1) The document discusses the chemistry of delignification during bleaching processes. It describes reactions involving both electrophilic and nucleophilic species that degrade lignin structures. 2) Electrophilic bleaching reagents like ozone and peroxyacetic acid initiate reactions by attacking activated sites on lignin. This is followed by nucleophilic additions or displacements. 3) Nucleophilic bleaching steps involve additions of hydroxide ions, hydroperoxide ions, and hypochlorite ions to quinone and enone structures on lignin, further degrading it.

1. Chemistry of delignification
Part 2: Reactions of lignins
during bleaching
By Audrey Zahra

2. Ozonation of aromatic and olefinic structures
Ozone may be represented as a resonance hybrid comprising four mesomeric structures (Scheme 33). The
terminal positively charged oxygens constitute the electrophilic sites ofthe molecule being more reactive
than the nucleophilic sites.
Thus, as was shown for the other oxidation reactions, the initial step of ozonation involves clectrophilic
attack of the activated positions by the oxidant (Schemes 34 and 35). This is true not only for the oxidative
hydroxylation (Scheme 34, reaction 1 a) and demethoxylation (reaction 1 b) proceeding via elimination of
oxygen, but also for the 1,3-dipolarcycloadditions (2 a and 2 b) which are followed by formation of
hydrogen peroxide.

3. While the oxidative opening of phenolic and non-phenolic nuclei by hydrolysis of the cycloaddition
product is straight-forward (2 a), the cleavage of olefinic structures takes a more complicated course (2b).

4. As can be seen in Schemes 34 and 35, ozonation of aromatic and olefinic structures is accompanied by
the generation of molecular oxygen (1, 3 and 4) or hydrogen peroxide (2). Alkaline decomposition of the
latter (see below) and of the starting oxidant provides intermediary hydroxy and hydroperoxy radicals,
known to be powerful oxidants.

5. Oxidation of aromatic and olefmic structures with peroxyacetic acid
The reacting species in this oxidation is the hydroxonium ion, HO+, arising by heterolytic cleavage of the
peroxidic bond ha the oxidant. HO+ ions attack the same activated sites as the previously treated
bleaching reagents. Schemes 36 and 37 summarize the main reaction types of phenolic and non-
phenolic lisnin-related structures, studied in model experiments.
These reaction types are:
(1) ring hydroxylation
(2) oxidative demethylation
(3) oxidative ring cleavage
(4) displacement of side chains
(5) cleavage offl-aryl ether bonds
(6) epoxidation

7. Liwnin-degrading bleaching by the action of hydroxonium ions can also be achieved using acidic
solutions of hydrogen peroxide. Although no detailed studies concerning the mechanism of liLgnin
breakdown by this reagent have been reported so far, it may be assumed that the reactions accounting
for this process are similar to those observed when peracetic acid is used.

8. Nucleophilic addition and displacement reactions following initial electrophilic attack
During the introductory electrophilic steps of lignin-degrading bleaching described in section A,
quinonoid and other enone structures are generated (Schemes 24, 26, 27, 29, 32, 34, 36, 37)
which are susceptible to subsequent attack by nucleophlles. The nucleophilic reactions may take
place during the same bleaching step as the initial electrophilic reaction and involve nucleophilic
species originally present or generated either from the electrophilic bleaching reagent or from
intermediary lignin structures (cf. e.g. formation of hydroperoxide and peroxide anions during
oxygen bleaching, p. 11). Nucleophilic reactions also participate in the delignification process
during later steps of conventional bleaching sequences, where added nucleophiles are the
reacting species.

