1. Pulping and Bleaching PSE 476/Chem E 471
Lecture #9
Kraft Pulping Lignin Reactions

2. Agenda Lignin Reactions Cleavage of b-O-4 linkages b-O-4, a-carbonyl
Effect of phenolic group
Effect of nucleophile (OH-/HS-)
b-O-4, a-carbonyl
b-1 linkages
b-5 linkages
4-O-5, 5-5, b-b linkages
Condensation Reactions

3. Kraft Reactions of b-O-4 Linkage (Free Phenolic Hydroxyl)
Formaldehyde
Under alkaline conditions, groups containing free phenolic hydroxyls and an ether linkage on the a carbon readily form quinone methides. Hydroxyls will readily add to the quinine methide forming a benzyl hydroxyl. Therefore, an equilibrium is established when prohibits the cleavage of the b-O-4 linkage.
Proton abstraction under the strongly alkaline conditions will result in the formation of structures known as vinyl ethers. Abstraction of a proton from the gamma carbon results in the formation of formaldehyde. This chemical can then cause harmful condensation reactions which we will discuss in next times lecture.
Notice that the
b-O-4 bond is
not cleaved.
Vinyl Ether
Notes

4. Reaction of Phenolic b-aryl Ether: HS-
HS- is a very strong nucleophile which readily adds to quinone methides. Therefore, β-O-4 structures with free phenolic hydroxyls and a benzyl hydroxyl group are very reactive. The thiol derivative which is formed is also a strong nucleophile and reacts with the β-carbon forming a three member thiirane structure with simultaneous cleavage of the β-O-4 linkage. The thiirane structure will breakdown generally releasing the sulfur into solution. In kraft lignin, therefore, there is very little incorporated sulfur.
This reaction is very rapid resulting
in the cleavage of the b-O-4 linkage.
Notes

5. Reactions of b-O-4 Linkages: Etherified Phenolic Hydroxyl/Benzyl hydroxyl
Reaction with OH-
The b-O-4 linkage is cleaved but only very slowly. This reaction requires elevated temperatures.
Cleavage of the b-O-4 linkage generates a free phenolic hydroxyl.
Reaction with HS-
HS- will only react with groups containing a free phenolic hydroxyl so therefore there is not cleavage.
* Reaction mechanisms on following pages

6. Reaction of Nonphenolic b-aryl Ether: OH-
In the first structure in this figure, there is no free phenolic hydroxyl. Therefore, a quinone methide is not formed and therefore there is not site for nucleophilic attack. Abstraction of a proton from the benzyl hydroxide results in the formation of a reactive alkoxide anion. In this structure, the β carbon is somewhat positive due to the electron withdrawing effect of the oxygen group in the β-O-4 linkage. Therefore, this carbon will undergo an internal nucleophilic attack by the alkoxide anion forming a three membered oxirane structure and cleaving the b-O-4 linkage. This results in the formation of a new phenolic hydroxide group. The rate determining step of this reaction is the formation of the alkoxide group which is quite slow under kraft pulping conditions.
Notes
In this reaction, the b-O-4 linkage is cleaved. This is,
however, a very slow reaction.

7. Reaction of Nonphenolic b-aryl Ether: HS-
No phenolic hydroxyl = no reaction

8. A Quick Review OH- (alone) HS-(with OH-)
Reaction of b-O-4 ether linked
structure with benzyl hydroxyl
OH- (alone)
HS-(with OH-)
Phenolic Hydroxyl
No cleavage
Formation of vinyl ethers.
Rapid cleavage
Non Phenolic
Slow cleavage

9. Reactions of b-O-4 Linkages: Alpha Carbonyl Group
These reactions occur whether there is a free or etherified phenolic hydroxyl group.
Reaction with OH- (alone).
No reaction
Reaction with OH-/HS-
Rapid cleavage of linkage.
Mechanism on next page

10. Alkaline Pulping Activation of g Carbon by Carbonyl
A: Keto-enol tautomerism

11. Alkaline Pulping b-O-4 , a-carbonyl
HS-
OH-

12. Alkaline Pulping Mechanisms β-1/free phenolic (OH- alone)
CH2O
No fragmentation of dimer
Stilbene structures

13. b-1 Reaction Mechanisms (OH- alone)
CH2O

14. b-1 Reaction Mechanisms (OH- alone)
Formation of Stilbene

15. Alkaline Pulping Mechanisms b-5 /free phenolic (OH- alone)
Formation of
Stilbenes
HO-
No fragmentation of dimer
Mechanism similar to b-1

16. Retroaldo Cleavage of Carbon-Carbon Bonds
Groups containing double bonds (vinyl ethers, stilbenes, QM) will cleave very slowly under alkaline conditions through a retro aldo reaction. This reaction requires elevated temperatures.

17. Alkaline Pulping Mechanisms b-1 /free phenolic (OH- /HS-)
No fragmentation
of dimer
HS-
HO-
2HS-
OH-

18. Alkaline Pulping Mechanisms b-5 /free phenolic (OH- /HS-)
Same reactions as for
The b-1 linkage
HS-
OH-

19. Alkaline Pulping Mechanisms b-1/etherified phenolic OH (HS-/OH-)
No Reaction

20. Alkaline Pulping Mechanisms b-5/etherified phenolic OH (HS-/OH-)
No Reaction
HS-
OH-

21. Alkaline Pulping Mechanisms 4-O-5 (HS-/OH-)
No Reaction
HS-
OH-

22. Alkaline Pulping Mechanisms 5-5 (HS-/OH-)
No Reaction
HS-
OH-

23. Alkaline Pulping Mechanisms Methyl Aryl Ether Cleavage
This reaction requires very high temperature and are therefore very slow. The low molecular weight products are very odiferous compounds which give kraft pulp mills their characteristic odor. Roughly 5-10% (I need to check this number) of the methoxyl groups are cleaved by this reaction.
Notes

24. Alkaline Pulping Lignin Carbohydrate Bonds
Esters and glycosidic linkages easily cleaved.
Ether linkages.
Free phenolic hydroxyl: easily cleaved.
Etherified phenolic hydroxyl: no reaction.
Residual lignin problem.

25. Lignin Condensation
We have been discussing the reactions of quinone methides with OH- and HS-. In solution, these structures along with resonance structures of the phenoxy anion can react with lignin structures. This type of reaction is known as condensation and is a problem because it increases the molecular weight of lignin and forms a difficult to cleave bond.

26. Condensation Reactions (C5)
As shown below, a quinone methide can react with a C5 anion to form a new carbon-carbon bond. Remember, this new bond is not cleaved during pulping. The result of this reaction is higher molecular weight lignin (this is bad).

27. Condensation Reactions (C1)
C1 is another site for condensation reactions forming an a-1 linkage. In was seen with the b-1 free radical coupling reaction, this requires side chain elimination.

28. Condensation with Formaldehyde
Formaldehyde, which is formed from g carbons during the formation of vinyl ethers and stilbenes, will cross link 2 molecules. The linkage formed is known as a diphenyl methane linkage (same linkage in a-1 & a-5).

29. Reactivities of Lignin Linkages
The yes and no answers in this table refer to whether the listed linkages will be
cleaved with the specific ions. It is obvious that in a kraft cook that both OH- and HS- are present. Sometimes, however, there won’t be any HS- present because of diffusion issues.
* No if a hydroxyl, yes if a carbonyl
Short Note