The kinetics have been studied for the nitration of 3-hydroxy-5-methyl-4-nitro-I -phenylpyrazole and its Nand O-methyl derivatives at the para-position and of 3-hydroxy-5-methyl-I -p-nitrophenylpyrazole and its Nand O-methyl derivatives at the 4-position. All the compounds undergo a mechanistic changeover from nitration of the free base a t low acidity to nitration of the conjugate acid at high acidity. Rates are compared within the series studied and with those of other heteroaromatic compounds.FOLLOWING our work on l-phenyl-A3-pyrazolin-5-ones in the azolone series, we now report the kinetics of nitration of l-phenyl-A3-pyrazolin-3-ones. In view of the susceptibility of 3-hydroxy-phenylpyrazoles to nitration both at the 4-position of the pyrazole ring and at the para-position of the phenyl ring (cf. l-phenyl-A3-pyrazolin-5-ones) ,2 we studied the second nitration of the two mononitro-derivatives (3) and ( 5) of 3hydroxy-5-methyl-l-phenylpyrazole. l-Phenyl substituted-3-hydroxypyrazoles are tautomeric [ (9a) -(9b)],3 but at equilibrium the hydroxy-form is normally favoured. We included in our study the N-methyl compounds (4) and ( 6) [as models for the tautomeric form (9a)J and the methoxy-derivatives (11) and ( 12) [as models for (Sb)]. Attempts to prepare quaternised derivatives failed, as in the A3-pyrazolin-5-one series2 Preparation of Compounds.-Compounds of the 4-1 Part XL, A
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