A FRESHMAN EXPERIMENT : PREPARATION ANDPROPERTIES OF ETHYLENE GUIDO H. STEMPEL, JR. Carnegie Institute of
Technology, Pittsburgh, Pennsylvania
THE method for preparing sm%llquantities of ethylene described here requires little apparatus, a minimum of time, and is safe enough and simple enough to be used in the freshman laboratory. This makes it feasible for tint-year students to prepare ethylene and to compare its properties with those of methane and acetylene, for which simple methods of preparation are well known. This synthesis of ethylene proceeds according to the equation: GH4Braf Zn = ZnBr2 G& The reaction is most easily carried out in a 50-cc. Erlenmeyer flask equipped with a gas-delivery tube. The following amounts of reactants are sufficient to prepare about 250 cc. of ethylene: one cc. of ethylene bromide (2.18 g.), 1.5 g. of Zu dust (excess), and five cc. of alcohol. After the reactants are mixed there is an induction period, sometimes as long as three minutes, before the reaction starts. The induction period can be considerably reduced by warming the mixture to about 60°C. Once the reaction begins it proceeds smoothly without further heating or attention and at such a rate
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that the ethylene is easily collected over water in test tubes. The middle fraction of the ethylene so prepared is better than 95 per cent pure. Ethylene can be used to demonstrate the usual properties of olefins. For example, it rapidly decolorizes a one per cent aqueous solution of potassium permanenate as well as a one per cent solution of bromine in either CC14or water. A simple instructive experiment is to invert one test tube of ethylene in a beaker of water and a second in a beaker of bromine water. The gas slowly reacts with the bromine to form liquid products, thus permitting the water to rise in the test tube. By agitating the test tube the absorption can be completed in about ten minutes. The tube of ethylene inverted in the beaker of water shows no change. In general, the laboratory preparation of large quantities of ethylene is more economically camed out by the standard method of alcohol dehydration. When only small quantities are required, the simplicity of the preparation from ethylene bromide gives this method preference.
