Quantitative aspects of Lewis acidity. Part XIV. Comparison of the acidity of tin tetrachloride, tellurium tetrachloride, and zirconium tetrabromide towards substituted anilines in dioxan

Abstract

Whereas TeCl₄ and SnCl₄ form 1 : 1 adducts with nitroanilines in dioxan solution, ZrBr₄ forms 2acid: 1 base adducts in which both the amino- and the nitro-group of the base are probably involved. The strengths of the acids, judged by the values of K_1,1 for 1 : 1 adduct formation and by the concentration of acid required to convert the base into the adduct, follow the sequence ZrBr₄ > TeCl₄ > SnCl₄, and it can also be concluded that the sequence ZrCl₄ TelCl₄ > SnCl₄ obtains. This sequence supports our earlier predictions concerning the effects of the electronic structure of the metal atom on the acidity of covalent halides. Adduct formation between SnCl₄ and a serIes of unhindered anilines in dioxan solution is related to the protonation of the bases by the equation pK_1,1=–0.96pK_a–0.21, Although p-nitro-substitution provides anomalous high K_1,1 values. This result is discussed. Toward anilines SnCl₄ provides slightly more acidic solutions in dioxan than in diethyl ether.