Lignin 3-keto groups

The reaction of the Scheme 8.23 results in the formation of a substitnted phenolate ion and a radical. The radical is further reduced by the next AQ (Dimmel et al. 1985). The keto groups in lignin can be involved in the analogous reaction with AQ. This leads to formation of the ketyl fnnctions which originate analogous fragmentation processes. 

Infrared spectra of isolated lignins exhibit an absorption band in the region 1700-1720 cm.-1 While doubt still exists as to precisely which groups in dioxane or milled wood lignins are responsible for this absorption, clearly 16) there is at least some contribution from nonconjugated keto groups in the 0-position of the phenylpropane side chain. 

Of course one cannot neglect the possibility that the keto group may arise, not from a bond between lignin and carbohydrate but from the hydrolysis of internal bonds in the lignin 21) y and we intend to pursue this point in later studies. However, since there seems to be a relationship... 

Searching for evidence on the nature of the bond, we conducted a spectroscopic study of lignins isolated from wood by ethanolysis (P) and found, as expected, that 0-keto groups did not appear in the lignins until... 

Possibilities include (a) the hydrolysis of the lignin-carbohydrate bond by small amounts of water present in the wood despite all precautions to make the system anhydrous, (b) the dehydration of carbohydrates under these conditions to produce water and hence hydrolytic conditions, (c) transacetalization of the extracting acetal with carbohydrates to liberate alcohol and create conditions of alcoholysis, and (d) transacetalization of the extracting acetal with free or bound keto groups in the wood to liberate acetone and create conditions of acetonolysis. 

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