Condensation with Phenols

Condensation with other phenolic compounds can occur in the sulfite cooking of

tannin-damaged sapwood, and with the heartwood of certain species. In the first

case, the phenolic compounds originate from the bark and have diffused into the

wood, mainly by wet storage conditions of unbarked wood.

Phenols predominantly originate from lignin fragments, extractives and reaction

products from the acid-catalyzed conversion of low molecular-weight carbohydrates.

In particular, furfural and hydroxymethylfurfural – which are formed

under acidic conditions from carbohydrates by intramolecular dehydration – are

very prone to intra- and intermolecular condensation reactions of many types,

leading also to polymeric products [56]. All intermediate compounds exhibit a pronounced

tendency to condense either with lignin fragments, or with themselves.

These condensation pathways may even contribute a larger share to the overall

condensation reactions in the final cooking stage as compared to the reactions

with sulfur compounds involved [57].

424 4 Chemical Pulping Processes

The condensation reactions of lignin-model compounds with phenols under

acidic conditions have been studied extensively by Kratzl and Oburger [58,59].

Condensation reactions occur after protonation of the benzylic hydroxyl group

and cleavage of water. The formed carbonium ion attacks the phenol as an electrophile,

leading to formation of stable C–C-bonds. The neighboring b-substituent is

removed by acid catalysis, and subsequent rearrangements finally also yield

another stable C–C-bond at C-b.