Hydrogen-bond basicity pKHB scale of aliphatic primary amines

Using 4-fluorophenol as a reference hydrogen-bond donor equilibrium constants, K-f,for the formation of 1 : 1 hydrogen-bonded complexes have been obtained by FTIR spectrometry for 22 aliphatic primary amines, in C2Cl4 at 298 K. The pK(HB) (log K-f) scale shows that most primary amines are weaker hydrogen-bond bases than many oxygen bases. The pK(HB) scale of primary amines extends from 2.31 for adamantan-1-amine to 0.67 for CF3CH2NH2. The main effects explaining the pK(HB) variations are (1) field-inductive effects (e.g. in CF3CH2NH2), (ii) resonance effects (cyclopropylamine), (iii) polarizability effects (alkylamines), and (iv) intramolecular hydrogen bonding (e.g. in 2-methoxyethylamine). Except for intramolecularly hydrogen-bonded methoxyamines and diamines, the pK(HB) and pK(a) scales are correlated. The pK(HB) scale also correlates with the minimum electrostatic potential on the nitrogen lone pair.