Aluminium bromide

Aluminium bromide [7727-15-3] M 266.7, m 97°, b 114°/10mm. Refluxed and then distilled from pure aluminium chips in a stream of nitrogen into a flask containing more of the chips. It was then distd under vacuum into ampoules [Tipper and Walker 7 Chem Soc 1352 1959], Anhydrous conditions are essential, and the white to very light brown solid distillate can be broken into lumps in a dry-box (under nitrogen). Fumes in moist air. 

Aluminium bromide and chloride Ammonium bifluoride and other bifluorides... 

Tarbell and Petropoulos159 measured the rate of formation of phenol from benzyl phenyl ether catalysed by aluminium bromide and found it to be a first-order process. Moreover, the rate coefficient was the same as that for the formation of phenol from ortho benzyl phenol. Since the ratio of products (phenol ortho benzyl phenol) was unaffected by a large change in the temperature it was argued... 

Cross-alkylations have been reported on a number of occasions. Thus, ethylbenzene when treated with aluminium bromide and hydrogen bromide at 0 °C forms some benzene and diethylbenzene168, and in the sulphonation of durene some trimethyl- and pentamethyl-benzenesulphonic acids are formed as well as the tetra-methyl compound. It has been suggested169 that these transfer reactions involve an SN2 type process... 

The aluminium bromide/dichloromethane mixtures are only stable at a low temperature (perhaps for the same reason). 

Solutions of aluminium bromide in dichloromethane used as a catalyst in hydride-transfer equilibrium experiments should be kept cold, as a potentially dangerous exothermic halide exchange reaction occurs on warming. 

Bromochloromethane was being prepared in a 400 1 reactor by addition of liquid bromine to dichloromethane in presence of aluminium powder (which would form some aluminium bromide to catalyse the halogen exchange reaction). The reaction was started and run for 1.5 h, stopped for 8 h, then restarted with addition of bromine at double the usual rate for 2.5 h, though the reaction did not appear to be proceeding. Soon afterwards a thermal runaway occurred, shattering the glass components of the reactor. 

A ubiquitous co-catalyst is water. This can be effective in extremely small quantities, as was first shown by Evans and Meadows [18] for the polymerisation of isobutene by boron fluoride at low temperatures, although they could give no quantitative estimate of the amount of water required to co-catalyse this reaction. Later [11, 13] it was shown that in methylene dichloride solution at temperatures below about -60° a few micromoles of water are sufficient to polymerise completely some decimoles of isobutene in the presence of millimolar quantities of titanium tetrachloride. With stannic chloride at -78° the maximum reaction rate is obtained with quantities of water equivalent to that of stannic chloride [31]. As far as aluminium chloride is concerned, there is no rigorous proof that it does require a co-catalyst in order to polymerise isobutene. However, the need for a co-catalyst in isomerisations and alkylations catalysed by aluminium bromide (which is more active than the chloride) has been proved [34-37], so that there is little doubt that even the polymerisations carried out by Kennedy and Thomas with aluminium chloride (see Section 5, iii, (a)) under fairly rigorous conditions depended critically on the presence of a co-catalyst - though whether this was water, or hydrogen chloride, or some other substance, cannot be decided at present. 

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 ... 

Note added in proof. Marek and Chmelir [40b, c] found that the polymerization of isobutene in heptane by aluminium bromide is greatly accelerated by addition of titanium tetrachloride. They suggested that the polymerization by aluminium bromide only is initiated by a cation such as AlBr2+ which adds to the isobutene and which is formed by self-dissociation of the catalyst. The enhancement of the rate by titanium tetrachloride they attribute to an increase in the concentration of ions by the reaction... 

The catalytic event mentioned above was our observation that under some conditions the formation of ions from aluminium bromide in methyl bromide is a comparatively slow reaction . The slowness of this reaction is not now an essential feature of my theory, but it set off the train of thought which resulted in the theory. 

