THERMODYNAMIC AND ELECTROCHEMICAL PROPERTIES OF IMIDAZOLE-2-THIOLS (IMIDAZOLE-2(3H)-THIONES)

Abstract

The oxidation and reduction potentials for eight imidazole-2-thiols [imidazole-2-thiol (1a), 1-methyl-(1b), 4,5-dimethyl- (1c), 1,4,5-trimethyl- (1d), 4,5-diphenyl- (1e), benz- (1f), 5-methylbenz- (1g), and 5-nitrobenzimidazole-2-thiol (1h)] have been determined in ethanenitrile-hydrogen chloride solution. Substituent effects on E(ox) are observed and are discussed. The pK_a's for thiols 1a, 1b, and 1c have been determined as a function of pH in buffer solutions at five temperatures and constant ionic strength of 0.10 M using ultraviolet spectroscopy. The ΔH^o (kJ/mol) and ΔS^o (J mol⁻¹ K⁻¹) for imidazole-2-thiol (1a), 1-methylimidazole-2-thiol (1b), and 4,5-dimethylimidazole-2-thiol (1c) are 38.2 and –92.9, 34.1 and –107.5, and 45.2 and –80.3, respectively.

Key Words:

• Imidazole-2-thiol
• 1-methylimidazole-2-thiol
• 4,5-dimethylimidazole-2-thiol
• 1,4,5-trimethylimidazole-2-thiol
• 4,5-diphenylimidazole-2-thiol
• 1,3-dihydro-2H-benzimidazole-2-thiones
• benzim-idazole-2-thiol
• 5-methylbenzimidazole-2-thiol
• 5-nitrobenzimidazole-2-thiol
• oxidation and reduction potentials
• electrochemistry
• pK_a's for thiols
• equilibrium concentrations of thiols and thiolate anions
• ultraviolet spectroscopy
• thermodynamic parameters (ΔG^o, ΔH^o, ΔS^o)