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Amidines-XVIII: Tautomeric equilibria and pka values of N,N'-disubstituted amidines. Substituent effects
Authors:E Raczyńska  J Oszczapowicz
Institution:Department of Chemistry, University, 02-093 WarsawPoland
Abstract:The influence of substitution of amidine group on tautomeric equilibria constants and basicities is discussed. Equations based on correlation analysis methods are derived enabling predictions of both, microscopic pKa, values of individual tautomers, measured macroscopic pKa values of the tautomeric mixture, as well as the tautomeric equilibrium constant (as pKT). It is shown that pKarn values of unsymmetrically N,N'-disubstituted amidines should obey a non-linear relation with σ° constants, and only for symmetrically N,N'-disubstituted amidines obey the linear Hammett equation. Tautomeric equilibrium constants of N,N'-disubstituted amidines correlate withσ° substituent constants. The methods of prediction of pKa value of both tautomers and pKT value are proposed.Derived relations are applied to the series of N,N'-diphenylacetamidines and benzamidines.
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