Aluminium bromide

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Aluminium tribromide (anhydrous)
Aluminium tribromide
IUPAC name aluminium tribromide
tribromoalumane
tribromidoaluminium
Other names aluminium(III) bromide
Identifiers
InChI InChI=1/Al.3BrH/h;3*1H/q+3;;;/p-3
InChIKey PQLAYKMGZDUDLQ-DFZHHIFOAT
Standard InChI InChI=1S/Al.3BrH/h;3*1H/q+3;;;/p-3
Standard InChIKey PQLAYKMGZDUDLQ-UHFFFAOYSA-K
CAS number [7727-15-3]
EC number 231-779-7
RTECS BD0350000
ChemSpider 22818
Properties[1]
Chemical formula AlBr3
Molar mass 266.72 g mol−1
Appearance white to pale yellow solid, hygroscopic
Density 2.64 g cm−3
Melting point

97.5 °C

Boiling point

265 °C

Solubility in water reacts violently
Solubility soluble in ethanol, acetone, carbon disulfide
Thermochemistry (Al2Br6, gas phase)[2]
Std enthalpy of formation ΔfHo298 −937.22 kJ mol−1
Standard molar entropy So298 547.18 J K−1 mol−1
Hazards[3]
GHS pictograms Skin Corr. 1C, Eye Dam. 1Acute Tox. (oral) 4
GHS signal word DANGER
GHS hazard statements H302, H314
GHS precautionary statements P260, P264, P270, P280, P301+312, P301+330+331, P363, P304+340, P310, P321, P305+351+338, P405, P501
Flash point non-flammable
Related compounds
Other anions Aluminium fluoride
Aluminium chloride
Aluminium iodide
Other cations Boron tribromide
Gallium(III) bromide
Indium(III) bromide
Thallium(III) bromide
Other Lewis acids Iron(III) bromide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)

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:[4]

6/n "[AlBr]n" → Al2Br6 + 4Al

This reaction is reversed at temperatures above 1000 °C.

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:

2Al + 6HBr → Al2Br6 + 3H2

Alternatively, the direct bromination occurs also:

2Al + 3Br2 → Al2Br6

Structure

AlBr3 has an almost molecular structure in the solid state, with the Al3+ centers occupying adjacent tetrahedral holes of the close-packed framework of Br ions. "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:[4]

Al2Br6 → 2AlBr3
ΔHodiss = 59 kJ/mol

Representative 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 AlBr3:L.

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.

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 °.

Chemical Reactions

Reacts with carbon tetrachloride at 100 °C to form carbon tetrabromide:

4AlBr3 + 3CCl4 → 4AlCl3 + 3CBr4

Reacts with phosgene to yield carbonyl bromide and aluminum chlorobromide:

AlBr3 + COCl2 → COBr2 + AlCl2Br

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.

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.

References

  1. CRC Handbook of Chemistry and Physics, 62nd ed.; Weast, Robert C., Ed.; CRC Press: Boca Raton, FL, 1981; p B-73. ISBN 0-8493-0462-8.
  2. Dialuminum hexabromide. In NIST Chemistry WebBook; National Institute for Standards and Technology, <http://webbook.nist.gov/cgi/cbook.cgi?ID=C18898345&Units=SI&Mask=7>. (accessed 8 January 2011).
  3. HSNO Chemical Classification Information Database, <http://www.ermanz.govt.nz/Chemicals/ChemicalDisplay.aspx?SubstanceID=12437> (accessed 22 August 2009), New Zealand Environmental Risk Management Authority.
  4. 4.0 4.1 Greenwood, Norman N.; Earnshaw, A. Chemistry of the Elements; Pergamon: Oxford, 1984; pp 262–66. ISBN 0-08-022057-6.

Further reading

  • Dohmeier, C.; Loos, D.; Schnöckel, H. Aluminum(I) and Gallium(I) Compounds: Syntheses, Structures, and Reactions. Angew. Chem., Int. Ed. Engl. 1996, 35, 129–49.
  • Gugelchuk, M. Aluminum Bromide. In Encyclopedia of Reagents for Organic Synthesis; Paquette, Leo A., Ed.; John Wiley: New York, 2004. DOI: 10.1002/047084289.
  • Armour, M. A. Hazardous Laboratory Chemicals: Disposal Guide, 2nd ed.; CRC Press: Boca Raton, FL., 1996.

External links

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