Process for preparing benzophenone-azines

A process for continuously preparing benzophenone-azines comprising oxidizing benzophenone-imines with molecular oxygen or a molecular oxygen-containing gas in the presence of a copper halide catalyst, wherein the oxidation reaction is carried out in a multistage system under an oxygen partial pressure decreasing from the former stage to the latter stage and at a conversion gradually decreasing from the former stage to the latter stage, while controlling the conversion so that the molar ratio of the benzophenone-imines to the copper halide in the final stage be maintained at from 1 to 3, wherein the partial pressure of oxygen is lower in the latter stage than in the former stage within such a range that the partial pressure of oxygen in the final stage is from 0.1 to 1 atm and that in the former stage or stages is from 0.3 to 10 atm, and wherein the conversion in the final stage is from 5 to 30% and wherein the molar ratio of the benzophenone-imines to the copper halide in the former stage or stages is from 2 to 20.