Acid-Base Kinetics of Pyridine Studied with a Slow Spectrophotometric Indicator in Methanol

Article abstract of DOI:10.1021/ja00543a029

The protonation-deprotonation kinetics of pyridine in methanol has been studied by the electric field jump technique.Kinetic measurements of the electric field insensitive equilibrium B + H⁺ (k₁) <*> BH⁺ (k_-1), where B denotes pyridine, were obtained by coupling the equilibrium with a field-sensitive, visibly colored indicator equilibrium.Use of the "slow" indicator 2,2',4,4',6,6'-hexanitrodiphenylamine allowed observation of the slower of the two relaxation times of the coupled system in a time range where measurements could be made with sufficient precision for a reliable extraction of the rate constants k₁ and k_-1 from the data.The values obtained are k₁ = 1.57 +/- 0.32 * 10¹⁰ dm³mol^-1s^-1 and k_-1 = 8.43 +/- 1.69 * 10⁴ s^-1.The value for k₁ is within the expected limits for a diffusion-controlled reaction in this solvent.An ionic reaction radius of 3.9 Angstroem is calculated from the protonation rate constant value, which indicates that the neutral species involved is probably a hydrogen-bonded pyridine-methanol complex rather than free pyridine.The value for k_-1 is 1 order of magnitude greater than the constant for methyl-substituted pyridines, reflecting unsubstituted pyridine's lower basicity in methanol.