Modification of methadone synthesis process step

In a process for preparing a mixture of 2,2-diphenyl-4-dimethylaminovaleronitrile and 2,2-diphenyl-3-methyl-4-dimethylaminobutyronitrile by the reaction of diphenylacetonitrile and 1-dimethylamino-2-chloropropane, the improvement which consists in carrying out the reaction in an inert polar, aprotic solvent having a dielectric constant greater than 28 at 25.degree. C. by forming the sodium salt of diphenylacetonitrile with sodium hydroxide and then adding thereto 1-dimethylamino-2-chloropropane.