Building blocks
Lignin is built from three similar molecules. Each has a benzene ring and a short tail of three carbon atoms. They differ only in the number of methoxyl groups on the ring: none, one or two. The units they form are labelled H, G and S.
The monolignols are p-coumaryl, coniferyl and sinapyl alcohols, giving p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) units in the polymer. Each is built on a C6-C3 phenylpropane unit. Softwood lignin is almost entirely guaiacyl (with a little H), hardwood lignin is guaiacyl-syringyl, and grass lignins contain all three.
How the polymer grows
Lignin has no blueprint. Enzymes turn the alcohols into reactive radicals, and these stick together at random. Every lignin molecule is unique, so a lignin formula in a textbook is always just a sketch.
Monolignols are oxidised by peroxidases and laccases to phenoxy radicals that couple combinatorially, without a template and, in the accepted theory, without enzymatic control of stereochemistry; the chain grows mainly by adding a monomer to the polymer end. Lignin is therefore optically inactive and has no single structure; it is described by the shares of unit and bond types.
Bonds between units
Most often the units are linked through an oxygen atom, the β-O-4 bond. It is relatively easy to break, and pulping relies on that. There are also strong carbon-carbon bonds that hardly break at all.
The β-O-4 bond (arylglycerol β-aryl ether) makes up roughly half of the inter-unit linkages in softwood lignin and about 60% in hardwood lignin. The rest are C-C bonds (β-5, 5-5, β-β, β-1) and 4-O-5 diaryl ethers. In S units the C5 position carries a methoxyl, so they cannot be linked through C5: they form no 5-5 bonds and can take part in β-5 and 4-O-5 bonds only from the other side. Hardwood lignin is therefore less condensed and easier to remove in pulping.
How lignin is removed
To make paper, wood is cooked with chemicals until the lignin dissolves. What remains is almost pure cellulose fibre. The dissolved lignin is burned at the mill to power the mill itself.
Kraft pulping (NaOH and Na2S) cleaves β-O-4 bonds and lignin fragments dissolve; the spent (black) liquor is burned in a recovery boiler that also regenerates the chemicals. In sulfite pulping lignin is sulfonated and leaves as lignosulfonates. For analysis, lignin is measured as the residue insoluble after treatment with 72% sulfuric acid followed by boiling in dilute acid (the Klason method).
Colour and yellowing
Lignin stains easily and darkens in light. That is why newspaper yellows in the sun, while white office paper, with almost no lignin, lasts.
Chromophores in lignin and its photo-oxidation products cause yellowing of mechanical pulp and high-lignin papers. The classic qualitative test for lignin, phloroglucinol with hydrochloric acid (the Wiesner reaction), stains lignified walls red-violet through cinnamaldehyde end groups (coniferaldehyde and sinapaldehyde).
Key facts
- monomers
- H, G, S
- unit
- C6-C3 phenylpropane
- main bond
- β-O-4
- Scots pine
- 27.7%
- silver birch
- 22.0%
- softwood
- G (+ some H)
Sources for this chapter
- Азаров В.И., Буров А.В., Оболенская А.В. Химия древесины и синтетических полимеров: учебник. 2-е изд. СПб.: Лань, 2010.
- Sjöström E. Wood Chemistry: Fundamentals and Applications. 2nd ed. San Diego: Academic Press, 1993.
- Ralph J. et al. Lignins: natural polymers from oxidative coupling of 4-hydroxyphenyl-propanoids. Phytochemistry Reviews 3, 29-60 (2004).
- Ek M., Gellerstedt G., Henriksson G. (eds.) Wood Chemistry and Wood Biotechnology. Pulp and Paper Chemistry and Technology, vol. 1. Berlin: De Gruyter, 2009.