Abstract
Nitrations of 1-phenylpyrazole (I), 1-p-biphenylylpyrazole (II), and 1,5-diphenylpyrazole by "acetyl nitrate" (nitric acid – acetic anhydride) occur selectively in the 4-position of the pyrazole ring, as do brominations of I and II in chloroform solution. These results are in agreement with R. D. Brown's calculations of localization energies for electrophilic substitution in pyrazole.However, nitration of I by mixed acids at 12° yields 1-p-nitrophenylpyrazole, and bromination of I by bromine in concentrated sulphuric acid in the presence of silver sulphate yields 1-p-bromophenylpyrazole.The variations in orientation of substitution can be rationalized if the reacting species of I in strongly acidic solvents is the conjugate acid, in which the pyrazole ring is deactivated by protonation.
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