9. Nucleophilic reactions during the introductory bleaching step
Thus, in oxidative dealkylation by chlorine (Scheme 24) and by chlorine dioxide
(Scheme 27), the initial electrophilic addition of the oxidant is followed by hydrolytic,
i.e. nucleophilic, processes. In chlorination reactions of olefinic structures (schemes
25 and 26), the addition of electrophilic chloronium ions is followed by that of
nucleophilic chloride ions.
The nucleophile and the appropriate enone substrate may also be formed in the
same molecule. In such instances, the subsequent intramolecular attack gives rise to
cyclic intermediates. This is illustrated by the formation of dioxetane structures
during oxygen bleaching (Schemes 30 and 32 a).

11. Nucleophilic reactions during subsequent bleaching steps
Illustrative examples are given in the following sections: Reactions involving hydroxide ions. In
conventional bleaching, the introductory step, i.e. treatment with chlorine/chlorine dioxide in acidic
media, is followed by extraction with alkali. Chlorine linked to side chains or to quinonoid moieties
can be replaced by hydroxide ions via participation of a neighbouring hydroxyl group (formation of
oxiranes) or via nucleophilic addition (formation of cyclohexadienone intermediates), respectively
(Scheme 38).
H
H
H
H
H
H H
H

12. Similar nucleophilic addition of hydroxide ions, resulting in increased solubility, may also
take place in non-chlorinated quinonoid structures to give hydroxy-substituted catechols
or, via a benzylic acid type of rearrangement, yield a-hydroxy carboxylic acids of the
cyclopentadiene type (Scheme 39).

13. Oxidation of enone structures by hydroperoxide ions. In analogy to hydroxide ions, hydroperoxide ions add to
quinonoid and other enone structures (schemes 40 and 41) to give hydroperoxide-, and subsequently oxirane-,
dioxetane- or hydroxy quinone intermediates. Further alkaline and/or oxidative degradation affords end products
mainly of the carboxylic acid type.
Hydroperoxide ions may be used in different stages of bleaching sequences. In lignin-degrading bleaching they
usually complete the effect of electrophilic reagents, operative in earlier steps. In lignin-retaining bleaching they
remove chromophores from residual litmins in high yield pulps.
Phenolic structures are virtually stable towards alkaline hydrogen peroxide, provided homolytic decomposition of the
oxidant to give hydroxy and hydroperoxy (or superoxide anion) radicals can be effectively inhibited. These radicals,
together with their reaction product, the biradical oxygen, are responsible for the degradation of aromatic substrates
observed when non-stabilized hydrogen peroxide solutions are used. The same species are considered as being
involved in the oxidation of hydroxyl groups in wood polysaecharides to give carbonyl groups. This reaction initiates
alkaline carbohydrate degradation by "peeling" during hydrogen peroxide and oxygen bleaching (cf. also reaction with
chlorine radicals, p. 6).

14. Oxidation of enone structures by hypochlorite ions. As is true for hydroperoxide ions, hypochlorite ions constitute strongly
nucleophilic species adding readily to enone structures, in particular to quinonoid structures. The reaction proceeds via
hypochlorite esters to intermediates of the oxirane type (Scheme 42).
Reactions involving hypochlorite ions (Scheme 42) bear strong resemblance to those of hydroperoxide ions (cf. Schemes 40 and
41). Both nucleophilic species can be used to render the alkaline extraction step more effective with regard to lignin removal
and increase of brightness, and to continue lignin degradation in later bleaching steps.

17. The generation of these radical species from nucleophiles, like the opposite
process involving the formation of nucleophiles such as HOO- from radicals
such as ‘O2’ (p. 11), makes the differentiation between nucleophilic and
electrophilic bleaching steps less distinct. As shown above, nucleophiles and
electrophiles present in a bleaching liquor may be thought to co-operate in
the degradation of residual lignin. The efficiency of a particular bleaching
sequence may thus depend, at least in part, on a well balanced alternate
action of electrophilic and nucleophilic species.

