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 pK_HB (log K), Δν(OH), and pK_a scales points to the sensitivity of pK_HB to steric effects, and of Δν(OH) to the p character of the nitrogen lone pair. The pK_HB scale of secondary amines extends from 2.59 for pyrrolidine to −0.45 for (Me₃Si)₂NH. The main effects explaining the pK_HB 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 pK_HB 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.