Quantitative aspects of Lewis acidity. Part XI. Cadmium iodide–aniline equilibria in acetone

Abstract

Cadmium iodide and substituted anilines form two types of 1:1 adduct in acetone. One is formed very rapidly on mixing the two components in acetone; the second is formed relatively slowly. The equilibrium constants for the formation of each type of adduct are related to the dissociation constant (K_a) of the protonated base in water. For the adduct formed rapidly pK₁=–0·91 pK_a+ 1·65, and for the adduct formed slowly pK₂=–0·79pK_a+ 0·53. The visible spectra indicate that the lone pair of electrons on the nitrogen atom of the base is involved in the formation of each adduct. The structural difference between the adducts is best explained in terms of an iodine atom as the acceptor centre in the first adduct, and the cadmium atom as the acceptor centre in the second adduct. A Brønsted-type relation is found between the rate constant and the equilibrium constant for the slow adduct formation with α= 1·66. Both on the basis of constants and equilibrium constants cadmium iodide provides, in acetone, a less acidic system than the zinc halides.