Xylan peeling reaction

The end group which is produced contains a carboxylic acid functionality which has an influence on the anionicity of pulp fibres (Chapter 6) but, in this form, it is resistant to further alkaline degradation. The hemicelluloses are also able to undergo the same type of peeling reaction but at different rates from each other and from cellulose. The /3-1,4-xylans, for example, are more stable to alkaline degradation than the glucomannans. 

Fig. 8-19. Peeling reaction of polysaccharides during oxygen-alkali treatment. R is polysaccharide chain. Cellulose and glucomannans (R = CH,OH) 1,3,4-Dihydroxybutanoic acid 2, glycolic acid 3, formic acid 4, glyceric acid. Xylan (R = H) 1, 3-Hydroxypropanoic acid (2-deoxyglyceric acid) 2, glycolic acid 3, formic acid 4, glyceric acid.

Figure 6.13 Possible mechanism for the formation of 2-hydroxybutanoate from the peeling reaction of xylan.

Hardwood hemicelluloses are represented by xylans and a small proportion of glucomannans. Hardwood xylans are linear polymers, constituted of [l,4]-linked xylanopyranosyl units that constitute the main skeleton. Every tenth D-xylanopyranosyl unit is substituted by a 4-0-methyl-D-glucuronic acid residue, linked to the birch xylan chain by [1,2] linkages, that has been found to retard the alkaline peeling reaction. Analysis of partially hydrolyzed xylan indicates that 4-0-methyl-D-glucuronic acid units linked to the C2 position are randomly distributed along the birch xylan backbone [9]. 

Fig. 7 -31. Peeling and stopping reactions of polysaccharides (Sjostrom, 1977). R = polysaccharide chain and R = CH2OH (cellulose and glucomannans) or H (xylan). Cellulose and glucomannans (R = CH2OH) (1) 3-Deoxyhexonic acid end groups (metasaccharinic acid), (2) 2-C-methylglyceric acid end groups, (3) 3-deoxy-2-C-hydroxymethylpentonic acid (glucoisosaccharinic acid), (4) 2-hydroxypropanoic acid (lactic acid), and (5) 3,4-dideoxypentonic acid (2,5-dihydroxypentanoic acid). Xylan (R = H) (3) 3-Deoxy-2-C-hydroxymethyltetronic acid (xyloisosaccharinic acid), (4) 2-hydroxypropanoic acid (lactic acid), and (5) 2-hydroxybutanoic acid. 1977. TAPPI. Reprinted from Tappi 60(9), p. 152, with permission.

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