Phenolic Subunits

The following reactions form the basis of the degradation/

dissolution of phenolic lignin moieties during pulping under alkaline conditions:

_ Ionization of phenolic groups

_ Cleavage of a-aryl-ether bonds and the most abundant b-O-4-

ether links

_ Liberation of free phenolic groups

OH

OMe

The b- O –4 and a- O –4-ether links (cf. Scheme 4.2) taken together represent the

most abundant connections between lignin units (up to 65%) [1]. Hence, the behavior

of these moieties in the pulping process have been extensively analyzed in

model compound and lignin studies.

The key-intermediate is the para- quinone methide (3), which is formed from

the b-aryl ether structure upon ionization of the phenolic residue and elimination

of the aryl substituent in a-position by a vinylogous b-elimination [2]or by alkaliinduced

cleavage of the cyclic a-aryl ether bonds, for example cleavage of the phenylcoumaran-

type substructure via quinone methide. The reaction is reversible,

but addition of a nucleophile (e.g., HS–) in a subsequent reaction step leads to rearomatization

by nucleophilic addition of HS– to the C-a of the quinone methide,

which is the driving force. A common feature of all major pulping processes is

the reaction of nucleophiles (nucleophilicity increases in the order OH– < HS–

< SO32–) with electron-deficient centers at the lignin molecule, resulting in cleavage

of inter-lignin linkages and a higher hydrophilicity of the resulting lignin fragments

and thus a better dissolution in the pulping liquor.

4.2.4.1.2 Lignin: General Structure

Once the para- quinone methide has been formed, a number of reactions may proceed

as outlined in Scheme 4.3, which can be divided according the type of transformation

into addition, elimination and electron transfer reactions. The electron

4.2 Kraft Pulping Processes 165

density distribution at the quinone methide intermediate, as illustrated in Scheme

4.4, finally determines the pathway of subsequent processes. The size of the

respective atomic orbital [Lowest Unoccupied Molecule Orbital (LUMO) distribution;

Scheme 4. 4, left]denotes the nucleophilicity – that is the probability of a

nucleophilic attack, whilst red zones (Scheme 4.4, right) denote centers of high

electron density. The electron-deficient sites are marked in Scheme 4.3 by d+, situated

at alternating carbons starting from the keto carbon.

CH2OH

OH

OAr

OMe

ArO

CH2OH

O

OAr

OMe

B

C

O

O

OMe

HOH2C

OMe

OH

CH2OH

O

OAr

OMe

ArO

1 2

Quinone methide-Intermediate

A

Addition of

nucleophiles

Elimination

reactions

Electron transfer

reactions

ä+

ä+

ä+

ä+

-

-ArO

-

Scheme 4.3 Formation of quinone-methide (3) and subsequent reaction pathways.

Scheme 4.4 LUMO-distribution (left) and electron density

distribution of the quinone-methide intermediate [4].

166 4 Chemical Pulping Processes

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