Aluminum bromide is any chemical compound with the empirical formula AlBrx . Aluminum tribromide is the most common form of aluminum bromide. [3] It is a colorless, sublimable hygroscopic solid; The older samples are therefore hydrated, mostly in the form of aluminum tribromide hexahydrate (AlBr 3 6H 2 O).
structure
The dimeric form of aluminum tribromide (Al 2 Br 6 ) occurs in the solid state, in non-coordinating solvents (such as CS 2 ), in the melt and in the gas phase. Only at high temperatures do these dimers break down into monomers:
Al 2 Br 6 → 2 Albr 3 H ° Dis = 59 kJ/mol
The reaction of HBr with Al metal at high temperature produces aluminum monobromide species. It is disproportionate near room temperature:
6/n “[AlBr] n ” → Al 2 Br 6 + 4 Al
This reaction reverses at temperatures above 1000 °C. Aluminum monobromide is depicted crystallographically as the tetrameric adduct Al4Br4 ( Net3 ) 4 ( At = C2H5 ) . This species is electronically related to cyclobutane. The theory suggests that diatomic aluminum monobromide forms a dimer and then a tetrahedral cluster Al4Br4 , similar to the analogous boron compound.
Al2Br6 consists of two LBr4 tetrahedra that share a common edge . The molecular symmetry d2h .
The monomer AlBr3 , observed only in vapor, can be described as a tri-planar, D3H point group . The atomic hybridization of aluminum is often described as sp 2 . The Br – Al – Br bond angles are 120°.
synthesis
By far the most common form of aluminum bromide is Al 2 Br 6 . This species exists as a hygroscopic colorless solid under standard conditions. Typical impure samples are yellow or reddish-brown in color due to the presence of iron-containing impurities. It is prepared by the reaction of HBr with Al:
2 Al + 6 HBr → Al 2 Br 6 + 3H 2
Alternatively, direct bromination also occurs
:2 Al + 3 Br 2 → Al 2 Br 6
Reactions
Al2Br6 dissociates readily to give the strong Lewis acid , AlBr3 . With regard to the dimerization tendency of Al 2 Br 6 , it is common for the heavier main group halides to exist as larger aggregates than implied by their empirical formulae. Lighter main group halides such as boron tribromide do not show this trend, in part because of the smaller size of the central atom.
Consistent with its Lewis acidic character, water hydrolyzes with the evolution of Al 2 Br 6 HBr and the formation of Al-OH-Br species. Similarly, it also reacts rapidly with alcohols and carboxylic acids, although less rapidly than water. With simple Lewis bases (L), Al2Br6 forms adducts, such as LBr3L .
Aluminum tribromide reacts with carbon tetrabromide at 100 °C to form carbon tetrachloride: 4 AlBr 3 + 3 CCl 4 → 4 AlCl 3 + 3 CBr 4
and yields of cyanide with carbonyl bromide and aluminum chlorobromide: AlBr 3 + COCl 2 → COBr 2 + AlCl 2 Br
Al 2 Br 6 is used as a catalyst for the Friedel-Crafts alkylation reaction. [3] Related Lewis acid-promoting reactions include epoxide ring opening and dissociation of dienes from iron carbonyls. It is a stronger Lewis acid than the more common Al 2 Cl 6.
Security
Aluminum tribromide is a highly reactive material.
