In cotton and linen, the cellulose molecules occur as long fibers that are suitable for conversion into thread and fabric. In wood the cellulose does not differ chemically from that in cotton and linen, but the cellulose molecules are tangled and bunched. Wood cellulose, therefore, cannot be directly used for making cloth, but it is so cheap that much effort has been expended to convert it into useful forms.

The main difficulty with the modification of cellulose is its insolubility. Cellulose contains too many hydroxyl groups to be soluble in organic solvents, but it is too nonpolar to dissolve in water. In the synthesis of viscose rayon, this insolubility is overcome. First cellulose is mixed with strong alkali solution and then allowed to react with carbon disulfide (Fig. 10-20). A xanthate derivative of cellulose is formed by a reaction exactly analogous to the reaction of base with carbon dioxide. This xanthate salt is ionic; therefore, cellulose xanthate dissolves in water. The thick viscous xanthate solution of cellulose is forced through tiny holes into a solution of sulfuric acid. Just as sodium bicarbonate, when treated with acid, regenerates carbon dioxide, so cellulose xanthate regenerates carbon disulfide and precipitates cellulose in the form of a continuous thread that is drawn off and dried. Viscose rayon is then chemically identical to ordinary cellulose, although the treatment with acid and base usually results in shortening the polymer chain.

Figure 10-20. In the preparation of viscose rayon, cellulose (Fig. 10-17) is brought into solution by means of its xanthate salt. Cellulose is regenerated in long threads as viscose rayon by forcing the xanthate through small holes into an acid solution.

Cellulose can be made soluble in organic solvents by converting its hydroxyl groups to less polar derivatives. A cellulose ether is the simplest derivative that meets this condition. The sodium salt of cellulose can be treated, for instance, with methyl chloride to form methyl cellulose (Fig. 10-21). The resultant product does not resemble cotton or linen but is used to impart oil and grease resistance to paper. Acetate rayon (cellulose acetate) is formed by the reaction of cellulose with acetic anhydride to yield esters of the hydroxyl groups. Guncotton is an ester of cellulose with nitric acid.