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Abstract
The ultraviolet spectra of heterocyclic N-oxides of pyridine N-oxide series and the conjugated cationic acids (simple cations of protonated N-oxides) in acetonitrile as the representative of polar aprotic solvents, were determined. The obtained spectra of N-oxides studied (mainly tri-substituted pyridine N-oxides) and their cations obtained by the protonation of free N-oxides by the excess of perchloric acid, have been collected and discussed. Taking into account the spectroscopic results, a scheme of acid-base equilibria in polar aprotic solvents has been discussed. The influence of the traces of water on acid-base equilibria in nonaqueous media has been stressed.
Highlights
It is well known [1,2,3,4,5] that the derivatives of pyridineN-oxide in polar aprotic solvents such as acetonitrile, in addition to proton-exchange equilibria with the solvent leading to the formation of simple BH+ cations, Eq (1), form hydrogen-bonded complexes containing two molecules of base and one proton; the so-called cationic homocomplexes BHB+, Eq (2), and heterocomplexes BHB +, where B and B1 stand forN-oxide molecules.hetero-cyclic N-oxides form two types of salts with strong inorganic acids in the solid state i.e. simple salts containing cations of protonated N-oxides, BH+, and basic salts which contain homocomplex cations, BHB+ [6,7]
In the first step of this study the acetonitrile spectra for the simple perchlorates of N-oxides studied were recorded and compared with those obtained in acetonitrile solutions containing respective N-oxide and the excess of perchloric acid
It turned out that the spectral characteristics obtained for the acetonitrile solutions of simple cations in two various ways were different
Summary
Even in the cases of most basic N-oxides studied, the bands which are characteristic for free N-oxides were observed in the spectra of simple perchlorates. Since it is not possible that the free N-oxides appear in solutions studied in the case of alkyl-substituted pyridine N-oxides such as 2,6Me24MeOPyO, 2,4,6Me3PyO and 4C11H23PyO (for which the acidity constant in acetonitrile is expected to be of the order of that characteristic for pyridine N-oxide i.e. 10-11 and even lower, being too low to provide sufficient amounts of Noxide even for the lowest concentrations studied) the presence of the N-oxide bands can be explained in terms of the equilibrium between protonated N-oxides and water present in the non-aqueous solvent: BH+ + H2O →← B + H3O+
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