Samuel H. Wilen Chester 6. Kremer and Irving Waltcher
The City College New York
Polystyrene-A
For the undergraduate organic chemistry laboratory
A n unusually stimulating preparative sequence carrird out in the second semester organic laboratory is the multistep synthesis of polystyrene, starting effectively from benzene. Benzene is first acetylated with acetic anhydride. This standard acetophenone preparation is followed by a sodium horohydride reduction to phenylmethylcarbinol (l-phenylethanol).1,2 The latter is isolated and purified by vacuum distillation. Student yields in the reduction are of the order of 6&65y0. Dehydration of phenylmethylcarbinol with potassium hydrogen sulfate leads to styrene in yields of 70-8570'o. Attempts to dehydrate the alcohol with sulfuric acid, phosphoric acid, or anhydrous zinc chloride lead directly to polystyrene. If desired the styrene may be purified by vacuum distillation, but in practice the crude monomer is dissolved in an arene, e.g., toluene or xylene, and dried conventionally or by azeotropic distillation (e.g., distillation of about 10% of the solvent or until the distillate is clear); and the solution is used
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CHAIKIN, S. W., A N D BROWN, W. G., J. Am. Chem. Sm., 71, 122 11!349>~ --\-.- ,"Sodium Borohydride, Potassium Borohydride," Metal Hydrides Inc., Beverly, Mass., 1958.
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Multistep Synthesis
in the next step. In either case, styrene is characterized through formation of the solid adduct obtained by addition of bromine. The styrene is then polymerized by refluxing its solution in the arene with a small amount of dibenzoyl peroxide. A one hour reflux period is sufficient to give reasonable conversion to polystyrene. The reaction mixture is added to excess methanol and the precipitated polymer isolated by decanting the solvent mixture, filtering if necessary, and triturating the solid with fresh methanol. The final step gives from 2 to 2.5 g of product from an average of 6.5 g of styrene. Tine conversion depends upon the length of t,imeallowed for reflux. The sequence requires approximately four 4-hr laboratory periods when benzene is the starting mat,erial. If desired, acetophenone may serve as the starting material with a reduction in time of at least one laboratory period. This time can be reduced further by not drying the styrene before polymerization. Although the sequence is somewhat lengthy, the time invested in it is felt not to be excessive inasmuch as it exposes students to a wide variety of techniques, reactions, and compounds: FriedelLCrafts acylation,
reduction with a metal hydride, dehydration of an alcohol, controlled polymerization, vacuum distillation of an easily dehydrated compound, drying by azeotropic distillation, and trituration. It may be mentioned that in the authors' classes in which this sequence has been carried out, student interest has been high and good results have been obtained by nearly all students. The sequence (simplified)is summarized as follows:
The Experiment
Phmylmethylcarbinol. Over a period of approximately 15 minutes, 12.0 g of acetophenoneais added by drops to a solution of 1.20 g of sodium borohydride' in 25 ml of ethyl alcohol contained in a 125-ml Erlenmeyer flask. The flask is swirled and the temperature is maintained below 50' by external ice cooling if necessary. A white precipitate forms during the course of the reaction. After the reaction mixture has stood for fifteen minutes at room temperature, a total of 10 ml of a 10yo hydrochloric acid solution is added by drops. During the course of this addition, excess sodium borohydride is decomposed with evolution of hydrogen gas and most of the white solid dissolves. The supernatant is decanted, 5 g of solid anhydrous potassium carbonateE is 'Far typical directions see AD AM^, R., AND JOHNSON, J. R., "Laboratory Experiments in Organic Chemistry," 4th ed., The R. Maemillan Co., Inc., New York, 1949, p. 311; BREWSTER, Q., VANDERWERF, C. A,, AND MCEWEN,W. E., "Unitiaed Experiments in Organic Chemistry," D. Van Nostrand Co., Ino., Orgrtnio Princeton, N. J., 1960, p. 163; VOGEL,A. I., "Pr&ctic&l Chemistry," 3rd ed., Longmans, Green and Co., New York, 1956, p. 729. 'Metal Hydrides Inc., Beverly, Mass. Current price is 510/100 g. The hydride should be kept off the skin, and goggles should be worn when it is handled (see footnote 2).
added, and the solution is concentrated on a water-bath until two layers are visible in the distilling flask. At this point, most of the ethanol will have distilled. The two layers (one aqueous, the other consisting principally of the reduction product) are extracted with ether (40 ml total) and the ether extract dried over anhydrous potassium carbonate. After separation from the drying agent, fresh anhydrous potassium carbonate is added to the ether solution and the ether is removed by distillation on the steambath.% With the solid potassium carbonate still present,qhe residue is vacuum distilled from an oil bath, bp 102.5-103.5°/19 mm, 97'/13 mm. Styrene. A mixture of 10 g of phenylmethylcarbinol, 1.0 g of potassium hydrogen sulfate and 0.1 g powdered copper metal7 is placed in a 125-ml distilling flask fitted with a short condenser. The mixture is distilled from an oil bath whose temperature is gradually (10 minutes) brought up to 200-220". The vapor temperature should not exceed 130'. The distillate is collected in a 10-ml graduate cylinder. The yield of crude styrene may be estimated from the volume of the organic layer (density of styrene = 0.91) or from the volume of the aqueous layer. For isolation of the styrene, the organic layer is taken up in 30 ml of ether, the ether extract washed with saturated aqueous sodium carbonate, dried over anhydrous calcium chloride and distilled from copper powder, first on the steam-bath and then under vacuum, bp 4S0/20 mm. If styrene is not to be isolated, the original distillate containing the crude styrene is diluted with three times its volume of toluene or xylene (commercial mixture), the small aqueous layer separated, and the styrenearene solution dried either over anhydrous calcium chloride or by distillation of the arene until the distillate is clear (distilling 10-20 ml is generally sufficient). Polystyrene. To a solution of styrene in an arene containing approximately 6.5 g of styrene, 0.1 g of dibenzoyl peroxide8 is added. After refluxing for one hour (longer reflnxing leads to a greater conversion), the solution is poured slowly into 150-200 ml of methanol. The methanol is decanted from the white solid material or filtered with suction if necessary. The solid is triturated with fresh methanol until it is tractable, then filtered and dried. A small portion of the polystyrene so prepared may be redissolved in toluene and a film cast by allowing the solvent to evaporate on a glass or metal plate. Styrene dibromide (I$-Dibromophenylethane). To two or three drons of crude styrene in a test tube there is added a 5% biomine in acetic acid solution until an orange color persists. The orange solution is poured into 5-10 ml of water to precipitate the solid styrene dibromide. The solid is isolated by suction filtration with a Hirsch funnel, washed with cold water, and recrystallized from aqueous ethanol, mp 73'. Traces of acid would result in dehydration of the alcohol. At this stage, two students usually combine their samples and perform the vacuum distillation together. 7 Addition of copper powder to the dehydration mixture reportedly reduces the tendency to polymerize; BERGMANN, E. D., AND KATZ, D., J. Chem. Soc., 3216 (1958). 8 It is safer to purchase dibenzoyl peroxide in, and dispense it from, cardboard containers than screw cap jam. Volume 38, Number 6, June 1961
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