Как пишется бромид алюминия

Бромид алюминия
Общие
Систематическое наименование	Бромид алюминия
Химическая формула	AlBr3, Al2Br6
Эмпирическая формула	AlBr3
Физические свойства
Состояние (ст. усл.)	твёрдое
Молярная масса	266,69 г/моль
Плотность	3,205[1] г/см³
Термические свойства
Температура плавления	97,5[1] °C
Температура кипения	255[2] °C
Энтальпия образования (ст. усл.)	− 514; − 422 (AlBr3, газ); − 971(Al2Br6, газ)[3] кДж/моль
Структура
Кристаллическая структура	моноклинная
Классификация
Рег. номер CAS	7727-15-3
Рег. номер EINECS	231-779-7

Броми́д алюми́ния (бромистый алюминий) — это неорганическое бинарное соединение. Химическая формула . Вещество представляет собой соль алюминия и бромоводородной кислоты. В твердом и жидком состоянии существует в форме димера: Al₂Br₆.

Физические свойства

Безводный бромид алюминия представляет собой бесцветное кристаллическое вещество, плавящееся при температуре 97,5 °C; температура кипения: 255 °C.

В твёрдой и жидкой фазе существует в форме димера Al₂Br₆, частично диссоциирующего в AlBr₃, в газовой фазе масс-спектры показывают наличие ди-, тетра- и гексаформ: Al₂Br₆, Al₄Br₁₂, Al₆Br₁₈ соответственно.

Структура молекулы бромида алюминия Al₂Br₆ представляет собой сдвоенные тетраэдры, в центре которых расположены атомы алюминия, ковалентно связанные с атомами брома^[4].

Координационное число алюминия в молекуле бромида равно 4^[5].

Энергия разрыва связи Al—Br в молекуле бромида алюминия составляет примерно 358 кДж/моль^[6].

Вещество очень гигроскопично: на воздухе расплывается, легко поглощая влагу с образованием гексагидрата AlBr₃•6H₂O^[7]. Хорошо растворимо в воде, спирте, сероуглероде, ацетоне^[8]; плотность водного раствора при 20 °C составляет: 1079,2 кг/м³ (10%-ный раствор), 1172,5 кг/м³ (20%-ный раствор)^[9].

Химические свойства

• Безводный бромид алюминия очень энергично реагирует с водой выделяя при растворении много тепла и, частично гидролизуясь:

При нагревании водного раствора гидролиз можно провести полностью:

• Вступает в реакцию со щелочами:

• При пропускании безводного сероводорода через раствор бромида алюминия в сероуглероде выпадает осадок комплексного соединения^[10]:

• При высокой температуре разлагается:

При нагревании бромида алюминия с алюминием в газовой фазе (1000 °C) образуется нестабильный монобромид алюминия^[2]:

• С гидридом лития образует алюмогидрид:

• Бромид алюминия — сильный акцептор электронных пар (кислота Льюиса) — легко присоединяет молекулы доноры (на этом, в частности, основано его применение в органическом синтезе)^[7]:

Получение

Безводный бромид алюминия получают взаимодействием элементов (Al и Br₂) при нагревании:

Водный раствор можно получить реакцией алюминиевой стружки с бромоводородной кислотой:

Применение

Коммерческое применение бромида алюминия в настоящий момент относительно небольшое.

Бромид алюминия входит как основной компонент в состав ксилольных электролитов для электроосаждения алюминиевых покрытий^[11].

Безводный бромид алюминия используется в органическом синтезе, в частности, в реакции алкилирования по Фриделю-Крафтсу, по аналогии с хлоридом алюминия.

Соединение может выступать катализатором в реакции изомеризации бромалканов, например^[12]:

Также бромид алюминия может выступать в качестве бромирующего агента, например в реакции с хлороформом^[13]:

Опасность для здоровья

При контакте с кожей бромид алюминия может вызывать ожоги.

