Quantitative aspects of Lewis acidity. Part IX. Equilibria between aluminium bromide and substituted anilines in ether solution

Abstract

In ether solution aluminium bromide and substituted anilines form 1:1 adducts in reversible equilibria. Equilibrium constants (K) have been measured at 25° for nine anilines. For unhindered anilines pK=–0·75pK_a–1·15, where K_a is the dissociation constant of the corresponding anilinium ion in water. Alkyl and halogen substituents ortho to the amino-group hinder adduct formation. Aluminium bromide is an appreciably stronger acid towards anilines than are boron fluoride, stannic chloride, and the gallium halides. The ultraviolet spectrum of an adduct always resembles that of the corresponding anilinium ion except that, for certain (4-NO₂) anilines, new long-wavelength absorption, not typical of the anilinium ion, is exhibited. This effect, found previously in ether only with zinc halides and stannic bromide, is attributed to a charge-transfer type of absorption by the adduct.