Specific interactions of pyridinium ions and pyridine molecules in nitrobenzene. Substituent effects

Abstract

From conductance measurements of a series of pyridinium picrates in nitrobenzene solutions, the equilibrium constants of the following hydrogen bonding reactions were estimated for each system: 1. the molecular acid-base association constant ▪ 2. the ionic association constant of K a, ▪ ( 3) the homoconjugation constant K + 1 ▪ In addition to these, the formation of 1:2, complexes between the pyridinium ion and the pyridine molecule is observed in the case of 3,4-dimethylpyridine and 4-dimethylamino pyridine. This may be explained in terms of π-π interactions between the second pyridine molecule and that already bound to the cation by a hydrogen bond. Plots of log K f, log K a and log K + 1 vs. aqueous pK a s values of the pyridines yield straight lines with slope of 1.167, −0.218 and 0.061 respectively. The results show that the hydrogen bond formation between pyridine and picric acid in nitrobenzene increases with increasing basicity of the pyridine molecule. The value of the first slope indicates that the proton transfer is important. The ionic association between the pyridinium cation and the picrate increases with increasing acidity of the cation. In the case of the formation of the homoconjugate cation PyH +⋯Py it was expected that the substituent effects will be small, because the increasing proton donor ability of the cation will be compensated by the decreasing proton acceptor ability of the conjugate base. It is interesting to note that, contrary to this expectation, the stability of the non- ortho substituted homoconjugate cation increases, although to a small extent, with the basicity of the pyridine. This is explained by considering the potential energy curve of the proton for the homoconjugate cation. The increase of the depth of the potential well resulting from the hydrogen bond formation is more important when the pyridine is more basic.