Quantitative aspects of Lewis acidity. Part VII. Equilibria between gallium trihalides and aniline bases in ether solution

Abstract

Gallium chloride and bromide and substituted anilines interact in ether solution to give 1:1 adducts. Equilibrium constants, K, at 25° have been determined for both the chloride and the bromide with a series of anilines. For both acids pK appears to be linearly related to pK_a, where K_a is the dissociation constant of the corresponding anilinium ion in water. For gallium chloride pK=–0·99pK_a+ 0·01, and for gallium bromide pK=–1·01pK_a–0·05. However, as found previously in similar systems, o-methyl groups sterically hinder adduct formation, and data for p-nitroanilines are sometimes anomalous. The addition of water to ether solutions of gallium halides reduces the observed acidity whereas the addition of acetic acid increases it. Towards anilines gallium chloride and bromide are almost equally acidic and come just below boron trifluoride as two of the most powerful Lewis acids yet studied quantitatively in such interactions.