Amidines. Part 30. Influence of Substitution at Amino Nitrogen Atom on pKa Values of N2-Phenylacetamidines and N2-Phenylamidines
A series of N2-phenylacetamidines and N2-phenylformamidines (22 compounds) containing variable substituents at amino nitrogen atom have been synthesized, and their pKa values in 95.6percent ethanol (azeotrope) measured.The pKa values obtained for N1-methyl-N1,N2-diphenylacetamidines were correlated with Hammett type constants.Applicability of various ? values is discussed and it is shown that for substituents on the phenyl ring at amino nitrogen atom ?0 values should be used.It is also shown that the pKa values of amidines correlate well with the pKa values of corresponding secondary amines, and that this correlation can serve the prediction of the pKa of amidine. Comparison of the correlations obtained for studied acetamidines and formamidines 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.