Aluminium bromide
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| Names | |
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| Preferred IUPAC name
Aluminium bromide | |
| Other names
Aluminic bromide
Aluminium(III) bromide | |
| Identifiers | |
3D model (JSmol)
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| ChemSpider | |
| ECHA InfoCard | 100.028.891 |
| EC Number |
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PubChem CID
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| RTECS number |
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| UN number | 1725 |
CompTox Dashboard (EPA)
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| Properties | |
| AlBr3 Al2Br6 | |
| Molar mass | 266.69 g/mol |
| Appearance | white to pale yellow crystalline solid |
| Density | 3.205 g/cm3 |
| Melting point | 97.8 °C (208.0 °F; 370.9 K) |
| Boiling point | 265 °C (509 °F; 538 K) |
| reacts | |
| Solubility | slightly soluble in methanol, diethyl ether, acetone |
| Hazards | |
| NFPA 704 (fire diamond) | |
| Lethal dose or concentration (LD, LC): | |
LD50 (median dose)
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1598 mg/kg (oral, rat) |
| Related compounds | |
Other anions
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aluminium trichloride aluminium triiodide |
Other cations
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boron tribromide |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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.
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
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 D2h.
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 °.
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
Reactions
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.
Aluminium tribromide reacts with carbon tetrachloride at 100 °C to form carbon tetrabromide:
- 4 AlBr3 + 3 CCl4 → 4 AlCl3 + 3 CBr4
and with phosgene yields carbonyl bromide and aluminum chlorobromide:[citation needed]
- AlBr3 + COCl2 → COBr2 + AlCl2Br
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.
Uses
The anhydrous form is used as a catalyst for the Friedel-Crafts alkylation reaction. Its catalytic activity is similar to anhydrous aluminum chloride. However, commercial applications are rare.
References
- “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.
- Gugelchuk, M. "Aluminum Bromide" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.
- Armour, M.A., Hazardous laboratory chemicals : disposal guide. 2ème. Boca Raton, FL. : CRC Press. (1996). [RT-435017]