Three series of N'N'-dimethylamidines, propionamidines, isobutyramidines, and pivalamidines, each containing the same set of 25 substituents Rx at the imino nitrogen atom have been synthesized and their pKa values in 95.6% ethanol (azeotrope) measured. The pKa values were correlated with o and oo constants as well as with the pKa values of the corresponding primary amines RXNH,. Regression parameters for the series studied were compared with those for N'N'-dimethyl-formamidines and -acetamidines. It is shown that the sensitivity of the amidine group to substitution at the imino nitrogen atom depends on the polar effects of substituents at the amidino carbon atom. A good linear correlation between the slope of the correlation line and the o* value of the substituent at the amidino carbon atom is found.

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137ChemInform Abstract Three series of N1N1-dimethylamidines (I), each containing the same set of substituents R1 at the imino N atom are synthesized by known procedures and their pKa values in 95.6% ethanol (azeotrope) measured. The pKa values are correlated with σ and σ0 constants as well as with the pKa values of the corresponding prim. amines R1-NH2. Regression parameters for the series studied are compared with those for N1N1-dimethyl-formamidines and -acetamidines, the hitherto not studied members of these series (II) and (III) now being prepared. It is shown that the sensitivity of the amidine group to substitution at the imino N atom depends on the polar effects of substituents at the amidino C atom. A good linear correlation between the slope of the correlation line and the σ* value ofthe substituent at the amidino C atom is found. 1H chemical shifts and B.p./M.p. for all compounds (I)-(III) are tabulated.