86 PRACTICAL ORGANIC CHEMISTRY Acetylene. C 2 H 2 . Acetylene can be readily obtained by the action of water on calcium car-bide: since, however, commercial calcium carbide contains traces of calcium CaC 2-f- 2HOH = Ca(OH) 8 + C 2 H 2 sulphide, phosphide and nitride, the acetylene should be purified before use. If a specially pure sample of acetylene is required, it should be obtained by the action of alcoholic potassium or sodium hydroxide on i,2-dibromoethane: this reaction should be compared with that of aqueous potassium or sodium hydroxide, which merely hydrolyses i,2-dibromoethane to ethylene glycol. C 2 H 4 Br 2- aKOH (ethanolic) CJI 2-f aKBr- 2H 2 O C 2 H 4 Br 2 + 2KOH (aqueous) = C 8 H 4 (OH) 1-f zKBr If acetylene is required either in large quantities or in a steady regular current for several hours, it should be obtained from a cylinder, in which the acetylene is compressed in acetone solution. The cylinders can be purchased and when empty can then be exchanged for refilled ones. The acetylene in
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