E9_Synthesis_of_Rayon - EXPERIMENT 9 Synthesis of Rayon...

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83 EXPERIMENT 9 Synthesis of Rayon Objective: Rayon will be synthesized and some of its physical properties examined. Introduction: Polymers are compounds in which chains or networks of small repeating units form very large molecules. For example, the polymer polypropylene is a long chain molecule, Polypropylene H 2 C CH H 2 C CH H 2 C CH H 2 C CH H 2 C CH CH 3 CH 3 CH 3 CH 3 CH 3 . . . . . . Figure 9.1 composed of the repeating unit —CH 2 —CHCH 3 —, so the formula of polypropylene is often written as Polypropylene H 2 C CH CH 3 n Figure 9.2 where n is a very large number. Polymers are composed of small repeating units because of the manner in which they are formed. A polymerization reaction involves the repetitive bonding together of many smaller molecules, called monomers. Before chemists realized how to create synthetic polymers of industrial value, they converted natural polymers into useful products. One of the first successes was the conversion of cellulose from cotton residues into a modified cellulose fiber know as Rayon. Rayon had a luster similar to that of silk, but could be produced much more cheaply than natural silk.
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Experiment 9: Synthesis of Rayon 84 This experiment demonstrates the most important steps in the production of Rayon by the cuprammonium process. Today, most commercially produced Rayon is made according to a more sophisticated variation of the method used in this experiment. Cellulose (see Figure 9.3) is a major constituent of wood and cotton fibers and is one of the most abundant organic compounds on earth. The molecular weight of native cellulose is several hundred thousand amu (so n is several thousand). Native cellulose is useful for the production of paper, but not as a fiber for the production of textiles. Cellulose n O H O H H OH H OH H HOCH 2 Figure 9.3 Native cellulose is not soluble in common solvents. It does dissolve in an aqueous ammoniacal solution of tetraamminecopper(II) hydroxide, Cu H 3 N H 3 N NH 3 NH 3 2+ 2 OH - Tetraamminecopper(II) hydroxide Figure 9.4 All the processes that lead to dissolution of native cellulose in a solution of tetraamminecopper(II) hydroxide are not known in detail. However, two of the essential ones are known. The first of these is the deprotonation of some of the C—OH groups in cellulose by the very basic tetraamminecopper(II) hydroxide solution to create alkoxide ions,
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Experiment 9: Synthesis of Rayon 85 O H O H H OH H OH H HOCH 2 O H O H H OH H OH H HOCH 2 + OH - O H O H H OH H OH H HOCH 2 O H O H H OH H OH H - OCH 2 + H 2 O alkoxide ion The cellulose chain becomes electrically charged and capable of interacting strongly with the water molecules in the solvent via hydrogen bonds, O H O H H OH H OH H HOCH 2 O H O H H OH H OH H - OCH 2 H O H
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Experiment 9: Synthesis of Rayon 86 Secondly, it is known that dissolution of cellulose is aided by cleavage of large cellulose molecules (300,000 amu) into smaller molecules (60,000 amu). The cleavages occur when water reacts with the so-called glycosidic C—O—C bond, O H O H H OH H OH H HOCH 2 O H O H H OH H OH H HOCH 2
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