Spectroscopic and thermodynamic study of charge transfer interaction between vitamin B6 and p-chloranil in aqueous ethanol mixtures of varying composition

Charge transfer complexes of 1:1 stoichiometry have been found to form between vitamin B6 (pyridoxine hydrochloride) and a series of electron acceptors including p-chloranil. Since vitamin B6 is soluble in water while the electron acceptors are insoluble in water but soluble in ethanol, the medium chosen for study is water–ethanol mixture. From the trends in the CT absorption bands the vertical ionization potential of vitamin B6 has been determined to be 8.12 eV. The enthalpy and entropy of formation of the complex between p-chloranil and vitamin B6 have been determined by estimating the formation constant (K) spectroscopically at four different temperatures in 75% ethanol–water mixture. Again, the magnitude of K has been found to decrease noticeably with decrease in dielectric constant of the medium (as the percentage of ethanol in the aqueous–ethanol mixture is increased). A plausible explanation for this has been given in terms of hydrolysis of pyridoxine hydrochloride.