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Electron-transfer Reaction
In the alkaline cooking liquor, reducing compounds such as carbohydrate moieties,
or pulping aids such as AHQ (Anthrahydroquinone), are also present.
Hence, the quinone methide can be reduced by different compounds, as was demonstrated
by model compound studies (Scheme 4.9). The process is postulated to
proceed according to a single-electron transfer mechanism involving radical intermediates
(12) [23]. The structural analysis of residual lignin in pulp also supports
the occurrence of such mechanisms to a minor extent [24].
O
OAr
OMe
OR
O
OAr
OMe
OR
O
OMe
OR
O
OAr
OMe
OR
.
+e
-
-e
- -ArOH
3 12 13 14
Scheme 4.9 Reduction of the quinone methide, and formation of coniferyl-type structures.
Nonphenolic Units
Non-phenolic units are more difficult to cleave than units with a free phenolic hydroxyl
group. To a small extent, fragmentation proceeds via oxirane intermediates
(13) formed by a neighboring group-assisted mechanism (Scheme 4.10).
170 4 Chemical Pulping Processes
OR
OMe
The cleavage of nonphenolic lignin units requires more drastic conditions (temperature,
alkalinity), and the reaction is consequently assigned to the bulk delignification
phase of a kraft cook.
CH2OH
OAr
OMe
O
OMe
CH2OH
OMe
O
OMe
CH2OH
OMe
HO
O
OMe
HO
-
Or HS
-
CH2OH
OMe
HO
O
OMe
O
HO
OH
HO OMe
O
HO
OH
HO OMe
OH
(HS)
A
B
Scheme 4.10 Intramolecular formation of epoxides and
nucleophilic opening of the oxirane, either by hydroxyl or
hydrosulfide ions (path A) or by carbohydrate hydroxyl groups
and subsequent formation of stable LCC. (path B)
Starting from epoxide 16, hydroxyl groups of carbohydrate moieties can also
attack C-a, resulting in opening of the oxirane ring and formation of lignin-carbohydrate
ether bonds. These linkages are stable to some extent under the conditions
of bulk delignification, and are thus considered to be one of the reasons for
alkali resistant inter-unit linkages and incomplete removal of lignin [25]. Example
reactions, carried out with model compounds, also underline the necessity of high
alkalinity towards the end of a kraft cook to prevent the increasing formation of
stable lignin–carbohydrate-complexes (LCC) (Scheme 4.10, Path B).
4.2 Kraft Pulping Processes 171
For a-keto lignin units (frequency of 19 in MWL of spruce approx. 0.2 per C9
unit [26,27]), a sulfidolytic cleavage as depicted in Scheme 4.11 is also possible.
Nonphenolic a-ether structures are stable in all phases of the kraft cook [128].
CH2OH
OAr
OMe
OMe
O
CH2
OAr
OMe
OMe
O
-HS
-
+HS
-
OMe
O
OMe
S
OMe
O
OAr
OMe
S
-H2O
20 22
-OAr
Scheme 4.11 Sulfidolytic cleavage of the b-O-4 ether at a-carbonyl structures.
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Reaction Path B | | | Residual Lignin Structure (see Section 4.2.5) |