Hydrogen-bond basicity pKHB scale of aliphatic primary amines

Abstract

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 (i) 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.