L'échelle p<i>K</i>_HB de basicité de liaison hydrogène des amines tertiaires aliphatiques

Abstract

The hydrogen bond acceptor strength of 40 tertiary amines has been measured by Fourier transform – infrared (FTIR) spectrometry from their 1:1 complexation constant towards 4-fluorophenol in CCl 4 at 25°C (the pK HB scale). Also measured was the frequency shift, Δν(OH), of the ν(OH) band of methanol hydrogen-bonded to these amines. The comparison of the thermodynamic hydrogen bond basicity scale, pK HB , with the spectroscopic one, Δν(OH), and with the Brønsted pK a scale, points to the great sensitivity of pK HB to steric effects. The pK HB scale of tertiary amines extends from 2.71 for quinuclidine to –0.34 for N,N-diisopropyl-3-pentylamine. The main factors governing this important variation (17 kJ·mol –1 on the Gibbs energy scale) are the electron-withdrawing inductive effect and various kinds of steric effects (e.g., opening of the CNC angles and hindrance to OH fixation on the nitrogen lone pair). Infrared (IR) spectra show the attachment of 4-fluorophenol to the nitrile nitrogen of Me 2 NCH 2 C[Formula: see text]N and Me 2 NCH 2 CH 2 C[Formula: see text]N, to the oxygen of N-methylmorpholine, and to the π electrons of (HC[Formula: see text]CCH 2 ) 3 N and (PhCH 2 ) 3 N, in addition to the attachment to the amino nitrogen. In (PhCH 2 ) 3 N, the electron-withdrawing effect of the three benzyl substituents and, mainly, the very important congestion of the nitrogen lone pair reduce the nitrogen hydrogen-bond basicity almost to nothing, so that tribenzylamine, a nitrogen Brønsted base, turns to a π base in hydrogen bonding. From this example, the large differences between the pK HB and pK a scales of organic bases are emphasized.Key words: basicity, hydrogen bonding, tertiary amines, pK HB scale.