Thermal properties of 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone.

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Other Title
  • １‐フェニル‐３‐メチル‐４‐ベンゾイル‐５‐ピラゾロンの熱的性質
  • 1 フェニル 3 メチル 4 ベンゾイル 5 ピラゾロン ノ ネツテキ セイシ
  • 1-フェニル-3-メチル-4-ベンゾイル-5-ピラゾロンの熱的性質

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Abstract

1-Phenyl-3-methyl-4-benzoyl-5-pyrazolone(PMBP) is a β-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 (0---H) distances are associated with a strong increase in the delocalization of the electrons in the O=C-C=C-OH π-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)Å] 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 2550cm -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 3100cm-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°C and 121°C respectively, due to a phase change on melting. The obtained enthalpies were found to be 27 and 24kJ/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.

Journal

  • BUNSEKI KAGAKU

    BUNSEKI KAGAKU 44 (1), 25-28, 1995

    The Japan Society for Analytical Chemistry

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