Соединение умеренно ядовито: ЛД₅₀ (крысы) ≈ 1600 мг/кг (перорально); ЛД₅₀ (крысы) ≈ 815 мг/кг (внутрибрюшинно)^[14].

Литература

1. Downs A.J. Chemistry of aluminium, gallium, indium, and thallium. — First edition. — London: Chapman & Hall, 1993. — 526 p. — ISBN 0-7514-0103-X

Примечания

Aluminum, one of the components of aluminium bromide, accounts for over 8.1 percent of the Earth’s crust, making it one of the most prevalent minerals on the planet. Despite the fact that it is present in a wide range of compounds on Earth, aluminium is rarely discovered in its free metal state. Instead, it is frequently found in minerals such as bauxite and cryolite. Bromine, the other component of aluminium bromide, is a poisonous, oily, crimson liquid. Bromine, in addition to being employed in the manufacture of dyes, pesticides, and fertilizers, is occasionally detected in trace amounts in living species, although having no known biological purpose in humans.

What is Aluminium Bromide?

Aluminum bromide is a pale yellow chemical solution, also known as tribromoalumane or aluminium tribromide (the most common form). It is a colorless, hygroscopic solid that may be sublimated. Its molecular or chemical formula is AlBr₃. 

It appears in its anhydrous state as a white to yellowish-red compound with a lumpy solid and a strong odour. It appears as a liquid in its watery form. It is extremely damaging to the eyes, mucous membranes, and skin. It is frequently utilized in the production of many compounds. It is a solution in a wide range of organic solvents, including benzene, toluene, simple hydrocarbons, nitrobenzene, and xylene.

Structure of Aluminium Bromide

The formula of Aluminium Bromide is made up of one aluminium cation (Al³⁺) and one bromide anion (Br^–). Therefore, the molecular or chemical formula of Barium sulfate is AlBr₃. Its chemical formula reveals that it is composed of one aluminium atom and three bromine atoms. It has a molecular mass of 266.694 g/mol as an anhydrous chemical and 374.785 g/mol as a hexahydrate. It has a monoclinic crystalline structure with a positive dipole moment.

Physical Properties of Aluminium Bromide

• Aluminium Bromide is a white to pale yellow powder with a pungent smell.
• It is a colorless, sublimable hygroscopic solid which occurs mostly in the form of Aluminium tribromide.
• The density of Aluminium Bromide is 3.2 g/cm³ as anhydrous and 2.54 g/cm³ as a hexahydrate.
• The melting point of Aluminium Bromide is 97.5 °C as an anhydrous and 93 °C as a hexahydrate.
• The boiling point of Aluminium Bromide is 255 °C as an anhydrous and 252 °C as a hexahydrate.
• The heat capacity of Aluminium Bromide is 100.6 J J/mol K.
• The standard molar entropy of Aluminium Bromide is 180.2 J J/mol K.
• The standard enthalpy of formation of Aluminium Bromide is −572.5 kJ/mol.
• Its aqueous solution is neutral in nature with a pH of 6 at 14 °C.
• It is slightly soluble in methanol, diethyl ether, acetone but entirely soluble in water.

Chemical Properties of Aluminium Bromide

• At 100 °C, aluminium tribromide combines with carbon tetrachloride to produce carbon tetrabromide.

4 AlBr₃ + 3 CCl₄ → 4 AlCl₃ + 3 CBr₄

• It reacts with phosgene gas to produce carbonyl bromide and aluminium chlorobromide.

AlBr₃ + COCl₂ → COBr₂ + AlCl₂Br

• Water hydrolyzes Al₂Br₆ in line with the properties of Lewis acid. Furthermore, water hydrolysis occurs during the development of HBr and the production of the Al-OH-Br species. It also reacts fast with alcohol and carboxylic acids.

Uses of Aluminium Bromide

• It is used as a catalyst in Friedel–Crafts reactions.
• It is used as a medication in the field of health.
• As it is a more powerful Lewis acid than Al₂Cl₆, it is employed in industries to create various substances.
• It acts as an electron pair acceptor to increase the reactivity of a substrate.
• It has a number of specific chemical uses in synthesis and extractive metallurgy.

