Aluminium bromide

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Aluminium bromide
Ball and stick model of dimeric aluminium bromide
Identifiers
CAS number 7727-15-3 YesY, 7784-11-4 hexahydrate
PubChem 24409, 11062022 tertikis(tetrachloromethane), 10865226 hemi(acetyl bromide), 11324936 ethanethiol, 6093832 tris(pyridine)
ChemSpider 22818 YesY, 9040513 hemi(acetyl bromide) YesY, 9499890 ethanethiol YesY
EC number 231-779-7
UN number 1725
RTECS number BD0350000
Jmol-3D images Image 1
Properties
Molecular formula 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)
Solubility in water reacts
Solubility slightly soluble in methanol, diethyl ether, acetone
Hazards
EU classification not listed
NFPA 704
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g., canola oil Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g., calcium Special hazards (white): no codeNFPA 704 four-colored diamond
LD50 1598 mg/kg (oral, rat)
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 (77 °F), 100 kPa)
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Infobox 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 higher than 1000 °C.

Structure[edit]

"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[edit]

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[edit]

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, water hydrolizes Al2Br6 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 aluminium 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 acid 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[edit]

The anhydrous form is used as a catalyst for the Friedel-Crafts alkylation reaction. Its catalytic activity is similar to anhydrous aluminium chloride. However, commercial applications are rare.

References[edit]

  • Dohmeier, Carsten; Loos, Dagmar; Schnöckel, Hansgeorg (1996). "Aluminum(I) and Gallium(I) Compounds: Syntheses, Structures, and Reactions". Angewandte Chemie International Edition in English 35 (2): 129. doi:10.1002/anie.199601291. 
  • Encyclopedia of Reagents for Organic Synthesis. 2001. doi:10.1002/047084289X. ISBN 0471936235. 
  • Renfew, Malcom M. (1991). "Hazardous laboratory chemicals: Disposal guide (Armour, M.A.)". Journal of Chemical Education 68 (9): A232. doi:10.1021/ed068pA232.2.