Diketene: Acetone Pyrolysed in Copper at 650 C, Ketene Caught Cold in Acetone and Left to Dimerise (Law, Carbide, US 2,019,983)

Hot acetone splits into ketene, a very reactive gas that turns to tar if it stays warm. Law chilled the gases at once so the ketene dissolved in the cold unchanged acetone, only a few per cent strong, and then let the solution warm slowly: in that dilute state the ketene pairs up into diketene instead of tar. Result: diketene, a water-white liquid boiling at 30 C under 12 mm. Source: Law, US 2,019,983 (Carbide and Carbon, 1935). Not given: yield and batch size. Picture drawn by Youblob.