Reactivity values pyridinium ions

One-third of all pyridine electrochemical citations deal with the electrolysis of quaternary salts of pyridines two out of five cathodic reports are concerned with them. Moreover, the products of reduction and the salts themselves are commercially valuable. A whole class of biochemical transformations depends on the reactivity of pyridinium ions. Agricultural products are also derived from these salts, and the value of bipyridiniiim herbicides is directly linked to their redox chemistry. 

However, an evaluation of the observed (overall) rate constants as a function of the water concentration (5 to 25 % in acetonitrile) does not yield constant values for ki and k2/k i. This result can be tentatively explained as due to changes in the water structure. Arnett et al. (1977) have found that bulk water has an H-bond acceptor capacity towards pyridinium ions about twice that of monomeric water and twice as strong an H-bond donor property towards pyridines. In the present case this should lead to an increase in the N — H stretching frequency in the o-complex (H-acceptor effect) and possibly to increased stabilization of the incipient triazene compound (H-donor effect). Water reduces the ion pairing of the diazonium salt and therefore increases its reactivity (Penton and Zollinger, 1971 Hashida et al., 1974 Juri and Bartsch, 1980), resulting in an increase in the rate of formation of the o-complex (ik ). 

The DSP approach nicely answers the controversial question about which substituent parameters should be employed to correlate pKa data for 4-substituted pyridinium ions. Statistically, the best correlation is given by Eq. (9), which has values to measure the resonance contribution of a substituent, a result in keeping with chemical intuition. This correlation is statistically superior to a Hammett treatment, where both resonance and inductive effects of a group are combined into a single parameter, p or ap.53,54 Moreover, now it is possible to rationalize why a simple Hammett treatment using ap works so well. Equation (9) reveals that the protonation equilibrium is much more sensitive to an inductive effect (p, — 5.15) than to a resonance effect (p = 2.69). Hence, substituent parameters, such as erp, which are derived from a consideration of the dissociation constants for benzoic acids where resonance contributions are small serve as a useful approximation. The inductive effect is said to have a larger influence on pKa values for pyridinium ions than for benzoic acids because the distance between the substituent and the reactive site is shorter in the pyridine series.53... 

Acidity data for 2-substituted pyridinium ions may be correlated using a Hammett equation and am values. Although it is not obvious that am parameters ought to be applied to a reaction series in which a substituent and the reactive site are in an ortho relationship, the correlation clearly shows that inductive effects have an important influence on acidities. 

Overall, the DSP correlations of the three sets of pKa data for pyridinium ions lead to a most interesting conclusion. The ratio of p values for the resonance to the inductive effect decreases from 0.52 to 0.43 to 0.13 for the 4-, 3-, and 2-series, respectively. The inductive effect increases in importance over the resonance effect as the distance between substituent and reactive site diminishes. 

Spectro H2O t = 25.0 1=0.005-0.025 to fully deprotonated D, neutral to fully deprotonated. Also reported the corresponding data for picolinic and isonicotmic acids. Jellinek HHG and Urwin JR, Ultraviolet absorption spectra and dissociation constants of picolinic, isonicotmic acids and Uieir amides, /. Phys. Chem., 58,548-550 (1954). Used glass electrode to measure pH values and recorded spectra with a Hilger Uvispek spectrophotometer. Ionic strengtti was extrapolated to zero wiBi Bie Debye-Huckel equation. Fischer A, Galloway WJ and Vau an J, Structure and reactivity in the pyridine series. I. Acid dissociation constant of pyridinium ions, /. Chem. Soc. B, 3591-3596 (1964). All data were cited in Perrin Bases no. 1076-77 Perrin Bases suppl. no. 5081. 

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