Influence des effets de substituants sur la protonation de la pyridine

Standard thermodynamic values of proton ionisation of 3 substituted (Cl?, Br?, I?, C₂H₅? and CH₂CN?) pyridine derivatives are determined at 25°C, in an aqueous medium of ionic strength 0.5 M KNO₃.Free energies are deduced from equilibrium constants log K potentiometrically calculated. Enthalpies are obtained from calorimetric measurements.ΔG^o, ΔH^o and ΔS^o values are discussed in the context of the results of our earlier studies on this subject.The Hammett plot corresponding to the thirteen systems examined gives the proton ionisation constant of a substituted pyridine from the equation log K = 5,48 – 5,94Σσ.Besides, a linear relationship is found between ΔG^o and ΔH^o, which confirms the observation made previously in other series of pyridine derivatives.