Page 104 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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412 Carbohydrates

stitution, which in turn determines the appropriate usage
for the product; higher DS products are used as explosives
and lower ones as cements and lacquers.
Carboxymethyl Cellulose: This ether of cellulose is ob-
FIGURE 19 Structure of cellulose. tained by a modified Williamson synthesis whereby cel-
lulose is first treated with sodium hydroxide and the alco-
amylose in their anomeric configuration; β for cellulose holate formed is reacted with the halide, namely sodium
and α for amylose. The DP of cellose, determined by monochloroacetate. Although a DS of 3 is attainable (po-
4
physical methods is around 10 , much higher than the sitions 3,4, and 6 can all be etherified), a DS of only 0.4 is
value deduced chemically by end group analysis, which all that is needed to achieve the desired viscosity and sol-
is only 200. The difference is attributed to its degradation ubility needed in the commercial products. Carboxmethyl
during chemical manipulation. A similar difference was cellulose is widely used in the form of alkali metal salts
observed with amylose. Upon methylation and hydrol- as surfactant in detergents, in oil drilling, mining, and in
ysis, cellulose affords 2,3,6-tri-O-methyl glucopyranose the food, paper, and textile industries.
in high yield (>90%). The presence of (1 → 4)-linkages Hydroxyethyl Cellulose: Ether alcohols such as
can be confirmed by partial hydrolysis to the disaccha- hydroxyethyl cellulose are used industrially to solubi-
ride, cellobiose, whose β-D-configuration was confirmed lize cellulose and to facilitate its handling. Hydroxy-
byenzymatichydrolysis(forapartialstructureofcellulose ethyl cellulose is prepared by swelling the cellulose with
see Fig. 19). The actual shape of the cellulose aggregates sodium hydroxide and reacting it with ethylene glycol.
was studied by X-ray diffraction, which revealed that there As in the previous example, a low DS of 0.3 is sufficient
exist two types of cellulose: cellulose I and cellulose II. to solubilize the product in alkali.
Cellulose I is formed naturally and is composed of highly
Cell-OH + CH 2 -CH 2 −→ Cell-O-CH 2 -CH 2 -OH
ordered microfibrils that are held together by hydrogen ❙
bonds and are further associated to form fibers. Bundles O
of cellulose I all have their reducing ends in the same side Cellulose Hydroxyethyl Cellulose
of the macromolecules and the nonreducing end in the
Hydroxypropyl cellulose is prepared in the same manner
opposite side. A different type of bundle occurs in cel-
except that propylene glycol is used in place of ethylene
lulose II, found in regenerated cellulose such as viscose
glycol.
rayon. The aggregates of cellulose II have their reducing
Cellulose Xanthate: Cellulose xanthate or viscose is an
ends arranged in an antiparallel manner, i.e., their reduc-
intermediate in the production of viscose rayon, used in
ing ends occur at both ends of the bundle. Cellulose II
the manufacture of textile fibers and cellophane wrapping.
types can be further divided into III I and III II and IV I and
The manufactureof viscosestarts by swellingthe cellulose
IV II , according to the conformation of their unit cells; the
with sodium hydroxide. The mixture is then treated with
I family forms bent chains, whereas the II families adopt
carbon disulfide to form the sodium xanthate of cellulose,
bent-twisted conformations.
which is known as viscose. It was given this name be-
cause its viscosity increases with time. When the desired
a. Chemically modified cellulose. The most im-
consistency is reached, the mass is extruded into an acid
portant derivatives of cellulose are the esters, such as
bath to form fibers or sheets depending on the extrusion
cellulose acetate and nitrate, the ethers, such as car-
orifice shape (circular holes or slits). The acid bath decom-
boxymethyl, hydroxyethyl, and hydroxypropyl cellulose,
poses the xanthate groups and regenerates cellulose now
and the rayons, of which the most important are the acetate
called viscose rayon to distinguish it from other rayons.
rayon, which is discussed next and viscose rayon obtained
from cellulose xanthate. Cell-OH −→ Cell-ONa + CS 2 −→ Cell-OCSSNa
Cellulose Acetate: Cellulose acetate, also known as ac- Cellulose Viscose
etate rayon is obtained from wood pulp in three steps:
(1) pretreatment (soaking with acetic acid and sulfuric −→ Cell-OH
acid); (2) acetylation with a mixture of acetic anhydride Viscose rayon
and acetic acid, to form the 2,3,6-tri-O-acetyl derivative;
and (3) partial hydrolysis to afford a product having a DS
5. Mannans and Galactomannans
of about 2.6, which forms the best fibers and films.
Cellulose Nitrate: Cellulose can be esterified with a Mannans are highly insoluble polymers of β-D-man-
mixture of nitric acid and some sulfuric acid added as nopyranose, found in the kernels of many palms, for ex-
catalyst. The amount of water controls the degree of sub- ample, ivory nut. This hard material is composed of nearly