Aluminium bromide
2493259
221874074
2008-06-26T13:52:28Z
Michał Sobkowski
5315381
iw:pl
{{Chembox new
| Name = Aluminium bromide
| ImageFile = Aluminium-bromide-3D-balls.png
<!-- | ImageSize = 200px -->
| ImageName = Aluminium bromide
| OtherNames = aluminium(III) bromide<br />aluminium tribromide
| Section1 = {{Chembox Identifiers
| CASNo = 7727-15-3
}}
| Section2 = {{Chembox Properties
| Formula = AlBr<sub>3</sub><br />Al<sub>2</sub>Br<sub>6</sub>
| MolarMass = 266.694 g/mol
| Appearance = white to pale yellow<br /> crystalline solid
| Density = 3.205 g cm<sup>−3</sup> as solid
| Solubility = hydrolyzes
| MeltingPt = 97.5 °C
| BoilingPt = 255 °C
}}
| Section7 = {{Chembox Hazards
| EUClass = not listed
| NFPA-H = 3
| NFPA-R = 2
| NFPA-F = 0
}}
| Section8 = {{Chembox Related
| OtherAnions = [[aluminium trichloride]]<br /> [[aluminium triiodide]]
| OtherCations = [[boron tribromide]]
| OtherCpds = [[iron(III) bromide]]
}}
}}
'''Aluminium bromide''' is any chemical compound with the empirical formula AlBr<sub>x</sub>. 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 [[disproportionation|disproportionate]]s near room temperature:
:6/n "[AlBr]<sub>n</sub>" → Al<sub>2</sub>Br<sub>6</sub> + 4 Al
This reaction is reversed at temperatures > 1000 °C.
==Synthesis==
By far the most common form of aluminium bromide is Al<sub>2</sub>Br<sub>6</sub>. 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 [[Hydrogen bromide|HBr]] → Al<sub>2</sub>Br<sub>6</sub> + 3 H<sub>2</sub>
Alternatively, the direct bromination occurs also:
:2 Al + 3 [[Bromine|Br<sub>2</sub>]] → Al<sub>2</sub>Br<sub>6</sub>
==Structure==
"Aluminium tribromide" is really dialuminium hexabromide with the molecular formula of Al<sub>2</sub>Br<sub>6</sub> in the solid state, solutions in noncoordinating solvents (e.g. CS<sub>2</sub>), and in the melt. Even upon evaporation, Al<sub>2</sub>Br<sub>6</sub> exists in the gas phase. At high temperatures, the gaseous molecules break up into monomers:
: Al<sub>2</sub>Br<sub>6</sub> → 2 AlBr<sub>3</sub> ΔH°<sub>diss</sub> = 59 kJ/mol
==Representative reactions of Al<sub>2</sub>Br<sub>6</sub>==
Al<sub>2</sub>Br<sub>6</sub> dissociates readily to give the strong [[Lewis acid]], AlBr<sub>3</sub>. Regarding the tendency of Al<sub>2</sub>Br<sub>6</sub> to [[dimer]]ize, it is common for heavier main group halides to exist as aggregates larger than implied by their empirical formulae. Lighter [[main group]] [[halide]]s 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, Al<sub>2</sub>Br<sub>6</sub> 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 base]]s (L), Al<sub>2</sub>Br<sub>6</sub> forms [[adduct]]s, such as AlBr<sub>3</sub>L.
Al<sub>2</sub>Br<sub>6</sub> is an excellent reagent for [[Friedel-Crafts reaction|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 [[Aluminium chloride|Al<sub>2</sub>Cl<sub>6</sub>]]. 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 Al<sub>4</sub>Br<sub>4</sub>(NEt<sub>3</sub>)<sub>4</sub> (Et = C<sub>2</sub>H<sub>5</sub>). This species is electronically related to cyclobutane. Theory suggest that the diatomic aluminium monobromide condenses to a dimer and then a tetrahedral [[cluster]] Al<sub>4</sub>Br<sub>4</sub>, akin to the analogous boron compound.
Al<sub>2</sub>Br<sub>6</sub> consists of two AlBr<sub>4</sub> tetrahedra that share a common edge. The [[molecular symmetry]] is C<sub>2v</sub>.
The monomer AlBr<sub>3</sub>, observed only in the vapor, can be described as [[trigonal planar]], D<sub>3h</sub> point group. The atomic hybridization of [[aluminium]] is often described as ''sp<sup>2</sup>''. The [[Bromine|Br]]-[[Aluminium|Al]]-[[Bromine|Br]] bond angles are 120 °.
==Safety precautions==
Al<sub>2</sub>Br<sub>6</sub> is corrosive and can cause skin burns if not washed off immediately after contact. It should be stored in a dry area.
==References==
<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]
[[Category:Bromides]]
[[Category:Aluminium compounds]]
[[Category:Metal halides]]
[[ar:بروميد ألومنيوم]]
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[[fr:bromure d'aluminium]]
[[ku:Bafûnbromîd]]
[[pl:Bromek glinu]]