COMPLETE THERMODYNAMIC ANALYSIS OF THE HYDRATION OF 13 PYRIDINES AND PYRIDINIUM IONS - SPECIAL CASE OF 2,6-DI-TERT-BUTYLPYRIDINE

An apparatus is described for measuring vapor pressures and deriving Henry's Law constants at a series of temperatures in order to obtain standard free energies and enthalpies and entropies of solution from the gas phase to water for a number of substituted pyridines. The results agree well with comparable published values. In view of its low volatility and low solubility, special strategies were required to study 2,6-di-tert-butylpyridine. By using these data for solution of the pyridine bases from the gas phase to water, and published values for the thermodynamics of protonation in the gas phase, a complete analysis of the ionization in water is presented for all 13 pyridines. The most interesting new data are those for the solvation of the 13 pyridinium ions. In terms of all these properties (and also the infrared hydrogen-bonding spectra of its salts presented here), 2,6-di-tert-butylpyridine is differentiated sharply from the rest of the series. All of the results support the proposal that steric hindrance to hydration of the 2,6-di-tert-butylpyridinium ion is largely responsible for the very low relative basicity in water in contrast to its high basicity in the gas phase. © 1979, American Chemical Society. All rights reserved.