Quantitative aspects of Lewis acidity. Part XIII. Basicity of substituted benzamides towards metal halides. The validity of nuclear magnetic resonance chemical shifts as measures of Lewis acid strength

Abstract

ZnCl₂, ZnBr₂, BF₃, GaCl₃, GaBr₃, PhSnCl₃, SnCl₄, AlCl₃, and AsCl₃ from 1:1-adducts with substituted benzamides in diethyl ether or in tetrahydrofuran. The values of K, the equilibrium constant for adduct formation in diethyl ether, follow the sequence AlCl₃ > SnCl₄ > GaCl₃ [graphic omitted] GaBr₃ > ZnCl₂ [graphic omitted] ZnBr₂ BF₃ > PhSnCl₃ > SbCl₃ > AsCl₃. The K values are larger in diethyl ether than in tetrahydrofuran. It is shown that the carbonyl oxygen atom of the benzamide is the donor atom. It is also demonstrated that neither standard heats of formation of the adducts nor the resulting ¹H n.m.r. chemical shifts of the N–H protons are reliable guides to the relative acidities of the covalent halides. The relative basicity of benzamides and anilines towards metal halides is very different to their relative basicity towards the proton in water. The pK_a value of 2-methylbenzamide is –1·88 and molecular weight determinations show that gallium chloride is monomeric in ether.