Apparent Complexation Constants Research Articles

The complexes formed between triethylamine and phenols have been studied by infrared spectrometry in carbon tetrachloride solution at 30°C. The phenols have been chosen so that their pKα vary from 10·30 to 3·50. The apparent complexation constant, calculated on the model of a 1:1 stoichiometric complex shows a marked variation with the concentration in free proton donor. Using a method based on the variation of this “apparent constant”, the equilibrium constants of 1:1 (K1) and 2:1 (K2) stoichiometric complexes have been determined. These values are related to the sum of the Hammett σ constants (Σσ) by the following relations: log K1 = 1.73 + 1.30Σσ log K2= 1.57 + 0.05 Σσ + 0.94 (Σσ)2 Log K1 is linearly related to Δμ, the increment of dipole moment, determined by Ratajcyak and Sobczyk, in benzenic solution. On the other hand, equation (2) is compared to an expression, obtained previously in the study of the complexation of substituted phenols-substituted anilines, showing that the logarithm of the complexation constant depends on an interaction term depending on the product of the substitution constants σaσb. The formation of a complex of 2:1 stoichiometry gives a characteristic vibration band lying at some 100 cm−1 lower than the dimeric phenol; this constatation is explained in terms of the greater basicity of the oxygen atom in the 1:1 complex than in the dimeric phenol. The CH stretching vibrations of the triethylamine are disturbed in frequency and intensity by the formation of a hydrogen bond; this perturbation increases with the phenol acidity.

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