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Aluminium bromide

Aluminium bromide
Ball and stick model of dimeric aluminium bromide
Aluminium bromide from 1967 In large ampoules
Names
Preferred IUPAC name
Aluminium bromide
Other names
Aluminic bromide

Aluminium(III) bromide

Aluminium tribromide
Identifiers
3D model (Jmol)
ChemSpider
ECHA InfoCard 100.028.891
EC Number 231-779-7
PubChem CID
RTECS number BD0350000
UN number 1725
Properties
AlBr3
Al2Br6
Molar mass 266.69 g/mol
Appearance white to pale yellow
crystalline solid
Odor pungent
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
Structure
monoclinic
Hazards
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
Lethal dose or concentration (LD, LC):
LD50 (median dose)
 1598 mg/kg (oral, rat)
Related compounds
Other anions
aluminium trichloride
aluminium triiodide
Other cations
boron tribromide
Related compounds
 iron(III) bromide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
YesY  (what is YesYN ?)
Infobox references

Aluminium(III) bromide

Aluminium bromide is any chemical compound with the empirical formula AlBrx. The species called "aluminium tribromide," is the most common aluminium bromide. It is a colorless, sublimable hygroscopic solid, although older samples tend to be hydrated.

The dimeric form of aluminium tribromide (Al2Br6) predominates in the solid state, solutions in noncoordinating solvents (e.g. CS2), in the melt, and the gas phase. Only at high temperatures do these dimers break up into monomers:

The species aluminium monobromide forms from the reaction of HBr with Al metal at high temperature. It disproportionates near room temperature:

This reaction is reversed at temperatures higher than 1000 °C. 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 °.

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:

Alternatively, the direct bromination occurs also:

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.


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