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

Abstract

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.

Substituent Effects on the Hydrolysis of p-Substituted Benzonitriles in Sulfuric Acid Solutions at (25.0± 0.1) °C

“…In para derivatives of benzene the inductive and resonance effects can be expressed as the sum of two components [10,11]. Therefore, the logarithms of the observed rate constants (k obs ) of p-substituted benzonitriles were correlated by using different inductive (localized) and resonance (delocalized) substituent constants.…”

Improved Yukawa-Tsuno equation and the substituent effect on pyridine basicity

“…Later, analysis by the dual substituent parameter treatment 32 assigned ' R constants to the pyridinium ion system, and from a remark by Ellam and Johnson 33 a long controversy ensued. 13,29,[34][35][36][37][38] Basically, Johnson and co-workers 33,36,38 consider the pyridine reactivity to be closer to the benzoic acid reactivity rather than to the sigma-plus reactivities. Their contention was then supported only by Sawada et al 13 More recently, Reynolds et al 39 suggested that ' R BA and ' R scales could be replaced by a new ' R scale such that ' R BA values would be generated by linear interpolation between ' R and the new ' R values.…”

“…13,29,[34][35][36][37][38] Basically, Johnson and co-workers 33,36,38 consider the pyridine reactivity to be closer to the benzoic acid reactivity rather than to the sigma-plus reactivities. Their contention was then supported only by Sawada et al 13 More recently, Reynolds et al 39 suggested that ' R BA and ' R scales could be replaced by a new ' R scale such that ' R BA values would be generated by linear interpolation between ' R and the new ' R values. This suggests that the controversy could be settled by resorting to the YukawaTsuno analysis, insomuch that different r values associated with a single scale for resonance contributions would embrace the reactivities of benzoic acid, pyridinium ion, benzyl cation and a,a-dimethylbenzyl (or tertcumyl) cation derivatives.…”

“…Thus, in terms of the linear substituent free energy (LSFE) treatment, 56 Sawada et al 13 found the ratio & for the pyridinium ion system to be 'virtually identical' with the corresponding ratio for the benzoic acid system. We note that the LSFE correlation 13,56 treats meta and para derivatives separately at the cost of eight adjustable parameters and yields 13 & p i a& m i = 0.85 for the reference system, against our value of 0.961 AE 0.005 (see the Appendix). In turn, on the basis of the Taft-Topsom treatment of substituent effects, 57 Abboud et al 43 have demonstrated that polarizability effects are practically absent in aqueous solution where they found the para/meta ratio for the field effect in the pyridinium ion system to be 0.90.…”

“…10 On the basis of this model, a constrained tetralinear version of the Hammett equation was developed 11 which yielded values for the para/meta ratio of the field or inductive effect in excellent agreement 11 with Dewar et al 's model. 12 Then we referred to difficulties encountered while fitting data for the pyridinium ion series with sigma-plus constants, and to Sawada et al 's contribution 13 to the controversy on the proper scale to describe the pyridine reactivity.…”