The Reactions of Lignin

 

Objective: a very general understanding of lignin does under standard pulping conditions.

Important Concepts
Nucleophile and electrophile
Ether cleavage/Depolymerization
Condensation Reactions
Sulfonation (lignosulfonates)
 
Wood chemists have spent an enormous amount of time looking at the reactions of lignin. Summarizing this work is somewhat difficult, especially when it needs to be condensed down to a sound bite. Approximations and generalizations abound here! Emphasis is placed on the reactivity of the beta-O-4 ether bond under acid sulfite, neutral sulfite and kraft conditions. What happens to lignin during bleaching will be addressed in a future section on bleaching.
Acid Sulfite (140 C, pH 1-2)
When sulfur dioxide (SO2) is mixed with water and base, and equilibrium develops where, under acidic conditions, H2SO3 (sulfurous acid) predominates (see sulfite pulping for more details). At high temperatures a proton (electrophile) attacks the benzylic oxygen (typically a hydroxyl group) with subsequent loss of water (or ROH if the alpha-position has some R-group attached). What is formed is an intermediate benzylic carbocation which will add a sulfonate group. The net result for an alpha-hydroxyl (benzyl alcohol) is replacement of the hydroxyl with a sulfonate group. This reaction can occur at any beta-O-4 ether, ie., the phenolic position can either be free or etherified.
 
 
Placing a sulfonate group at the alpha-position has the net result of increasing the polarity of the lignin macromolecule. If enough groups are substituted, one eventually gets dissolution of the polymer. While some depolymerization of lignin does occur, to a first approximation, this pulping process differs from the other two discussed below in that extensive sulfonation occurs without significant loss in polymer size, generating lignosulfonates. If care is not taken in the pulping process, the intermediate carbocation can also undergo condensation reactions with other lignin molecules leading to an increase in molecular weight and eventual precipitation. This is not a good thing.
Neutral Sulfite (140 C, pH = 7)
Under neutral sulfite conditions, the reactivity of the sulfurous acid solution has been reduced to the point that sulfonation reactions are limited to beta-O-4 ethers with free phenolic hydroxyls only. The major reactions that occur are at these positions are alpha-sulfonation and beta-ether cleavage. Pulps produced by this process are typically high-yielding (>75%).
The net result of these reactions is light sulfonation and some beta-ether cleavage. The combination results in a lignin that is somewhat "softened" and when neutral sulfite pulps are prepared, most of the lignin remains in the pulp but the lignin is such that when the pulp is processed under higher temperatures it "gives" like thick molassas and spreads out amongst the fibers. When made into a sheet and cooled down, the lignin hardens again resulting in a stiff sheet. This is the basis of cardboard type products.
Alkaline (Kraft Pulping)
The reactive ingredients in a kraft pulping solution are nucleophiles, and we will only consider (at this point) the hydroxide anion (OH minus). The anion removes a proton from the alpha position and the resulting alkoxide attacks the beta-position breaking the beta-O-4 bond and forming an intermediate epoxide (which eventually opens up again). The net result is extensive depolymerization of the lignin.
Several side reactions occur, including loss of the gamma-CH2OH as formaldehyde (H2CO), and cleavage of methoxyl groups forming mercaptans (methyl mercaptan, CH3SH) and disulfides (dimethyl disulfide, CH3SSCH3). These compounds stink!!! Other by-products are typically dark-colored (chromophores) and kraft pulps are typically quite dark (paper bags), and must be bleached for use in printing. If kraft delignification goes too long, condensation can occur resulting in a precipitation of the dissolved lignin onto the pulp fibers...not good.
The pdf file below summarizes the above discussion:
 
Lignin Reactions.
Hardwood Versus Softwood Lignin
From the above discussion we have seen that cleavage of the beta-O-4 linkage is key for the manufacture of low lignin pulps and breakage of carbon-carbon interunit bonds is not typically feasible via economically feasible chemical routes. Considering the structure of hardwood and softwood lignins, which lignin type do you think would be more susceptible to degradation?
 
Hardwood lignin is easier to break down. The reason has to do with the monomers used for formation of lignin. Remember, hardwood lignins have sinapyl alcohol as a monomer. Since it has two methoxyl groups there is no site on the aryl ring where free radical coupling will occur. Therefore, hardwood lignin has more ether interunit linkages than softwood lignin.
 
Questions to ponder: What might be some effects of reducing lignin contents of wood utilizing the tools of biotechnology? What about modifying softwoods to produce hardwood lignin?
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