Preparation of potassium acetylide
Alternative Names: potassium ethynide; ethynyl potassium
To a 2 litre three-necked flask fitted with a mechanical stirrer and surrounded by a cooling bath at —35° C, 1 liter of liquid ammonia and 0.2 g finely powdered hydrated ferric nitrate are placed. Then 1 g of clean potassium while stirring is introduced in small pieces, where a vigorous effervescence of hydrogen takes place, indicating the conversion of the metallic potassium into potassium amide, and the initial deep blue colour due to dissolved potassium rapidly changes to the black color of the suspension of finely-divided iron produced. 38 g of potassium are added in 1 g lumps over fifteen minutes (a useful alternative technique is to impale the lumps of potassium on the end of a stout iron wire and immerse them in the ammonia solution; in this way the possibility of the jamming of the potassium between the stirrer and the base of the flask is avoided). A pale grey suspension of potassium amide is thus obtained. A gaseous acetylene, which is purified by passage through a carbon dioxide-acetone-cooled trap and then through two half-filled 500 ml concentrated sulfuric acid wash bottles, preferably of the sintered glass type is passed via a simple mercury safety valve, into the stirred, cooled (—35° C) potassium amide suspension at a rate of 2-3 litres per minute until the grey reaction mixture becomes uniformly black. Sometimes this colour change is indefinite but passage of the acetylene at the above rate for 1 hour ensures complete conversion to potassium acetylide. The concentration of the solution of potassium acetylide in liquid ammonia produced by the above procedure is about 1M. The potassium acetylide solution thus prepared is ready for acetylene chemistry reactions.
Acetylenic compounds in organic synthesis, by R. R. Alexander, 193-194, 1955