt and co-workers have shown that model compounds of the biphenyl type also give rise to isohemipinic acid in addition to the expected bis-vanillic acid structure (7). The reason seems to be incomplete alkylation (pH 11, 24 h) with the result that the substrate is alkylated at only one of the available positions. To achieve a more complete alkylation, it is necessary to increase the pH to 13 or alternatively, to extend the alkylation time to 84 h. In both cases, the yield of the bis-vanillic acid structure is substantially increased as shown in Table 6.3.1 nevertheless, the concomitant formation of isohemipinic acid cannot be completely avoided. 

Specific Monomers Released by Thioacidolysis of Various Lignin and Non Lignin Substructures. Monomers can be Recovered if the Aromatic Ring of the Parent Structure is not Involved in C-C or Biphenyl Ether Bond... 

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