The tautomerism of 3(5)-phenylpyrazoles: an experimental (1H, 13C, 15N NMR and X-ray crystallography) study

3(5)-Phenyl- and 5(3)-methyl-3(5)-phenylpyrazole have been studied using multinuclear NMR spectroscopy at low temperature to determine the tautomeric equilibrium constants in the slow proton exchange regime by simple signal integration. In order to compare the results in solution with those in the solid state, the X-ray structure of a derivative of the first, namely 4-bromo-3-phenylpyrazole was determined [triclinic, P, a= 13.0867(8), b= 13.2546(7), c= 7.8079(3)Å, α= 100.015(4), β= 93.648(3), γ= 84.923(5)°, Z= 6]. The conclusions are that 3(5)-phenylpyrazoles exist in solution as mixtures rich in the 3-phenyl tautomer which is also the tautomer present in the solid state, whereas they form monomers which are hydrogen bonded to the solvent in liquids like THF and self associate in inert solvents.