Effects of Alkali on Oxidized Polysaccharides

Interest in the alkah lability of oxidized polysaccharides has been revived, as is evidenced by excellent reviews.  

In 1873, it was first reported that cellulose that has been oxidized with chromic acid or potassium permanganate is weakened in alkaline solutions, and several years later these findings were confirmed. By 1899, it was known that cellulose that had been oxidized with nitric acid, or bromine in the presence of calcium carbonate, afforded oxidized celluloses from which D-glucoisosaccharinates were formed on suitable treatment with lime-water.Hydrocelluloses, alkaline-hypobromite oxycelluloses, and alkaline-hypochlorite oxycelluloses have no internal, alkali-sensitive links. Cellulose oxidized with neutral hypochlorite, acidic hypochlorite, potassium dichromate and sulfuric acid, potassium dichromate and oxalic acid, periodic acid, or metaperiodate gives products which are alkali-sensitive. Periodic acid oxycellulose and metaperiodate oxycellulose are much more alkali-sensitive than the others.  

When oxycelluloses are placed in alkaline solution, large reductions in molecular weight take place because of the scission of glycosidic bonds. As a consequence, the oxidation of cellulose does not, in general, lead directly to rupture of the molecules, but renders those linkages near the point of attack extremely susceptible to alkaline cleavage. Several mechanisms have been proposed for explaining these effects. 

Periodate-oxidized celluloses are very sensitive to alkali, and, as the proportion of periodate used for oxidation is increased, the alkali-sensitivity increases. Alkali-sensitivity is manifested as a diminution in tensile strength when periodate-oxidized, cotton fiber is exposed to alkalis.  

Appropriate studies with methyl glycosides have shown that oxidation with periodate occasions scission between C2 and C3, with the consequent formation of a dialdehyde structure (XXII). Similarly, each oxi- 

---