18. Lignin-retaining bleaching
This bleaching variant is used in the production of high-yield mechanical, chemi-mechanical and chemical pulps
with the aim of removing chromophoric groups without degrading and dissolving lignocellulosic material.
Lignin-retaining bleaching can be achieved by the action of nucleophilic reagents. Best results are obtained
when the decomposition of the bleaching reagents, catalyzed by heavy metals, is minimized by the choice of
appropriate reaction conditions and by the use of complexing and other stabilizing agents.
Conversion of chromophoric enone structures, e.g. quinonoid groups, into colourless structures can also be
achieved by addition of nucleophiles. An example of this "additive" bleaching mode is the treatment of
mechanical and other high-yield pulps with sodium sulfite which adds to quinones to give aromatic sulfonic
acid structures (Scheme 43)

19. Selectivity of delignification
Pulping reactions
Acidic cleavage of benzyl-aryl and benzyl-alkyl ether linkages parallels acidic cleavage
of glycosidic bonds. Both processes proceed via the corresponding hydroxonium- and
carbonium ions (Scheme 44).

20. Alkaline cleavage of the same types of bond, requiring the participation of a free phenolic
hydroxyl group in the para-position, may be regarded as a vinylogous β-elimination, a reaction
type which constitutes the key step in alkaline peeling of carbohydrates (Scheme 45)

23. Bleaching reactions
The alkaline oxygenation of phenolic structures in lignins and enolic structures in
carbohydrates, here in their carbanion form, give the corresponding hydroperoxide
intermediates (scheme48).

24. These intermediates then undergo intramolecular nucleophilic attack of the carbonyl carbon with
formation of dioxetanes followed by rearrangement with cleavage of the carbon-carbon bond of the
dioxetane ring system (Scheme 49). In lignins, this results in rupture of the originally aromatic ring; in
carbohydrates, in shortening of the reducing end units by one carbon atom, released as formic acid,
and in formation of a stable aldonic acid end group.

25. Another striking analogy exists in the alkaline autoxidation of enediol structures (Scheme 50). Both
catechol structures in lignins and enediol structures in carbohydrates are readily autoxidized to give
the respective dicarbonyl structures, i.e. ortho-quinones and 2,3-diketones.

26. Hydrogen peroxide is thereby formed (cf. Scheme 32b) which adds in the known manner to one of
the two neighbouring carbonyl groups affording the corresponding hydroperoxide intermediate
(Scheme 51). The subsequent transformation of these intermediates via formation of dioxetanes and
cleavage of carbon-carbon bonds are also completely analogous.

27. Concluding remarks
Delignification during pulping is due essentially to nucleophilic reactions. Both the addition of
pulping chemicals and the intramolecular attack by ionized neighbouring groups are nucleophilic
processes. This is also true for the competing condensation reactions.
Delignification during lignin-degrading bleaching is initiated by electrophilic reactions which are
followed by nucleophilic processes either during the same or during subsequent bleaching steps.
Nucleophilic reactions generate new phenolic and enolic structures which maybe attacked by
electrophiles, while electrophilic reactions result in the formation of enone structures which may be
the substrate for subsequent nucleophilic attack. The elucidation of this interplay facilitates the
understanding of the problems encountered in technical delignification. Thus, the incomplete
removal of lignin during conventional pulping processes can be explained by the supposition that
after a certain period of time all enone structures have been consumed by addition and/or
elimination reactions and, thereby, all possibilities of nucleophilic attack have been exhausted.

28. Thus, the chemistry of delignification can now be described and summarized in terms of reaction
mechanisms generally accepted in organic chemistry. However, there are still gaps in our knowledge
which will be filled by future work. The following topics are suggested:
1. Investigation of the possibility of formation of limain-carbohydrate linkages during delignification
processes and of the behaviour of such linkages under the conditions of pulping and bleaching.
2. Confirmation of the delignitication mechanisms by isolation of further lignin degradation products
from pulping and bleaching spent liquors and by characterization of residual lignins.
3. Extension of the mainly qualitative studies hitherto carried out to include the kinetics of the
lignin- and carbohydrate-degrading reactions and their dependency on various parameters.