Studies on correlations of acid-base properties of substituted pyridine N-oxides in solutions. Part 1. Correlations of the pKa values in non-aqueous solvents and water

The strength of cationic acids conjugated with substituted pyridine N-oxides has been compared in a number of polar non-aqueous media including aprotic (protophilic and protophobic), as well as amphiprotic solvents. The solvents studied have been arranged in the following sequence of increasing acid strength of the protonated N-oxides: acetonitrile < benzonitrile < propylene carbonate < nitromethane < acetone < methanol < N,N-dimethylformamide < dimethyl sulfoxide. The sequence of the pKa values of the cationic acids in these solvents has been found to be the same as in water. Based on this observation linear relationships between the aqueous and non-aqueoues pKa values were established. Values of correlation coefficients r provided a criterion for the classification of the media into two classes. The first class involved those solvents in which a high accuracy of determination of the acid-base equilibrium constants could be achieved, while the other one encompassed the media which limited significantly the precision of results. Interpretation of the slope a and intercept b in these correlations has been given as well. Application of the parameters a and b as a criterion for differentiation of aprotic solvents (as protophilic and protophobic ones) has been suggested. Some examples for using the correlation of this type to the indirect determination of aqueous pKa values based on the non-aqueous ones have been presented. It has been concluded that the good correlations between the pKa values in aqueous and non-aqueous solutions enables the estimation of chosen value (aqueous or non-aqueous) with satisfactory accuracy even in cases when the experimental limitations make their precise determination impossible.