Ionisation of aluminium halides in alkyl halides

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’κi is established very rapidly. The difference κ–κi(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, κe, gives κe–κi 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.