Quantitative aspects of Lewis acidity. Part XVI. Acidity of covalent metal halides towards substituted anilines in dioxan. The validity of ΔH0 as a measure of Lewis acid strength

AlCl3, GaCl3, SnCl4. PhSnCl3, and ZnCl2 form 1 : 1 adducts with substituted anilines in dioxan. The equilibrium constant for adduct formation, K, has been determined for each acid with several anilines at, normally, several temperatures; ΔH0 and ΔS0 values have been derived. Whereas, for any given base. K25 values follow the sequence SnSl4 > AlCl3 > GaCl3 > PhSnCl3 > ZnCl2(MeSnCl3, Ph2SnCl2, HgCl2), the values of –ΔH0 follow the different sequence PhSnCl3 > SnCl4 > GaCl3 ZnCl2. The corresponding ΔS0 values vary irregularly from one acid to another. For most of the acids adduct formation probably involves the replacement, by an aniline molecule, of a solvent molecule co-ordinated to the metal atom, but for PhSnCl3 the reaction is probably an addition of a base molecule to the solvent–acid adduct. A comparison is made of the level of acidity exhibited by the various acids in dioxan with that exhibited in diethyl ether, and for each acid equations are given relating the pK in dioxan of unhindered anilines to the pKa values of the anilinium ions in water. The changes in pK found on moving from one substituted aniline to another, although always regularly related to changes in pKa, are not determined primarily by changes in either ΔH0 or ΔS0; both quantities are important. This fact is discussed.