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]] [[da:Aluminiumbromid]] [[de:Aluminiumbromid]] [[fr:bromure d'aluminium]] [[ku:Bafûnbromîd]] [[pl:Bromek glinu]]