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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
OH SO3H
R = H, CH2OH R = H, CH2SO3
-
11 27 28
Scheme 4.42 Reaction of phenolic phenylcoumarans under
neutral sulfite conditions.
4.3 Sulfite Chemical Pulping 413
Eliminated formaldehyde results in the formation of hydroxymethanesulfonic
acid. At neutral pH, methoxyl groups are also removed by mechanisms (SN2) similar
to the demethylation in kraft pulping, but with formation of the corresponding
methylsulfonic acid. The rate of delignification generally decreases with increasing
pH.
HC
HC
H C 2
O CH
O CH2
CH
OH
OH
OMe
OMe
CH
C
R CH
R1
C
OH
OH
OMe
OMe
CH
HC
R HC
R1
CH
OH
OH
OMe
OMe
SO3
O3S
9 29 30
R = R1=H
R=H, R1=CH2OH
R=R1=CH2OH
-
-
R = R1=H
R=H, R1=CH2SO3
-
R=R1=CH2SO3
-
Scheme 4.43 Reaction of phenolic pinoresinol structures
under neutral sulfite conditions.
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