Nucleophile concentration, pyridinium

The rate constants for the reaction of a pyridinium Ion with cyanide have been measured in both a cationic and nonlonic oil in water microemulsion as a function of water content. There is no effect of added salt on the reaction rate in the cationic system, but a substantial effect of ionic strength on the rate as observed in the nonionic system. Estimates of the ionic strength in the "Stern layer" of the cationic microemulsion have been employed to correct the rate constants in the nonlonic system and calculate effective surface potentials. The ion-exchange (IE) model, which assumes that reaction occurs in the Stern layer and that the nucleophile concentration is determined by an ion-exchange equilibrium with the surfactant counterion, has been applied to the data. The results, although not definitive because of the ionic strength dependence, indicate that the IE model may not provide the best description of this reaction system. 

When R is primary alkyl, the second-order rate constant k2 is obtained by taking the slope of kobs vs. concentration of the nucleophile. The plot passes through the origin, indicating a pure SN2 mechanism without SN1 participation. The reference pyridinium ion is the 2,4,6-triphenyl derivative (because pyrylium precursors with phenyl substituents are more easily prepared) (82AHC(Suppl 2)1) but numerous other substituents have been introduced into the ring. Rate constant values reported in Table XIX, where release of steric strain has a major influence, are in agreement with the role of structural factors discussed in Section IV,A. 

Vinyl pyridine is nucleophilic and so starts the polymerization of vinyl pyridinium salts. In aqueous media polymerization, high-molar-mass poly-(vinyl pyridinium salts), II, are formed at high monomer concentrations by the normal vinyl mechanism, but polyadduct formation to oligomers. III, occurs at low monomer concentration ... 

Nucleophilic replacement of hydrogen is presumably being observed when reduction of a 3-nitropyridine to a 3-aminopyridine by stannous chloride and concentrated hydrochloric acid is accompanied by nuclear chlorina-tion so, 241a, 772-4, Thus, 4-amino-3-nitropyridine gives 3,4-diamino-2-chloropyridine soa and 4-ethoxy-3-nitropyridine yields 5-amino-2-chloro-4-ethoxypyridine24ia. Clearly, the pyridinium cations must be involved. 

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