Structures of l-Phenyl-3-methyl-pyrazolone-5 and its Benzoyl Derivatives+

The structures of l-phenyl-3-methyl-pyrazolone-5 and its benzoyl derivatives have been investigated by means of IR, UV, NMR, and mass spectral spectroscopy. 1-Phenyl 3-methyl-pyrazolone-5 is found to exist as a zwitter ion in the solid state, in the 5-keto form in chloroform and dioxane, and in the 5-hydroxy form in pyridine. The 4-benzoyl derivative of l-phenyl-3-methyl-pyrazolone-5 is shown to exist in two different tautomeric enol forms and not an enol form and a keto form as was formerly reported. The structures of two other benzoyl derivatives were investigated and assigned as the C-5-0-benzoyl derivative and C-4-benzoyl-C-5-0-benzoyl derivative, respectively.