In the prepn. of 3-phenyl-6-pyridazone (I), ammonium 2-hdroxy-4-oxo-phenylbutyrate (C6H4-CO-CH2-CHOH-COONH4) (II) is reacted with hydrazine hydrate in aq. soln. at 80-120 degrees C. (II) is pref. used in the form of an aq. soln. obtd. in situ by neutralising 2-hydroxy-4-oxo-4-phenylbutyric acid with aq. gaseous NH3 (pref. obtd. from the reaction of (II) with N2H4). (I) is useful as an intermediate for pharmaceuticals and plant protection agents and is prepd. in a single-stage process giving (I) in high yields, and practically free from by products. In an example, (I) is obtd. in 91% yield by adjusting a suspension of 2-hydroxy-4-oxo-4-phenylbutyric acid (97.1g) in water (450 ml) to pH 7.8 with conc. aq. ammonia, adding 99.2% N2H4.H2O (25.2 g), and refluxing for 4 hrs.
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