Comparison to Sulfonation Reactions under Conditions of Neutral Sulfite Pulping

Sulfite and bisulfite ions are both strong nucleophiles, which are able to bring

about the cleavage of ether bonds. Hence, with increasing pH values the b-O-4-

ether groups become less stable and undergo a sulfitolytic cleavage. However,

under neutral conditions only phenolic structures are reactive so that the sulfonation

is more selective, proceeding moreover at a high rate. This leads to a much

lower degree of sulfonation and thus a lower rate of delignification (roughly 20%

of the lignin units react) [6]. Model reactions show the sulfonation to occur also at

other positions than the a-carbon atom (e.g., the c-C) [13]as well as the existence

412 4 Chemical Pulping Processes

of two sulfonic acid groups per phenylpropane unit [5,14], which are present in

different lignosulfonate fractions [13].

At higher pH values and long reaction times, phenolic b-O-4-ether groups can

be converted to styrene-a-sulfonic acids (Scheme 4.41).

OH

OMe

O

HO

OMe

HO

O

OMe

O

HO

OMe

OH

OMe

HO

O3S

-

HSO3

-

OH

OMe

O

HO

OMe

O3S

5 24 24 26

-

Scheme 4.41 Reactions of b-O-4 ether structures during neutral sulfite pulping.

The final products obtained upon sulfonation are often similar to the sulfonated

lignin fragments produced under acidic conditions (cf. b-O-4-units), but the

mechanism of their formation is quite different. In analogy to the lignin reaction,

under alkaline kraft conditions the reactive intermediate in neutral and alkaline

sulfite reactions is the quinone methide in contrast to the carbonium ion (benzylium

ion), which prevails under acidic conditions. The sulfite or bisulfite ions attacks

the quinone methide at the Ca as depicted in Scheme 4.41.

Phenolic a-ether bonds are most completely cleaved, but the nonphenolic a-aryl

ether units are stable, which supports the quinone methide being the reactive

intermediate. Phenolic phenylcoumarans yield the corresponding a-sulfonic acids

(Scheme 4.42), whereas the nonphenolic phenylcoumarans are again mostly

stable. Nonphenolic pinoresinol units are cleaved at the respective a-carbons and

sulfonated.

CH2OH

O OH

OH

OMe

MeO

CH2OH

R

OH

OMe

MeO

OH

CH2OH

R

OH

OMe

MeO