A series of N²-phenylanylacetamidines and N²-phenylformamidines (22 compounds) containging variable substitutents at amino nitrogen atom have been synthesized, and their pK_avalues in 95.6% ehonal (azeotrope) measured. The pK_a values obtained for N¹-methayl-N¹,N²-diphenylaceamidines were correlated with Hammett type constants. Applicability of various σ values is discussed and it is shown that for substituents on the phenyl ring at amino nitrogen atom σ⁰values should be used. It is also shown that the pK_a values of amidines correlate well with the pK_avalues of corresponding secondary amines, and that this correlation can serve in the prediction of the pK_a of amidine.

Comparison of the correlations obtained for studied acetamidines and formamedines indicates that sensitivity of the amidino group to substitution at the amino nitrogen atom depends to a certain degree on the substituent at the functional carbon atom.