Condensation reactions during sulfite pulping

Condensation reactions of lignin have been mentioned 22) in connection with sulfite pulping as being responsible for color formation. However, most of the likely condensation reactions occurring during a kraft cook do not lead directly to chromophore formation. One potential condensation reaction which would give a chromophore is the formation of a chalcone (XIX) from vanillin and acetoguaiacone, both of which have been isolated by Enkvist 12) from black liquor. 

Fig. 7-9. Examples of lignin condensation products formed during acid sulfite pulping (Gierer, 1970). Condensation results from the reaction of a carbonium ion with the weakly nucleophilic sites in the benzene nucleus.

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