Ionisation of aluminium halides in alkyl halides

Abstract

The ionisation of AlBr₃ in MeBr and of AlCl₃ in EtCl and CH₂Cl₂ between –78 and 0 °C have been studied by electrical conductance measurements in vacuo. Consistent results with these notoriously irreproducible systems were achieved by new purification methods, whose effectiveness was assessed by functional tests. The slow ionisation had previously been mistaken for, or obscured by, reactions of the aluminium halides with impurities. Always a randomly variable ‘impurity conductivity’κ_i is established very rapidly. The difference κ–κ_i(κ= conductivity at any time) for most systems obeys the kinetics expected for ionisation of AlX₃ to AlX₂^– and AIX₄^–, and the final equilibrium conductivity, κ_e, gives κ_e–κ_i values which depend rectilinearly on the concentration of AlX₃. The equilibrium constant for the ionisation of the chloride in EtCl is ca. 100 times greater than that of the bromide in MeBr; for the ionisation in the chloride system ΔH^⊖=–17.6 ± 0.2 kJ mol^–1 and ΔS^⊖=–217 J K^–1 mol^–1. The solubility of the chloride in CH₂Cl₂ at –78 °C (4.23 × 10^–4 mol dm^–3) shows that it is monomeric. Previous results are discussed.