THERMAL-PROPERTIES OF 1-PHENYL-3-METHYL-4-BENZOYL-5-PYRAZOLONE

1-Phenyl-3-methyl-4-benzoyl-5-pyrazolone (PMBP) is a beta-diketone and exists either as a yellow enol-form or as a colorless keto-form. The yellow PMBP exists as a mixture of two enol conformers. The enol form is known to form strong intramolecular hydrogen bonds where the shortening (O---O) and lengthening (O---H) distances are associated with a strong increase in the delocalization of the electrons in the O=C-C=C-OH pi-conjugated system. On the other hand, the characteristic structure in the keto form is the formation of a short intermolecular hydrogen bond [2.673 (3) angstrom] between an imino nitrogen atom and a carbonyl oxygen atom [N-H---O=C]. In the IR spectra of the PMBPs, the keto-form exhibits a broad strong band centered at 2550 cm-1, which is attributed to the NH stretching vibration of the intermolecular hydrogen bonds (N-H---O). The enol form also gives a strong band, at around 3100 cm-1, which is attributed to the OH vibrations of the intramolecular hydrogen bonds (OH---O). The DSC curves of the keto and enol tautomers show endothermic peaks at about 92-degrees-C and 121-degrees-C respectively, due to a phase change on melting. The obtained enthalpies were found to be 27 and 24 kJ/mol for the keto and the enol forms, respectively. The higher enthalpy and melting point of the keto form are due to the strong intermolecular hydrogen bonds.