Reaction Path C

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.