Estimates of microscopic ionization constants for heteroaromatic exocyclic amines including purine and pyrimidine nucleotides and amides based upon a reactivity-basicity correlation for N-hydroxymethylation reactions with formaldehyde¹(a),².

The weaker basicity at the 2-amino and 4-amino sites of pyridines, pyrimidines, and purines, when compared to the endocyclic nitrogen atoms, precludes estimates of the pK values for the ionization of the conjugate acids of exocyclic amino groups by direct titration. A kinetic method is proposed for obtaining estimates of these microconstants for the aromatic exocyclic amino groups of heterocyclic compounds [K(c) = (ArNH₂)a(H⁺)/(ArNH₃⁺)], based upon the rates of reaction with formaldehyde to form the N-hydroxymethylamine. The Bronsted equation, pK(c) = log k₀(u)- 1.61)/0.87, where k₀(u) is the pH-independent rate constant for N-hydroxymethylation with respect to unhydrated formaldehyde, is based on aromatic exocyclic amines (pK(c) = -6.0-2.0) and provides, with values of K₀(u), ranges of values of pK(c) at 25°C for series of adenine, guanine, and cytidine derivatives of -2.8 to -2.2, -1.8 to -1.6, and -2.2 to -1.7, respectively. From the same Bronsted equation, the estimates of the microscopic proton dissociation constants of amides for N-protonation of benzamide and urea are estimated at -8.4 and -3.7, respectively.