Aluminium bromide

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Aluminium bromide
Aluminium bromide
Other names aluminium(III) bromide
aluminium tribromide
Identifiers
CAS number [7727-15-3]
Properties
Molecular formula AlBr3
Al2Br6
Molar mass 266.694 g/mol
Appearance white to pale yellow
crystalline solid
Density 3.205 g cm−3 as solid
Melting point

97.5 °C
Boiling point

255 °C
Solubility in water hydrolyzes
Hazards
EU classification not listed
NFPA 704
0
3
2
 
Related compounds
Other anions aluminium trichloride
aluminium triiodide
Other cations boron tribromide
Related compounds iron(III) bromide
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)
Infobox disclaimer and references

Aluminium bromide is any chemical compound with the empirical formula AlBrx. The species called "aluminium tribromide," is the most common aluminium bromide. The species aluminium monobromide forms from the reaction of HBr with Al metal at high temperature. It disproportionates near room temperature:

6/n "[AlBr]n" → Al2Br6 + 4 Al

This reaction is reversed at temperatures > 1000 °C.

Contents

[edit] Synthesis

By far the most common form of aluminium bromide is Al2Br6. This species exists as hygroscopic colorless solid at standard conditions. Typical impure samples are yellowish or even red-brown due to the presence of iron-containing impurities. It is prepared by the reaction of HBr with Al:

2 Al + 6 HBr → Al2Br6 + 3 H2

Alternatively, the direct bromination occurs also:

2 Al + 3 Br2 → Al2Br6

[edit] Structure

"Aluminium tribromide" is really dialuminium hexabromide with the molecular formula of Al2Br6 in the solid state, solutions in noncoordinating solvents (e.g. CS2), and in the melt. Even upon evaporation, Al2Br6 exists in the gas phase. At high temperatures, the gaseous molecules break up into monomers:

Al2Br6 → 2 AlBr3 ΔH°diss = 59 kJ/mol

[edit] Representative reactions of Al2Br6

Al2Br6 dissociates readily to give the strong Lewis acid, AlBr3. Regarding the tendency of Al2Br6 to dimerize, it is common for heavier main group halides to exist as aggregates larger than implied by their empirical formulae. Lighter main group halides such as boron tribromide do not show this tendency, in part due to the smaller size of the central atom.

Consistent with its Lewis acidic character, Al2Br6 reacts vigorously with water with evolution of HBr and formation of Al-OH-Br species. Similarly, it also reacts quickly with alcohols and carboxylic acids, although less vigorously than with water. With simple Lewis bases (L), Al2Br6 forms adducts, such as AlBr3L.

Al2Br6 is an excellent reagent for Friedel-Crafts and related Lewis acid-promoted reactions such as epoxide ring openings and decomplexation of dienes from iron carbonyls. It is a stronger Lewis acidic than the more common Al2Cl6. Because it is hygroscopic, older samples tend to be hydrated and less useful. Impure samples can be purified by vacuum sublimation.

[edit] Structures of the aluminium bromides

Aluminium monobromide has been crystallographically characterized in the form the tetrameric adduct Al4Br4(NEt3)4 (Et = C2H5). This species is electronically related to cyclobutane. Theory suggest that the diatomic aluminium monobromide condenses to a dimer and then a tetrahedral cluster Al4Br4, akin to the analogous boron compound.

Al2Br6 consists of two AlBr4 tetrahedra that share a common edge. The molecular symmetry is C2v.

The monomer AlBr3, observed only in the vapor, can be described as trigonal planar, D3h point group. The atomic hybridization of aluminium is often described as sp2. The Br-Al-Br bond angles are 120 °.

[edit] Safety precautions

Al2Br6 is corrosive and can cause skin burns if not washed off immediately after contact. It should be stored in a dry area.

[edit] References

  1. “Aluminum(I) and Gallium(I) Compounds: Syntheses, Structures, and Reactions” Dohmeier, C.; Loos, D.; Schnöckel, H. Angewandte Chemie International Edition in English (1996) Volume 35, Pages 129 - 149.
  2. Gugelchuk, M. "Aluminum Bromide" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.
  3. Armour, M.A., Hazardous laboratory chemicals : disposal guide. 2ème. Boca Raton, FL. : CRC Press. (1996). [RT-435017]