Condensation

Condensations always compete with the sulfonation process, and counteract the

delignification by formation of new, stable carbon–carbon bonds.

Increasing acidity, for example, towards the end of cook, favors condensation

reactions between the benzylium cation (3a) and other weakly nucleophilic lignin

positions, which are present due to resonance at position C1 (Scheme 4.44) and

C6 (Scheme 4.45). Condensation decreases with increasing concentration of bisulfite

ions (bound sulfur dioxide, cf. also the Kaufmann diagram; Fig. 4.155). The

resulting stable carbon–carbon bonds cause an increased molecular weight and a

lower hydrophilicity, and therefore work against the delignification process. However,

the introduction of sulfonic acid groups considerably increases the solubility,

which can often compensate for the increase in molecular weight by the formation

of new carbon–carbon bonds [15].

Intramolecular condensations have been described for phenylcoumaran and

pinoresinol structures (cf. Scheme 4.35) [6]. Due to the additional methoxyl group

in the 5-position, lignins of hardwoods generally have a lower tendency for condensation

reactions as compared to softwoods. Also, the sulfonation reaction is

somewhat slower for hardwood than for softwood lignins. Methoxyl groups are

not cleaved under acid sulfite conditions to a large extent due to the too low

nucleophilicity of the cooking chemicals, whereas under neutral sulfite cooking

conditions a cleavage of methoxyl groups is observed.

414 4 Chemical Pulping Processes

O

OMe

OAr

HO

HO

O

OMe

OAr

HO

O

OMe

OAr

HO OH

MeO

H

+

-

O

OAr

OH

-

+

+

+

A 32

Scheme 4.44 Condensation of the reactive benzylium ion

with weakly nucleophilic resonance form at C1.

Lig

O

OMe O

OMe

Lig

O

O

OMe

MeO

H

+

31 33

-

+

+

Lig = Lignin moieties

-

Scheme 4.45 Condensation of the reactive benzylium ion

with weakly nucleophilic resonance form at C6.