Aluminium ions, solvation

Evaluation of NMR measurements on Al is facilitated by the fact that the spectrum is comparatively simple the line broadening caused by quadrupole relaxation is extremely small even in solvent mixtures. This permitted the use of this method to distinguish between the different solvates formed in solvent mixtures. The coordination number of the aluminium ion is known to be 6 in both dimethylformamide and dimethyl sulphoxide solution [Mo 67, Mo 68, Th 66c]. In a mixture of these two solvents, however, the solvating effect of DMSO is more marked than that of DMF [Gu 74]. The solvation conditions are also influenced by dilution with an inert solvent. Thus, for instance, if a solution containing DMSO and DMF as solvents is diluted with nitromethane, which can be regarded as inert from the aspect of solvation, then the earlier difference between the solvating powers of the two donor solvents no longer appears. This experiment also reflects the effect of the solvent-solvent interaction on the solvation of a solute. 

The dilution with acetone of aqueous solutions containing aluminium ion showed that the acetone is not coordinated to the aluminium in solutions with low acetone contents [Fr 69, Ma 68b]. Ethylene glycol and propylene glycol have been shown to have a solvating power in different solvent mixtures [Bo 79, Ka 80, No 80]. 

The equilibrium constant (K2) for reaction (ii) under various conditions has been measured it is very small, so that only a very small fraction of the aluminium halide is ionized [9]. If the system is insufficiently pure, the concentration of ions derived from impurities may exceed that of the A1X2+. The A1X2+ is certainly solvated by one or two molecules of RX, but this is unimportant in the present context. 

Normally electrode reactions take place in solutions, or sometimes in molten salts (e.g. aluminium extraction). In order to minimize the phenomenon of migration of the electroactive ions caused by the electric field (Chapter 2) and to confine the interfacial potential difference to the distance of closest approach of solvated ions to the electrode (Chapter 3), the addition of a solution containing a high concentration of inert electrolyte, called supporting electrolyte, is necessary. This has a concentration at least 100 times that of the electroactive species and is the principal source of electrically conducting ionic species. The concentration of supporting electrolyte varies normally between 0.01m and 1.0 m, the concentration of electroactive species being 5 mM or less. The... 

Because of the amphoteric character of aluminium, it is soluble in both acidic and basic solvents. In acidic solutions below pH = 2 aluminium will be present as solvated Al3+ ions, while in basic solutions above pH = 12 it is present in the form of aluminate ions (AlO -). 

At C(2) we apparently see "steric, approach control and product development control in direct conflict, for although the axial 2 -alcohol (12) is the predominant product when sodium borohydride is used in a polar solvent preference almost disappears when the reagent is lithium aluminium hydride in ether [12]. The anion in the latter reagent is effectively quite small because of the weak anion-solvating power of ether, so the "steric approach factor is small compared with the much more heavily solvated borohydride ion in a hydroxylic solvent. Even the sterically unhindered 3-ketones, and simple analogues like q-tert-butyl cyclohexanone [47], give the equatorial alcohols in greater... 

The reduction of the maleic anhydride adduct (303) with lithium aluminium hydride was previously reported to occur selectively to give the lactone (304). The lower selectivity now observed with sodium aluminium hydride (none at all with sodium borohydride) is interpreted as evidence for a complex (305) of the ester and anhydride carbonyl groups with a solvated lithium ion when lithium aluminium hydride is used, leading to selective reduction of the free carbonyl group.Sodium ions are considered not to form so stable a complex. 

The situation is different in solutions of aluminium chloride in phosphorus oxychloride, deriving from a higher affinity of aluminium chloride towards 0-coordination than in the case of ferric chloride. Polymeric species seem to be present according to the results of precise conductivity measurements , the high surface tensions and high viscosities. Conductometric, potentiometric, spectro-photometric and preparative investigations have shown that aluminium chloride reacts in phosphorus oxychloride and in phenylphosphonic dichloride with considerable formation of autocomplex ions. Chloride ions are released by ionization of aluminium chloride due to preferred 0-coordination by the solvent. These chloride ions may react with other solvate molecules to give the chloro-complex ... 

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