Aluminium halide

Aluminium Halides.— Aluminium reacts with MgFg at steel-smelting temperatures to give a volatile subfluoride of aluminium. On condensation this dis-proportionates, giving Al metal, which is then oxidized in air to the oxide or suboxide of Al.  

The matrix-isolated i.r. spectrum of OAIF has been reported and assigned, as follows 386 cm, Vg 1022 cm (Al—F stretching) was 

The degree of dissociation ()8) of AIF (considered to give AIF + 2F ) in the system LigAlFg-CaFg has been calculated to be 0.40. 

The electrical conductivities of LiF-AlFg and NaF-AlFg molten mixtures were analysed in terms of MF-MgAlF and MgAlFg-MAlF4 equilibria.  

Despite these results, however, there is a considerable body of evidence that the AIF ion does not exist as such. Thus, thermodynamic measurements on the NaF-AlFg system suggest that there is only a weak interaction between F and AlFg, and that no clearly defined AIF ions are present.  

Aluminium Halides.—Matrix-isolation techniques have allowed isolation of A1F3, A1F, (A1F)2, GaF3, and GaF, which have been examined by i.r. spectroscopy in the range 33—4000 cm 1.472 The methods used for generating the species were as follows (a) Knudsen-cell effusion from GaF3, GaF3 + Ga, or GaF3 + Al (b) codeposition of Ga or GaF and molecular F2 or F atoms. 

Cryoscopic measurements on the NaF-rich side of the system NaF-AlF3-Na20-Al203 suggest that the chief Al/O-containing species is an Al2OF2- complex (containing an A1—O—A1 unit).473 

The only bands seen in the Raman spectrum of the LiF-Li3AlF6 eutectic were assignable to AlFs- no evidence was found for A1I%, A1F3, or any other species derived from the dissociation of AIF -.474 

Values for the enthalpies of fusion of alkali-metal cryolites may be obtained, using an aneroid, inverse-drop calorimeter with adiabatic shields.475 

Some reports of phase relationships in [A1F6]3 -containing systems are given in refs. 476-478. 

Aluminium Halides.— The enthalpies and entropies of formation of AIF, AIF, and AIF3 have been calculated from measurements on aqueous solutions of A1(C104)3 containing NaF, at 25 °C, and 7=0.5 (Table 1).  

Phase diagrams have been constructed for the following systems LisAlFg-LiCl-LiF, 3Li,3Na lAlF6,3F, and 3Li,3Na AlF6,3Cl.  

Hexagonal-rhombohedral modifications of the elpasolites M2M AlFe (M = Rb, M = Li M = Cs, M = Na) can be converted into cubic forms at high pressures (15—100 kbar), in some cases via the hexagonal forms analogous to K2LiAlp6.  

Fifteen new hexafluoroaluminates(iii), of the elpasolite type, have been obtained by heating mixtures of the binary fluorides. 

A single-crystal X-ray study shows that AlCl3,2MeCN is in fact ionic [AlCl(MeCN)sP [AlCl4]2,MeCN. One of the MeCN molecules is present as a molecule of solvation.  

This difference means that a three-centre bond treatment in which the bridge bonds have an order much less than one is less appropriate for AI2CI6 the molecule is better represented by allowing each bridging chlorine to form one shared electron-pair bond, and one dative bond as in Structure 9.10 all bonds then have order one. A bridging chlorine can do this because it can use more than one of its orbitals to form the two bonds in the bridge hydrogen, by contrast, has only one orbital available - the Is. 

The structure of the AI2X6 molecules found in liquid and gaseous AICI3, and in solid AlBr3 and AII3. The bond length and bond angle data refer to the chloride. 

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Boron and aluminium halides show many similarities but also surprising differences. Table 7.2 gives the melting and boiling points of the MX3 halides. 

The melting and boiling points of the aluminium halides, in contrast to the boron compounds, are irregular. It might reasonably be expected that aluminium, being a more metallic element than boron, would form an ionic fluoride and indeed the fact that it remains solid until 1564 K. when it sublimes, would tend to confirm this, although it should not be concluded that the fluoride is, therefore, wholly ionic. The crystal structure is such that each aluminium has a coordination number of six, being surrounded by six fluoride ions. 

All the other aluminium halides are covalently bonded with aluminium showing a coordination number of four towards these larger halogen atoms. The four halogen atoms arrange themselves approximately tetrahedrally around the aluminium and dimeric molecules are produced with the configuration given below ... 

The monomers are electron pair acceptors, and donor molecules are often able to split the dimeric halide molecules to form adducts thus, whilst the dimeric halides persist in solvents such as benzene, donor solvents such as pyridine and ether appear to contain monomers since adduct formation occurs. Aluminium halides, with the one exception of the fluoride, resemble the corresponding boron halides in that they are readily hydrolysed by water. 

