Cellulose

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INTRODUCTION::

INTRODUCTION: Cellulose is the most widely distributed plant polysaccharide. It forms the main constituent of the cellwall of plants. Cellulose in association with hemicelluloses (lower molecular weight cellulose like polysaccharide) and lignin (a non carbohydrate aromatic polymer) constitute structural material of woody plants. The seed hairs of cotton plant are almost pull cellusose . The longer fibres are used for spinning into cloth. Where as the short hairs (linters) are worked for the chemical cellulose.

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Other important sources of cellulose are best fibres such as flax(80%) and jute(65%) leaf fibres such as hemp (80%) and cereal straws (45%) cellulose is a colourless solid , insoluble in water and organic solvents but readily soluble in ammonical cupric hydroxide solution ( Schweizer’s reagent) with which it forms a complex. It is also soluble in strong hydrochloric acid, but undergoes degradation to give products of lower molecular weights.

Cellulose from cotton linters::

Cellulose from cotton linters: The impurities (nearly 2%) of cotton cellulose consisting mainly of the protective film of wax,oil from the seed, pectic substances,mineral subatances such as sand ,and some colouring matter are removed carefully so as to avoid degradation of the cellulose molecule The heavy impurities like stones and leaves are removed by mechanical cleaning. After removing the heavy impurities the linters are boiled with dilute aqueous sodium hydroxide in an inert atmosphere to remove pectin,wax and oil. The linters are then bleached with dilute sodium hypochlorite and the product thus obtained is nearly 99.7% pure cellulose;the residue is found to consist of mainly silica.

Cellulose from wood (wood pulp)::

Cellulose from wood (wood pulp): Cellulose obtained from wood is mainly used in paper induster . Some amount is also used in fibre production. The fatty and waxy impurities are removed by organic solvent . Lignin and hemicelluloses are removed by treating the wood pulp either with dilute acids or with calcium and sodium sulphite ( sulphite process) or with sodium hydroxide or with sodium sulphode (sulphate process ) at high temperature and pressure The fibres are then bleached either with chlorine or with sodium hypochlorite. Finally the fibres may be purified by the use of 15-20% of sodium hydroxide.

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Although the purification steps in the two processes are relatively mild, some of the cellulose is hydrolysed . By changing time of the various steps in the purification processes, different forms of cellulose may be achieved differing very much in the average size of the molecule. The three important cellulose forms are α , β and γ . α -Cellulose is very less degraded and is insoluble in 17.5% aqueous sodium hydroxide, β -cellulose is soluble in 17.5% aqueous sodium hydroxide but insoluble in dilute acid, while the γ -cellulose is insoluble in both.

Constitution::

Constitution: 1) The elementary analysis indicates that cellulose has C 6 H 10 O 5 as its empirical formula. 2) On acidic hydrolysis cellulose gives quantitative yield (about 96%) of crystalline D-glucose indicating that cellulose is composed of only D-glucose units. 3) Acetylation , nitration and methylation of cellulose gives tri-substituted celluloses suggesting that only three hydroxyl groups are free per glucose unit.

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4 ) Hydrolysis of fully methylated cellulose gives 2,3,6-trimethylglucose[86%] and 2,3,4,6-tetramethylglucose[nearly 0.6%]without any dimethylglucose . The hydrolysis products led to the following results (a) The formation of 2,3,6-trimethyl glucose indicates that in cellulose free hydroxyl groups are present on C 2 , C 3 and C 6. Hence in cellulose the two glucose unites may be linked through C 1 and C 4 (if glucose uintes are present as pyranose ) or through C 1 and C 5 (if glucose units are present as furanose ). But since the cellulose is not easly hydrolysed ,the glucose must b in the from of pyronose ring i.e., the glucose units are linked via C 1 and C 4 .

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Thus the following part structure may be assign to cellulose However, note that if the linkage at C 4 and C 5 were interchanged ,the same trimethyl glucose would be obtained on similer treatment .

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(b) The yield (0.6%) of 2,3,4,6-trimethyle glucose indicates that cellulose has a chain length of about (100 to 200 glucose units. (c) The failure to isolate any dimethyl D-glucose indicates that cellulose is a linear polymer. 5) Acetolysis ( i.e simultaneous acetylation and hydrolysis )of cellulose with a mixture of acetic anhydride and H2SO4 gives cellobiose octa -acetate(50%yield).

