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561 g (5.5 mol) of acetic anhydride were cooled to 10° C. Then, a solution of 164 g (2.0 mol) of phosphorous acid and 300 ml of acetic acid were added dropwise in the course of one hour, with stirring and cooling. 302 g (2.0 mol) of N-hydroxymethylbenzamide were then added in portions in the course of 50 minutes at 10°-15° C., with cooling. Stirring was continued at 10° C. for 20 minutes. The reaction mixture was subsequently kept under reflux for 2.5 hours, followed by cooling. 61 g (3.39 mol) of water were then added dropwise at 25°-30° C., with cooling and stirring. After this, the mixture was refluxed again. After cooling, the mixture was stirred overnight and solids were then filtered off with suction. 320 g (approx. 75% of theory) of crude benzoylaminomethanephosphonic acid were obtained. Further crude acid could be obtained from the mother liquor.

1. temperaturecooling
2. temperaturewith cooling
3. stirringStirring
4. temperatureunder reflux for 2.5 hours
5. temperatureby cooling
6. temperaturewith cooling
7. stirringstirring
8. temperatureAfter this, the mixture was refluxed again
9. temperatureAfter cooling
10. stirringthe mixture was stirred overnight
11. filtrationsolids were then filtered off with suction