Acid Group, lignin

Substitution Reactions on Side Chains. Because the benzyl carbon is the most reactive site on the propanoid side chain, many substitution reactions occur at this position. Typically, substitution reactions occur by attack of a nucleophilic reagent on a benzyl carbon present in the form of a carbonium ion or a methine group in a quinonemethide stmeture. In a reversal of the ether cleavage reactions described, benzyl alcohols and ethers may be transformed to alkyl or aryl ethers by acid-catalyzed etherifications or transetherifications with alcohol or phenol. The conversion of a benzyl alcohol or ether to a sulfonic acid group is among the most important side chain modification reactions because it is essential to the solubilization of lignin in the sulfite pulping process (17). 

As pulping progresses, the lignin content decreases and the acid group content also decreases. Lignin is not usually measured directly but by the degree of oxidisability of the pulp using, for example,... 

Figure 6.1 The acid group content of pulps expressed as an ion exchange capacity as a function of kappa number (measure of lignin content) for Kraft and sulfite pulps.

Lignin contains phenolic and a few carboxylic acid groups it therefore dissolves in sodium hydroxide. Soda pulping is based on this fact. Here, at the high temperatures used, ester bonds arc hydrolyzed, and some of the carbohydrate-lignin ether bonds are split. Cold caustic soda is used for semichemical processes. 

The treatment of lignin by ozone, and so its functionalization by acid groups provided them outlets, knowing that lignin constitutes a voluminous by-product resulting from wood delignification in the paper industry [199— 202],... 

So-called accessory enzymes are also required for hydrolysis of hemicellulose in native plant tissue. These enzymes include a variety of acetyl xylan esterases (EC 3.1.1.72), acetyl esterases (EC 3.1.1.6), and esterases, such as feruloyl acid esterase (EC 3.1.1.73),178 that hydrolyze lignin glycoside bonds. They also include classes for cleavage of specific hemicellulose side chains, such as a-L-arabinofuranose, glucuronic acid, and 4-O-methyl-glucuronic acid groups. 

The procedure described below was developed by Pobiner (1983) and consists of the nonaqueous potentiometric titration of lignin with tetra-n-butylammonium hydroxide in the presence of an internal standard, p-hydroxybenzoic acid. As is true also for conductometric titrations of lignin (see Chap. 7.7), phenolic hydroxyl and sulfonic acid groups may be coincidentally determined by this procedure. 

Enkvist T, Aim B, Holm B (1956) Some experiments concerning the acidic groups of lignin. Pap Puu 38 1-8, 12... 

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