Hydrogen-bond basicity pKHB scale of secondary amines

Abstract

Using 4-fluorophenol as a reference hydrogen-bond donor, equilibrium constants K for the formation of 1 ∶ 1 hydrogen-bonded complexes have been obtained by FTIR spectrometry for 33 secondary amines in CCl4 and/or C2Cl4 at 298 K. A spectroscopic scale of hydrogen-bond basicity is constructed from the IR frequency shift Δν(OH) of methanol hydrogen-bonded to secondary amines. The comparison of the pKHB (log K), Δν(OH), and pKa scales points to the sensitivity of pKHB to steric effects, and of Δν(OH) to the p character of the nitrogen lone pair. The pKHB scale of secondary amines extends from 2.59 for pyrrolidine to −0.45 for (Me3Si)2NH. The main effects explaining the pKHB variations are (i) the opposite polarizability and steric effects in alkylamines, (ii) field-inductive effects (e.g.NCCH2NHMe), (iii) intramolecular hydrogen bonding, e.g. in (MeOCH2CH2)2NH, and (iv) the ring size giving the order: pyrrolidine = azetidine > piperidine > 2-methylaziridine > azepane. IR spectra show the attachment of 4-fluorophenol to the nitrile nitrogen of NCCH2NHMe and NCCH2CH2NHMe, to the oxygen of morpholine and (MeOCH2CH2)2NH, and to the sulfur of thiomorpholine and thiazolidine, in addition to attachment to the amino nitrogen. The correlation of pKHB with the minimum electrostatic potential on the nitrogen lone pair is used for unravelling the basicity of each nitrogen of 1-methyl-1,4-diazepane.