A solution of 45 g. (0.25 mole) of trans-stilbene in 750 ml. of ether is prepared in a 1-l. three-necked round-bottomed flask fitted with a reflux condenser, an efficient mechanical stirrer, and a dropping funnel. To the well-stirred solution there is added 13.8 ml. (43 g.; 0.27 mole) of bromine, during the course of 10 minutes. A solid begins to separate in 5 minutes, but stirring is continued for 1 hour. The product is collected on a Büchner funnel and washed with ether until it is white. The yield of stilbene dibromide, melting at 235–237°, is 65.8–69.1 g. (77–81%).

A solution of 90 g. of potassium hydroxide in 150 ml. of absolute ethanol is prepared in a 500-ml. round-bottomed flask fitted with a reflux condenser (Note 1). The solution is cooled somewhat, and the stilbene dibromide is added in several portions (Note 2). The mixture is then refluxed for 24 hours in an oil bath (Note 3). The hot mixture is poured into 750 ml. of cold water, and the product is removed by filtration and washed with 50 ml. of water. The crude diphenylacetylene is dried over calcium chloride in a vacuum desiccator for 18 hours at room temperature. The chunky, pale yellow crystals melt at 58–60° and weight 33.7–37.8 g. By recrystallization of this product from 50 ml. of 95% ethanol there is obtained 29.2–30.5 g. (66–69% based on the stilbene) of pure diphenylacetylene, which forms white needles melting at 60–61° (Note 4).