1. Lecture 11 Lignin Structure
Wood Chemistry PSE 406
Lecture 11
Lignin Structure
PSE 406 Lecture 11

2. Class Agenda Functional groups Lignin structures
Methoxyl, phenolic hydroxyl, aliphatic hydroxyl, carbonyl
Lignin structures
Lignin – carbohydrate complexes
Lignin analytical procedures
Lignin trivial facts
Appendix
PSE 406 Lecture 11

3. Lignin Functional Groups Phenolic Hydroxyl
15-30 free phenolic hydroxyl/100C9: Softwood
10-15 free phenolic hydroxyl/100C9: Hardwood
Reactivity
Units containing free phenolic hydroxyl groups much more susceptible to cleavage reactions - hydrolysis of a and b aryl ether linkages
Structures much more reactive towards modification reactions
PSE 406 Lecture 11

4. Lignin Functional Groups Methoxyl
~0.95/C9 in softwoods
~1.5/C9 in hardwoods
Generally resistant to acid and alkali
HS cleaves to form thiols, mercaptans (Kraft mill odor)
PSE 406 Lecture 11

5. Lignin Functional Groups Aliphatic Hydroxyl
Majority of aliphatic hydroxyl groups are primary: on g carbon
Site relatively non-reactive
In some species, g carbon oxygen linked through ester linkage to r-coumaric acid, etc
Benzyl alcohols
Debated amount: 16-40/100 C9 in spruce
Play dominant role in delignification reactions
PSE 406 Lecture 11

6. Lignin Functional Groups Carbonyl Groups
Total carbonyl groups 20/100C9 in spruce
1/2 Conjugated Structures
Coniferaldehyde and a-keto structures
Play important role in delignifcation reactions
1/2 Non-conjugated Structures
Glyceraldehyde from b-1 coupling
Larger amount in certain hardwoods and grasses due to esters.
PSE 406 Lecture 11

7. Lignin Structure Sakakibara
This structure was generated by Sakakibara over 20 years ago. He developed it by compiling linkage and functional group information obtained by a variety of researchers. Putting this information together into a structure is a time consuming and frustrating experience. Your intrepid lecturer did this himself 15 years ago generating Bob’s lignin. This structure is not included in these notes because of massive inaccuracies. If you go through this structure and compare the linkages to experimental numbers, you will see certain problems. For one thing, it is difficult to include all the linkages in a structure with 25 rings if the linkage frequency is less than 3-4%. For a second thing, it is known that lignin from different parts of the tree has different linkages and functional groups which this structure doesn’t address. Thirdly, this structure shows the possible linkages but not in any particular order. It is now known that there is sections of lignin that has multiple b-O-4 linkages. This structure takes none of this into account. There have been multiple other structures developed by other lignin chemists. Some examples of these structures are included in the appendix.
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PSE 406 Lecture 11

8. Lignin-Carbohydrate Complex
All purified holocellulose materials contain a certain amount of lignin
All purified lignin fractions contain a certain amount of monosaccharides
LCCs have been enzymatically prepared from lignin and monosaccharide model compounds
Significant work studying isolated LCCs
No definitive information on exact covalent bonding patterns
Generally accepted bonding patterns
PSE 406 Lecture 11

9. Lignin-Carbohydrate Complex Proposed Linkages
PSE 406 Lecture 11

10. Lignin-Carbohydrate Complex General Information
Mw of isolated LCCs 600®15,000
LCC linkage stability
Esters: alkali labile, acid labile
Ethers: selectively alkali labile, mildly acid labile
Glycosides: mildly alkali labile, acid labile
Formation during pulping processes possible
LCCs: residual lignin and bleaching
Removal of that last little bit of lignin
Enzyme assisted bleaching
PSE 406 Lecture 11

11. Lignin Structure Analytical Procedures
All analysis require model compound studies!
Linkages
Enzymatic dehydrogenation (test tube studies)
Degradation studies (see appendix)
NMR
Functional groups
Wet Chemistry techniques
Spectroscopy
PSE 406 Lecture 11

12. Lignin Trivial Facts I Solubility Molecular Weight
Native lignins: limited/no solubility in all solvents without modification
Molecular Weight
Average Mw for softwood ~20,000, lower for hardwoods
Polydispersity ~
Mw measured for lignosulfonates as high as 1,000,000
PSE 406 Lecture 11

13. Lignin Trivial Facts II
Elemental Composition MWL
Spruce C9H7.92O2.40(OCH3) C9H8.83O2.37(OCH3)0.96
Beech C9H8.50O2.86(OCH3) C9H7.93O2.95(OCH3)1.46
UV Absorption
Strong adsorption at 205 and 280nm
Carbohydrates do not adsorb at 280nm
Compression Wood (Softwoods)
High % lignin (~40%), high % of r-hydroxy units (to 70%)
Tension Wood (Hardwoods)
Reduced lignin content
PSE 406 Lecture 11

14. Lecture 11
Appendix
PSE 406 Lecture 11

15. Lignin Structure Elucidation Studies Nitrobenzene Oxidation
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Nitrobenzene oxidation is one of oldest form of lignin analysis. The major product of the oxidation of softwoods is vanillin. With hardwoods, a mixture of syringaldehyde and vanillin is obtained. This procedure, therefore, gives an indication of the methoxyl content of the lignin. It was also an important procedure in early lignin studies as it gave another indication of the phenolic nature of lignin. An important point to note is that the total yield of vanillin (by far the major product) from this procedure is at the most 25%. Therefore, the majority of phenolic units do not react.
PSE 406 Lecture 11

16. Lignin Structure Elucidation Studies Permanganate Oxidation
The permanganate oxidation procedure has been used to determine the the abundance of most of the lignin linkages. The procedure is as follows:
A lignin preparation is either methylated or ethylated with dimethyl or diethyl sulfate over a 3 day period at a pH of / Free and only free phenolic hydroxyl groups are alkylated. The derivitized lignin is reacted with potassium permanganate which chews up the lignin destroying most of the lignin linkages and rings. When the oxidation reaches a derivitized ring, the oxidation stops leaving an aromatic carboxylic acid structure. Quantification of these structures gives information on linkages. Dimeric structures and dicarboxylic acid monomeric structures allow the quantification of 5-5, 4-O-5, b-5, a-6, and other units. It is not possible to quantify b-O-4 linkages using this procedure because other linkages such as b-1 give the same product. This procedure will also give information on the syringyl/guaicyl ratio, the number of catechol functionalities, and the number of free phenolic hydroxyls. A problem with this analysis is that only those groups that originally contain a free phenolic hydroxyl group can be determined through this procedure. To get around this problem, lignin is often oxidized first with copper oxide to cleave b-O-4 linkages and increase the number of free phenolic hydroxyls.
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PSE 406 Lecture 11

17. Lignin Structure Elucidation Studies Acidolysis
Acidolysis and thioaidolysis procedures are used to quantify the number of b-O-4 linkages in lignin. The product of acidolysis are a number of aromatic compounds known as Hibbert’s ketones. By running model compound studies, it is possible to determine the structures which will produce specific ketones under these reaction conditions. Like permanganate oxidation, this procedure is very time consuming.
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PSE 406 Lecture 11

18. Lignin Structure: Adler
PSE 406 Lecture 11

19. Lignin Structure: Freudenberg
PSE 406 Lecture 11

20. Lignin Structure: Nimz
PSE 406 Lecture 11

21. Lignin Structure: Forss
PSE 406 Lecture 11