Peeling reaction

Peel-apart pack films, Fuji, 19 312 Peeler centrifuge, 5 544 22 390-391 Peeling reaction, of polysaccharides, 22 27-28... 

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. 

Figure 3.16 Depolymerisation of cellulose by alkaline peeling reactions.

The reducing ends are the starting points for the well-known peeling reaction. The conditions for the peeling to proceed require alkaline media and a free 2-OH group. In case of a substituted hydroxyl at C2, as it occurs for instance in hemicelluloses, the peeling rate is drastically reduced. 

Fig. 2-33. Alkaline peeling reaction of cellulose (R = cellulose chain). 1 — 2, Isomerization 2 — 3, 2,3-enediol formation 3 — 4, j3-alkoxy elimination 4 —> 5, tautomerization 5 — 6, benzilic acid rearrangement leading to glucoisosaccharinic acid.

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.

Fig. 16. -Elimination in common O-glycoside linkages with Ser or Thr residues in alkaline conditions and a plausible mechanism of subsequent peeling reaction. This figure is adapted from ref. 96.

The DP results rendered by most of these methods are low and inaccurate, especially when the celluloses are alkali-sensitive, i.e., contain carbonyl groups bringing about chain cleavage in alkaline solutions or active carbonyls, which initiate the stepwise depolymerization of the cellulose chains (known as the peeling reaction) according to the carbonyl elimination mechanism of Isbell et al. [416-420]. The most accurate method for DP determination in all celluloses is the nitration method, in which the cellulose is nitrated in a solution of nitric and phosphoric acids and phosphorus pentoxide, and dissolved in butyl acetate [420-422]. 

Initial enolisation of glucose yields the 1,2-enediolate. Expulsion of the 3-OH or 3-OR yields a l-aldehydo-3-deoxy-2,3-enol. This tautomerises to the ot-ketoaldehyde and addition of OH at Cl, followed by shift of HI as hydride, yields a pair of straight-chain, C2-epimeric, 3-deoxygluconic acids known as metasaccharinic acids. These are the predominant six-carbon products from the peeling reaction of 1 3-linked polysaccharides. 

Figure 6.11 Nef-Isbell-Richards mechanisms for degradation of glucose and fructose and the peeling reaction of 1 -> 3- and 1 ->4-linked polysaccharides. R or R can be another sugar residue, when the mechanism of the peeling reaction is illustrated.

Figure 6.12 Stopping the peeling reaction by formation of 3-0-cellulose glycosides of 2-C-methylglyceric acids.

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