STUDIES ON CORRELATIONS OF ACID-BASE PROPERTIES OF SUBSTITUTED PYRIDINE N-OXIDES IN SOLUTIONS .1. CORRELATIONS OF THE PK(A) VALUES IN NONAQUEOUS 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 includ ing aprotic (protophilic and protophobic), as well as amphiprotic solv ents. The solvents studied have been arranged in the following sequenc e of increasing acid strength of the protonated N-oxides: acetonitrile < benzonitrile < propylene carbonate < nitromethane < acetone < metha nol < N,N-dimethylformamide < dimethyl sulfoxide. The sequence of the pK(a) values of the cationic acids in these solvents has been found to be the same as in water. Based on this observation linear relationshi ps between the aqueous and non-aqueous pK(a) values were established. Values of correlation coefficients r provided a criterion for the clas sification of the media into two classes. The first class involved tho se solvents in which a high accuracy of determination of the acid-base equilibrium constants could be achieved, while the other one encompas sed the media which limited significantly the precision of results. In terpretation of the slope a and intercept b in these correlations has been given as well. Application of the parameters a and b as a criteri on for differentiation of aprotic solvents (as protophilic and protoph obic ones) has been suggested. Some examples for using the correlation of this type to the indirect determination of aqueous pK(a) values ba sed on the non-aqueous ones have been presented. It has been concluded that the good correlations between the pK(a) values in aqueous and no n-aqueous solutions enables the estimation of chosen value (aqueous or non-aqueous) with satisfactory accuracy even in cases when the experi mental limitations make their precise determination impossible.