Polarizability Effects on the Aqueous Solution Basicity of Substituted Pyridines

A quantitative dissection of polarizability (P), field (F) and resonance (R) substituent contributions to the relative gas-phase and aqueous solution basicities of 2-, 3-, and 4-substituted pyridines (XPy) is described in this work. The standard-free-energy changes for the reaction XPyH+ + Py ⇄ XPy + PyH+ (eq 1) in the gas-phase (δ⊣G°g) and in aqueous solution (δ⊣G°g) have been analyzed. (δ⊣G°g has been found to be the sum of F, R, and ? while δ⊣Gaq“, only depends on F and R. 5AG°g corrected for polarizability, (δ⊣G°g-P) is a linear function of δ⊣Gaq,. This relationship holds for substituents in any of the three positions (ortho, meta, para), including 2-mono- and 2,6-disubstituted pyridines. These results show that the fundamental differences between gas-phase and solution basicities of pyridine are (i) the essentially complete disappearance of polarizability effects in solution and (ii) an attenuation (by a factor of ca. 2.3) of field and resonance contributions. © 1988, American Chemical Society. All rights reserved.