Sulfite pulp acid groups

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). 

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.

In a later paper (7) they discussed the treatment of some lignin model compounds (e.g., vanillic acid and vanillyl alcohol) with sulfite liquors under varying pulping conditions. Their results indicated that pyrocatechol groups may be formed in lignin during semichemical sulfite pulping. 

Acid Sulfite (and Bisulfite) Pulping During acid sulfite pulping the a-hydroxyl and the a-ether groups are cleaved readily under simultaneous formation of benzylium ions (Fig. 7-6). This reaction takes place regardless... 

Fig. 7-6. Behavior of /3-aryl ether and open a-ether structures during acid sulfite pulping (Gellerstedt and Gierer, 1971). R = H, alkyl, or aryl group. The first reaction step involves cleavage of the a-ether bond with formation of a resonance-stabilized carbonium ion which is then sulfonated. Note that both the phenolic and nonphenolic structures are sulfonated, while the /3-aryl ether bonds are stable.

Fig. 7-12. Reactions of phenolic /8-aryl ether and a-ether structures (1) during neutral sulfite pulping (Gierer, 1970). R = H, alkyl, or aryl group. The quinone methide intermediate (2) is sulfonated to structure (3). The negative charge of the a-sulfonic acid group facilitates the nucleophilic attack of the sulfite ion, resulting in /8-aryl ether bond cleavage and sulfonation. Structure (4) reacts further with elimination of the sulfonic acid group from a-position to form intermediate (5) which finally after abstraction of proton from /8-position is stabilized to a styrene-/8-sulfonic acid structure (6). Note that only the free phenolic structures are cleaved, whereas the nonphenolic units remain essentially unaffected.

Katz S, Beatson RP, Scallan AM (1984) The determination of strong and weak acidic groups in sulfite pulps. Sven Papperstidn 87 R48-R53... 

Methods that attempt to measure the sulfonate content directly are based on retention of benzidinium ions by sulfonate groups (Sjostrom and Enstrom 1966), titration of the hydrogen ions released when a sulfite pulp in its acid form is washed with potassium chloride adjusted to pH 4 (Cappelen and Schoon... 

Brauns FE, Brauns DA (1960) The sulfonation of lignin In Brauns FE, Brauns DA (eds) The chemistry of lignin Supplement volume Academic Press, New York, 313-386 Canadian Pulp and Paper Association (1970) Standard Method G28 In Standard Testing Methods, Technical Section, Canadian Pulp and Paper Association, Montreal, Canada Canadian Pulp and Paper Association Useful Method G 7U In Standard Testing Methods, Technical Section, Canadian Pulp and Paper Association, Montreal, Canada Cappelen H, Schoon N-H (1966) Determination of sulfonic acid groups in unbleached sulfite pulps Sven Papperstidn 69 322-325... 

That carboxylic acid groups on the cellulosic chain play a minor role in the decay of the material is not surprising. As Davidson and Standing (10) and Davidson and Nevell (11) showed, acidic oxycelluloses are unstable because the carboxylic acids are converted to water-soluble, low molecular weight species. Thus, in old cellulosic textiles, the acidic species are removed from the fabric upon washing, and deacidification is not necessary. Kerr et al. (12) and more recently Hackney and Hedley (13) have provided independent evidence of the correctness of this view. Paper that has been subjected to sulfite pulping or that contains alum or rosin, which may form acids, responds well to deacidification because the additives are being neutralized. 

The recent studies of the Finnish group further emphasize the complexity of the xylan in wood cellulose. These workers have shown that the properties of the xylan in wood pulps depend not only on the method of pulping but also on the type of raw material. It was observed that, when pulps are prepared by the (acidic) sulfite process from the softwoods, pine and spruce, a substantial part of the xylan is of the acidic type (see Fig. 2, p. 303). Even when these sulfite pulps are refined further with hot alkali, some acidic xylan remains. However, when pulps are prepared from pine and spruce by the (alkaline) sulfate process, the acidic xylan is removed and only neutral xylan remains. 

---