Major Reaction Mechanisms

Under the prevailing acidic conditions, the oxygen of the a-ether or a-hydroxy

group is protonated. Subsequent release of the a-substituent (as water or as alcohol/

phenol), which is the rate-determining step, leaves behind a resonance-stabilized

benzylium cation. This intermediate immediately adds hydrogen sulfite by

nucleophilic addition. The electron density distribution of the benzylium cation is

shown in Scheme 3 (left), where areas of high electron density are marked in red,

and centers with a low electron density are marked blue. From theoretical calculations,

as well as from model reactions [6], the benzylium cation (3a) is favored

over the methylene quinone resonance form (3b). The latter resonance structure

can only come into play if the a-proton and the a-substituent are fully arranged in

the aromatic plane, which requires bond rotation around the benzylic carbon–carbon

bond. Rotation out of this plane breaks the resonance. This bond rotation

requires additional energy and time, and might be disfavored by steric factors

imposed by the surrounding lignin scaffold. All of these factors favor 3a over 3b.

A further stabilization of the intermediate is achieved by a 2-aryl substituent or by

a hydroxyl in para -position, the latter is favoring the formation of the oxonium

type resonance form (3b) [11].

Other nucleophilesmay also add to the benzyliumcation and competewith the sulfonation

reaction [5]. Such nucleophiles can either be ligninmoieties [6], carbohydrate

compounds, or extractives. The stereochemical outcome of the sulfonation reaction

was found to be consistent with a unimolecular SN1mechanism: the pure erythro and

threo forms of lignin-model compounds (e.g., b-O-4 ether models) always yielded a

mixture of the threo and erythro forms. The observed erosion of the stereochemistry

strongly supports the intermediacy of the carbonium ion – and hence the SN1

mechanism – and dismisses an SN2 mechanism with Walden inversion.

In the following, some examples on the reactions of different lignin units and

their conversion under acid sulfite conditions will be given.

Phenolic and non-phenolic b-O-4-lignin model compounds react exclusively by sulfonation

in the a-position; sulfonation of the c-carbon is not a relevant process [6]. No

free phenolic groups are required for reactivity. In alkaline pulping systems, a major

differencewas seen between phenolic and nonphenolic lignin substructures: the phenolic

groups were hereby always more reactive as compared to the nonphenolics.

This difference is practically absent under acidic sulfite conditions.

408 4 Chemical Pulping Processes

O

R

OMe

H

O R

H

+

O

R

OMe

H

O R

H

O

R

OMe

H

O

R

OMe

H

O

R

OMe

H

SO3H

HSO3

-

+

-ROH

+

+

R = Alkyl; Aryl; H

+

1 2

a 3 b 4

Scheme 4.32 Formation of the benzylium cation as the reactive

intermediate in acid sulfite cooking.

Scheme 4.33 Electron density distribution (left) and lowest

unoccupied molecule orbital (LUMO)-distribution (right) of

the benzylium cation intermediate (3).

The b-O-4-ether bond is rather stable under acidic conditions lacking strong

nucleophiles. Hence, no cleavage of the lignin macromolecule is accomplished at

this point, except for a-substituents (6–8% of all lignin links), although a-aryl-LCC

model compounds showed a high stability also in acid sulfite systems [7], as mentioned

earlier. A sulfidolytic cleavage of the b-O-4-ether bond can only be accomplished

at higher pH than acid sulfite conditions (e.g., neutral sulfite pulping [8–

10]), when stronger nucleophiles are present.

4.3 Sulfite Chemical Pulping 409

OR

OMe

O

HO

OMe

HO3S

OR

OMe

O

HO

OMe

OR

MeO

O

OH

OMe

OR OR

OMe

O

HO

OMe

OR

OMe

O

HO

OMe

RO

H

+

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