NH-acidities and Hammett correlation of 3-para substituted phenyl-1,2,4-oxadiazol-5(4 H)-ones and 1,2 λ^4 3,5-oxathiadiazole 2-oxides in nonaqueous media

NH acidities of some 3-( p -substitutedphenyl)-1,2,4-oxadiazol-5(4 H) -ones and 4-( p -substitutedphenyl)-1,2λ^4 3,5-oxathiadiazole 2-oxides were determined in methanol by means of potentiometric titration with sodium methoxide. pKa values of the title compounds calculated from the potentiometric data were interpreted on the basis of structural effects caused by para-substituted groups on the phenyl ring by plotting pKa values versus Hammett σ^+_p constants, which gave excellent linear correlations.