Kraft lignin condensed positions

The kraft lignin is constituted mainly of guaiacylpropanoid units (2) of which about half are condensed, i.e., substituted at the 5-position (16). The number of aromatic sites reactive towards base catalyzed electrophilic substitution reactions, therefore, is fairly low at about 0.3 per Cg unit (Table II). 

It has been suggested that the major part of condensation processes occurs at the unoccupied C-5 position of phenolic units. Thus, in isolated MWL preparations about half of the C-5 positions are unsubstituted, while in isolated kraft lignins only about one third of these positions remain free. The syringyl units of hardwood lignins cannot, of course, undergo condensation reactions of this type. 

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