Étude par spectrométrie infrarouge de la complexation entre les phénols substitués et les anilines substituées—I: Constantes de complexation et déplacement Δ v(O-H⋯N)

Abstract

The complexing constants ( K ab ) of disubstituted complexes phenol-aniline in carbontetrachloride are determined by i.r. spectrometry; one of the substituents is on the phenolic ring, the second on the anilinic ring. The frequency shift Δ v ab of the v OH hydroxylic stretching vibration is also measured. If σ a and σ b represent the Hammett substitution constant of the proton donor and the proton acceptor respectively, it is shown that log K ab obeys the following relationship: log K ab = log k 00 + ρ a 0σ a + ρ 0 b σ b + Bσ a σ b (1) where K 00 refers to the unsubstituted complex phenol-aniline and ρ 0 a and ρ 0 b are the ρ values when σ a and σ b are zero. The additional term Bσ a σ b is an interaction term where B = −1·30. Similarly, Δ v ab obeys the following equation: Δ v ab = Δ v 00 + ρ a '0σ a + ρ '0 b σ b + B'σ a σ b (2) where Δ v 00 refers to the unsubstituted complex. The constant B' is negative (−95). This shows that, when the acidity constant of the phenol increases (σ a ⪢ 0), the interaction term is positive when the basicity of the aniline increases (σ b < 0). On the other hand, this interaction term becomes negative when the basicity of the aniline decreases (σ b ⪢ 0). The comparison of expressions (1) and (2) substituting the coefficients ρ and B by the numerical values, shows that the complexing constant is obviously more sensitive to the substitution effects than the shift Δ v ab ; this last parameter being proportional to the enthalpy of complex formation with good approximation, the difference observed between these two magnitudes should be ascribable, at least partly, to an entropy effect. The shift Δ v ab can be related to the O⋯N distances. Hence the origin of the interaction term can be discussed in terms of polarisation effects and dipolar interaction effects which are more important when the hydrogen bond is compressed in a more reduced space. It also seems that the resonance effects existing in the phenolic and anilinic rings are disturbed by the introduction of a substituent on both sides.