SYNTHESIS OF DIPHENYLACETYLENE

CAUTION: Great care should be taken in the handling of hydrazine hydrate since it is highly corrosive and a suspected carcinogen; protective gloves and a face visor should be worn.

Benzil dihydrazone

• A solution of 105.1 g (0.5 mol) of benzil in 325 ml of propan-l-ol is prepared in a round-bottomed flask which is fitted with a reflux condenser. 

• To this solution 76 g (1.30 mol) of 85 per cent hydrazine hydrate is added and the mixture heated under reflux for 60 hours. 

• The solu­tion is cooled in an ice bath and the benzil dihydrazone is separated by suc­tion filtration. The crystals are washed with 200 ml of cold absolute ethanol and dried on the suction filter for 1 hour. The yield of benzil dihydrazone is 99-106 g (83-89%), m.p. 150-151.5 °C.

Diphenylacetylene:

STEP 1: Reagent grade copper(t) chloride (0.403 g, 4.0 mmol) is dissolved in a mixture of dry dichloromethane (20 ml) and pyridine (2 ml, CAUTION) to give a yellow—green clear solution. 

STEP 2: Oxygen gas from a gas burette is introduced to the solution with vigorous stirring. 

STEP 3: Rapid absorption of oxygen (about 20 ml) ceases in 15 minutes, the mixture becoming a dark green suspension. 

STEP 4: To the mixture is added dropwise a solution of benzil dihydrazone (0.237 g, 1.0 mmol) in dichloromethane (10 ml) over a period of 10 minutes at room temperature with stirring. 

STEP 5: The slightly exothermic reac­tion proceeds smoothly with nitrogen evolution. Stirring is continued for 1 hour under an oxygen atmosphere to ensure the completion of the oxidation. 

STEP 6: The mixture is hydrolysed with 3 M hydrochloric acid (40 ml), the organic layer is separated, and the aqueous layer extracted twice with dichloro­methane. 

STEP 7: The combined organic layer and extracts are washed with brine, dried over magnesium sulphate, and evaporated to leave the crude diphenyl­acetylene, which is purified by column chromatography (silica gel, hexane), to give 0.173 g (97%) of pure diphenylacetylene, m.p. 59-60 °C.