Abstract
Using 4-fluorophenol as a reference hydrogen-bond donor, equilibrium constants, Kf, 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 pKHB (log Kf) scale shows that most primary amines are weaker hydrogen-bond bases than many oxygen bases. The pKHB scale of primary amines extends from 2.31 for adamantan-1-amine to 0.67 for CF3CH2NH2. The main effects explaining the pKHB 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 pKHB and pKa scales are correlated. The pKHB scale also correlates with the minimum electrostatic potential on the nitrogen lone pair.
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