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This suggest that cellulose is composed of cellobiose units because it was shown that under the acetolysis conditions the back reaction ( i.e the conversion of glucose to celobiose octa -acetate) was not possible now the structure of cellose biose indicates that in cellulose also the glucose units are present in pyranose form and the two gluco pyranose units are linked to each other through the β - anomeric carbon(C 1 ) of one and C 4 of the other.

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But it must be noted that the formation of cellobiose derivative does not indicate that whether every two cellobiose units are linked to each other by β or α - glucosidic linkage.

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6) By the gentle acidic hydrolysis of cellulose , zechmeister in 1931 and miller in 1960 isolated ,in addition to cellobiose,cellotriose,-tetrose,-pentose,-hexose and – heptose and in all of these the C 1 -C 4 links have shown to be (from optical rotation calculations.) 7) Now since cellulose forms colloidal solution (property of high polymers) in the solvents in which glucose soluble, the cellulose is very large molecule. furthermore, since cellulose form fibers, e.g. rayon, it seems that the molecule is linear. The long length and linear structure of cellulose is confirmed by X-ray analysis. The absence of di -O-methyl glucose in the hydrolysis products of fully methylated cellulose further indicates that the cellulose molecule is linear polymer i.e. there is no branching in the chain.

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8) Thus on the basis of above points cellulose may be assigned the following structure.

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9) The cellulose is not planar but has a screw-axis, each glucose unit being at right angles to the previous one .the absence of free rotation about the C-O-C link due to steric effect and the close packing of atoms give rise to ridged chain molecule . the long chains are held together by hydrogen bonding, and thus cellulose has a three-dimensional brick work. This would produce strong fibers with great rigidity but no flexibility ,and consequently, although the fibers would have great tensile strength,they could not be knotted without shaping and thus they must possess flexibility.

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10) Now the only problem in establishing the structure of cellulose is the determination of the value of “n” i.e., the chain are the molecule size this is achieved by determining the molecular weight of cellulose It is important to note that although the first glucose unit in the upper structure and the last glucose unit in the lower structure have a free reducing group, cellulose does not exhibit the strong reducing properties of sugars since the reducing unit is present at the end of a very long chain and hence its properties are masked.

MOLECULAR WEIGHT OF CELLULOSE : :

MOLECULAR WEIGHT OF CELLULOSE : Due to insolubility of cellulose ,its molecular weight can not be determined by the common methods such as depression of freezing point and elevation of boiling point. However,some physical and chemical methods have been developed for determining the molecular weight of such polymers. A) CHEMICAL METHOD: All the commonly used chemical methods are based upon the determination of the proportion of end-groups(reducing or non-reducing or both) and comparing with the total number of units in the molecule. The important chemical method used for determining the molecular weight of cellulose are discussed.

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HAWORTH METHYLATION PROCESS: The method consists in complete methylation in an inert atmosphere, followed by hydrolysis with dilute acid to cleave all the glycosidic linkages.thus the non-reducing end will give 2,3,4,6-tetra-O-methyl D- glucose,while the all other units will be hydrolysed to 2,3,6-tri-O-methyl D-glucose.

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The two methylated sugars obtained by hydrolysis were separated by vaccum distillation,but column or paper chromatography is now used extensively. Hence by knowing the percentage (or ratio of tetramethyl / trimethyl derivative) of the tetramethyl derivative, the length of chain is estimated. The result obtained by this method indicate the presence of a chain having 100 to 200 D-glucose residues which correspond to the molecular weight of cellulose between 20000 to 40000.

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But the method always gives low molecular weight because it is not possible to methylate cellulose completely without degradation.however later on McGilvary (1953), using chromatographic methods,on the hydrolytic productsof methylated cellulose,obtained a value of 10000 units which is in agreement with the value obtained by physical methods. B) PHYSICAL METHODS: Several physical methods have been used for determining the molecular weight of polysaccharides. Among the important physical methods viscosity osmotic pressure, and the ultracentrifugal sedimentation measurements indicate that the untreated native cellulose has the molecular weight of the order of 300000 - 500000 and thus consists of a chain length of nearly 2000 - 3000 glucose units.

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From the foregoing account ,it is seen that the result obtained by different methods show some divergence.this indicates that the value of n is uncertain whichis due to the ease with which cellulose is degraded.furthermore,cellulose from different sources has a different chain length and actually the value of n may vary with the conditions under which the polysaccharides are synthesized by the plants

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