However, whilst our observations merely make an initiation without co-initiator appear probable for aluminium chloride in methylene dichloride, the experiments of Chmelir, Marek, and Wichterle subsequently proved this point fairly conclusively for aluminium bromide in heptane [11]. The Czech workers interpreted their findings by a scheme which is equivalent to reaction (4) or, more explicitly, to a combination of reactions (7) and (8) below. Their results occupy a central position in this field of enquiry, because they made it necessary to reassess old results and to re-examine the then current views, and therefore they were prominent among the stimuli which led to the evolution of the new theory. 

Supposition 1. In the paper in which we first described the self-ionisation of titanium tetrachloride in methylene dichloride or ethyl chloride, we also showed that the conductivity measurements on aluminium bromide solutions available at that time could be explained by a self-ionisation of the type... 

A scheme equivalent to our supposition 2 was put forward by Chmelir, Marek and Wichterle to explain the polymerisations initiated by aluminium bromide in heptane [11]. The fact that these reactions were of second order with respect to the initiator demanded an explanation in terms of a pre-initiation reaction between two molecules of initiator. 

Subsequently, Marek and Chmelir used a closely related set of ideas to explain their findings concerning two-component initiator systems, the most typical of which consists of aluminium bromide and titanium tetrachloride. In the presence of both these compounds the polymerisation of isobutene in heptane is much faster than when either of them is... 

Figure 1 The change of specific conductivity when isobutene was distilled slowly into 36 ml of a 1.4 x 10 4mol/l solution of aluminium bromide in methyl bromide at -74.5 °C. No polymer or oligomer was formed. The isobutene was added continuously...

Materials The methods used for the purification of aluminium bromide and chloride and for preparing phials of these catalysts, and the purification of methyl bromide, methylene chloride, and ethyl chloride, have been described [9]. The solvents were stored in a vessel coated with a sodium mirror and attached to the vacuum line, and they were metered into the observation cell by distillation from a hanging burette. 

A very rapid polymerisation accompanied each addition of monomer, and sometimes polymer precipitated out. The amount was exceedingly hard to judge, but with isobutylene and aluminium bromide it was normally very little. At -78 °C more polymer was seen to come out of solution than at -63 °C. Throughout the whole process the solutions always remained colourless, except in experiments with styrene, in which the solution and the precipitated polymer became yellow. 

Since most of our observations on the reacting systems were made by means of conductivity measurements it is necessary to remember that in these systems the only factor which increases conductivity is an increase in the concentration of ions, but that a decrease of conductivity could be due to any or all of the following effects increase of size of cation by polymerisation, increase of viscosity of solvent due to polymer, occlusion of ions in precipitated polymer, trapping of polymer between the electrodes. A similar list was given by Matyska in one of the earliest applications of conductivity measurements to a cationic polymerisation, that of isoprene by aluminium bromide in toluene solvent [19]. 

In order to study the changes of conductivity which accompany the polymerisations, a wide range of experimental conditions was explored with aluminium bromide in methyl bromide and with aluminium chloride in ethyl chloride these will be described separately. 

Figure 2 The change in conductivity (k) during the polymerisation of isobutylene by a solution of aluminium bromide in methyl bromide. In each experiment the isobutylene was added in several similar doses, as a solution in methyl bromide (3.3 mohdm 3). The polymerisations were finally stopped with a large excess of tritiated water conditions are given in the following Table...

Figure 5 Experiment R14 The change in conductivity during the polymerisation of styrene by 70 ml of a solution of aluminium bromide (36 mmoUdm 3) in methyl bromide at -78 °C. Styrene was added as a 5 moUdm"3 solution in methyl bromide, in six doses of 1.7 mmol each. There was a 99% yield of polymer of DP = 44...

The conductivity changes accompanying and following the polymerisation of five portions of norbornadiene added to an aluminium bromide solution at -63 °C are shown in Figure 6 (Experiment No. R4). The polymer, which was precipitated during the reaction, was subsequently found to be insoluble and therefore presumably crosslinked. Since this polymer was, therefore, unsuitable for radiochemical assay, another experiment (RIO) was done with norbornadiene in a mixture of methyl and ethyl bromide at -125 °C to prevent cross-linking. The polymer was soluble and the number of tritium atoms per molecule of polymer was much greater than for polyisobutylene. 

---