The Substituent Effect. 15. The LSFE Treatment of Substituent Effects on Basicities of Pyridines

The relative dissociation constants of meta and para substituted pyridinium ions in H2O at 25 °C were collected from a number of sources. To understand the actual substituent effects with a combination of inductive (I) and pi-electronic (Pi) effect components, the LSFE equation has been employed statistically for each of the meta and para series. ΔpK_a=ρ_iσ_i+ρ_π^+σ_π^++ρ_π^-σ_π^- The results illustrate (1) the I effect from the meta position is 1.2 times more effective than that from the para position, (2) the Pi effects from the para position have clearly different sensitivities for pi-donor (−R) and pi-acceptor (+R) classes of substituents; more exalted and more reduced Pi effects than that involved in a σp° constants for −R and +R groups, respectively. Compared with earlier results from the Taft’s DSP and relevant treatments, the soundness of the dual ρπ approach and, further, the superiority of the LSFE treatment have been demonstrated for the description of substituent effects on basicity changes in pyridines.