Amidines. Part 24. Influences of alkyl substituents at the amidino carbon atom on the sensitivity to substitution at the imino nitrogen atom. pKa Values of N1N1-dimethylamidines

Abstract

Three series of N¹N¹-dimethylamidines, propionamidines, isobutyramidines, and pivalamidines, each containing the same set of 25 substituents R_x at the imino nitrogen atom have been synthesized and their pK_a values in 95.6% ethanol (azeotrope) measured. The pK_a values were correlated with σ and σ⁰ constants as well as with the pK_a values of the corresponding primary amines R_xNH₂. Regression parameters for the series studied were compared with those for N¹N¹-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 σ* value of the substituent at the amidino carbon atom is found.