1) Prepare 2 solutions as follows - (A) Dissolve 5 parts N-(4-piperidyl) propionanilide, 6.85 parts sodium carbonate, 0.05 parts potassium iodide in 120 parts hexane. (B) Dissolve 3.8 parts B-phenylethylchloride in 24 parts 4-methyl-2-pentanone.
2) Place solution A in a 3 necked flask equipped with a dropper, a mechanical stirrer, and a reflux head. Turn on the stirrer and add solution B slowly dropwise until all is added. Plug the dropper neck, turn on the heating mantle, and reflux for 27 hours, with constant stirring.
3) Filter the mixture while hot and evaporate the liquid on a steam bath.
4) An oily residue remains which is dissolved in 160 parts diisopropyl ether and filtered several times until clear.
5) Concentrate this solution to a volume of about 70 parts.
6) Cool liquid for about two hours at a temperature of about 0°C to yield the fentanyl.