A Study of Acid-Base Equilibria in Acetonitrile Systems of 2-Halo(Cl,Br,I)-4-nitropicoline(3,5,6) N-oxides

Abstract

An attempt has been made to determine potentiometrically (1) acid dissociation constants of cations obtained by protonation of nine trisubstituted pyridine N-oxides, namely 2-halo(Cl, Br and I)-4-nitropicoline N-oxides with the methyl group at positions 3, 5, and 6, as well as (2) the cationic homoconjugation constants of these cationic acids with conjugated N-oxides in acetonitrile. On the basis of the substitution effect, variations of the acid dissociation constants of the trisubstituted pyridine N-oxide cations are discussed. The determined pK a values of the protonated 2-halo-4-nitropicoline N-oxides are compared with the previously determined equilibrium constants of the cationic acids conjugated with the mono-and disubstituted pyridine N-oxides in acetonitrile. Further, based on the pK a values of the protonated 2-halo-4-nitropicoline N-oxides in acetonitrile, supplemented with correlations between pK a 's of the protonated mono-and disubstituted pyridine N-oxides in acetonitrile and water, the pK a 's of the acids conjugated with the trisubstituted N-oxides studied in aqueous solutions have been estimated. Moreover, it has been concluded that the cationic homoconjugation constants cannot be determined by potentiometric titration in acetonitrile solutions of the 2-halo-4-nitropicoline N-oxide systems.

Potentiometric and ab initio studies of acid–base interactions of substituted 4-halo(Cl,Br)pyridine N-oxide systems

“…The differences in pK a values for the four pairs of the Noxides are (0.1-0.2) pK a units. These differences are consistent with predictable induction effects (chlorine is more electronegative than bromine) and the experimentally documented substituent effect [44] in pyridine N-oxides containing either the chlorine or bromine atom at position 2. Table 1 lists also pK W a values of the pyridine N-oxides in water.…”

“…The calculated correlation coefficients ranged from 0.62 to 0.84. Similar with pyridine N-oxide [44], the highest correlation coefficients were obtained for protonation energies at the MP2 level and the lowest for the free protonation energy at the RHF level. It should be emphasized that the quality of the correlations improves only slightly upon going from the RHF to the MP2 level.…”

DFT study of the molecular structures, infrared and Raman spectra of 1,5-dichloropenta-1,4-diyn-3-one, 1,5-dibromopenta- 1,4-diyn-3-one, 1,5-diiodopenta-1,4-diyn-3-one, their sulfur, and selenium analogues

“…It is interesting at this stage to compare the calculated parameters of the 1,5-dihalogenoketones with their parent ketones (4,5). This is because selective substitution of hydrogen by halogen is of considerable importance in organic chemistry (29). Analysis of the data from this work indicates that halogen for hydrogen substitution leads to only small steric and geometric perturbations with…”

Experimental and Theoretical Studies of Acid−Base Equilibria of Substituted 4-NitropyridineN-Oxides

“…The above linear relationship was derived from a previously established 36 equation: pK a AN ) 1.83pK a W + 8.56 after accounting for the literature 37,38 pK a W values for some trisubstituted pyridine N-oxides. The pK a W values obtained in this way are summarized in Table 1 together with those determined by the potentiometric method in acetonitrile, pK a AN .…”

“…In view of the lack of p K a values in aqueous solutions ( ) for the majority of the cationic acids investigated, these were estimated from linear relationships between p K a 's in acetonitrile, , and those in water, , of the cationic acids conjugated with mono- and disubstituted pyridine N -oxides: The above linear relationship was derived from a previously established equation: = 1.83 + 8.56 after accounting for the literature , values for some trisubstituted pyridine N -oxides. The values obtained in this way are summarized in Table together with those determined by the potentiometric method in acetonitrile, .…”