Halogen derivatives of silanes can be obtained but direct halogena-tion often occurs with explosive violence the halogen derivatives are usually prepared by reacting the silane at low temperature with a carbon compound such as tetrachloromethane, in the presence of the corresponding aluminium halide which acts as a catalyst. 

The compounds can therefore be used as nonaqueous ionizing solvent systems (p. 424). For example the conductivity of ICl is greatly enhanced by addition of alkali metal halides or aluminium halides which may be considered as halide-ion donors and acceptors respectively ... 

Baddeley and Voss also showed that the aluminium halide-catalysed acylation by acid anhydrides proceeds via the initial formation of the acyl halide, equilibrium (200)... 

Alkyl aluminium halides are used in many ways as coinitiators for the cationic polymerization. Due to presence of alkyl groups, which have the characteristics of potential carbanions, the alkyl aluminium halides and the counterions formed from them cause the following irreversible competing reactions whereby hydrocarbons are released — Termination by interaction of the cation with the alkyl group of the counterion, e-g-... 

The halogen carriers or aromatic halogenation catalysts are usually all electrophilic reagents (ferric and aluminium halides, etc.) and their function appears to be to increase the electrophilic activity of the halogen. Thus the mechanism for the bromination of benzene in the presence of iron can be represented by the following scheme ... 

Eley, D. D., and H. Watts Aluminium Halide Complexes with Pyridine, Trimethylamine and Triethylamine, Part I. J. chem. Soc. [London] 1952, 1914. 

Aluminium oxide, Carbon oxides See Aluminium Aluminium halides, Carbon oxides... 

Detailed studies showed that the concentration of ions was much smaller than c0, and therefore that of ion-pairs was negligible. I had thus been reminded of the BIE and could respond appropriately when shortly afterwards Grattan at the other end of the same laboratory found linear K - cQ plots in his work with A1X3 in alkyl halides RX [35, 36]. This indicated that in adequately pure systems the aluminium halides ionise thus ... 

D. W. Grattan, P. H. Plesch, Ionisation of Aluminium Halides in Alkyl Halides, J. Chem. Soc. Dalton Trans., 1977,1734. 

It is known that the conducting species in solutions of aluminium halides in alkyl halides are complex for instance solutions of aluminium bromide in ethyl bromide [70] contain the ions Al2Br5+ and Al2Br7 ... 

The thermochemical assessment of the feasibility of various possible reactions is developed further here, and there is a didactically useful example of this author being taken in by his own propaganda . In one calculation he shows thermochemically that the addition of the TiCl3+ ion to an alkene is unlikely, then shows that under different circumstances an initiation by the addition of an A1X2+ ion to a monomer is theoretically possible. That subject then did not surface again in his work until 1972 (92, 93) and it took until 1980 for the experimental solution of the problem of the initiation by aluminium halides to be published, in which the above-mentioned cationation is an essential feature (112). 

For the tungsten halides the trend in A/7°sub (MtXJ on changing from chloride to fluoride is the opposite to that shown by the aluminium halides. Thus, this is not a term with respect to which overall generalisation is possible and great care must be taken whenever numerical values for this term are not available. 

In the field of cationic polymerisation a notoriously intractable problem is the mechanism of initiation by aluminium halides. Despite much work on the polymerisation itself, there are few studies of the initiation mechanism. Existing theories are shown to be inadequate to explain the most characteristic features of the reactions when a solution of an aluminium halide in an alkyl halide is introduced into a solution of isobutene, there ensues a fast polymerisation which generally stops at incomplete conversion, and the number of moles of polymer formed is much smaller than the number of moles of initiator these features are found over a very wide range of conditions. 

On the other hand, an investigator interested in the fundamental chemistry rather than in the optimisation of the reaction conditions (from the commercial point of view) would naturally refrain from preparing, and then using after a variable storage-time, a solution of aluminium halide in alkyl halide, because the rapid reactions which can occur in such systems at all but the lowest temperatures would obscure largely the true nature of the initiator. For such studies it is necessary to introduce the monomer into a freshly prepared initiator solution, or to introduce into a solution of the monomer the solid initiator or a freshly prepared solution of initiator in a very pure and inert solvent. 

Current work in our laboratory [3] indicates that in methyl bromide solution the ionogenic reaction is of second order with respect to [AlBr3], and the same has been found [14] for the ionisation of A1I3, Gal3, and Inl3 in EtI. Since it is known that the aluminium halides are monomeric in these solutions, it follows that the rate-determining step for the selfionisation is the reaction (10) ... 

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