Sample Questions

Question 1: What is the process for the preparation of Aluminium Bromide?

Solution:

Aluminium bromide is prepared by the reaction of Hydrogen bromide (HBr) with aluminium metal (Al).

2 Al + 6 HBr → Al₂Br₆ + 3 H₂

The second method to obtain Aluminium Bromide is through bromination.

2 Al + 3 Br₂ → Al₂Br₆

Question 2: How does aluminium bromide exist in nature?

Solution:

In the solid state, solutions in noncoordinating solvents (e.g., CS₂), the melt, and the gas phase, the dimeric form of aluminium tribromide (Al₂Br₆) predominates. These dimers only break down into monomers at high temperatures.

Al₂Br₆ → 2 AlBr₃ (ΔH°diss = 59 kJ/mol)

Question 3: What are the side effects of aluminium bromide?

Solution:

Aluminum Bromide can harm you if inhaled. It is a highly corrosive chemical and direct contact can severely irritate and burn the skin and eyes, perhaps causing eye damage. It may irritate the nose, throat, and lungs, resulting in coughing, wheezing, and/or shortness of breath. Aluminum bromide is a reactive chemical that might cause an explosion.

Question 4: How can one check if he/she is exposed to aluminium bromide?

Solution:

The exposure of hazardous materials such as aluminium bromide should be assessed on a regular basis. This might entail taking personal and regional air samples. A worker’s employer can provide him with copies of the sample findings.

Question 5: Explain some methods of reducing exposure to aluminium bromide.

Solution:

Methods of Reducing Exposure to Aluminium Bromide are:

1. Enclose activities and employ local exhaust ventilation at the place of chemical discharge wherever practicable. Respirators should be worn if no local exhaust ventilation or enclosure is employed.
2. Wear appropriate job clothes.
3. Wash carefully after being exposed to aluminum bromide and at the end of the workshift.
4. Furthermore, as part of a continuing education and training effort, communicate to potentially exposed personnel all information on the health and safety concerns of Aluminum Bromide.

Aluminum, one of the components of aluminium bromide, accounts for over 8.1 percent of the Earth’s crust, making it one of the most prevalent minerals on the planet. Despite the fact that it is present in a wide range of compounds on Earth, aluminium is rarely discovered in its free metal state. Instead, it is frequently found in minerals such as bauxite and cryolite. Bromine, the other component of aluminium bromide, is a poisonous, oily, crimson liquid. Bromine, in addition to being employed in the manufacture of dyes, pesticides, and fertilizers, is occasionally detected in trace amounts in living species, although having no known biological purpose in humans.

What is Aluminium Bromide?

Aluminum bromide is a pale yellow chemical solution, also known as tribromoalumane or aluminium tribromide (the most common form). It is a colorless, hygroscopic solid that may be sublimated. Its molecular or chemical formula is AlBr₃. 

It appears in its anhydrous state as a white to yellowish-red compound with a lumpy solid and a strong odour. It appears as a liquid in its watery form. It is extremely damaging to the eyes, mucous membranes, and skin. It is frequently utilized in the production of many compounds. It is a solution in a wide range of organic solvents, including benzene, toluene, simple hydrocarbons, nitrobenzene, and xylene.

Structure of Aluminium Bromide

The formula of Aluminium Bromide is made up of one aluminium cation (Al³⁺) and one bromide anion (Br^–). Therefore, the molecular or chemical formula of Barium sulfate is AlBr₃. Its chemical formula reveals that it is composed of one aluminium atom and three bromine atoms. It has a molecular mass of 266.694 g/mol as an anhydrous chemical and 374.785 g/mol as a hexahydrate. It has a monoclinic crystalline structure with a positive dipole moment.

Physical Properties of Aluminium Bromide

• Aluminium Bromide is a white to pale yellow powder with a pungent smell.
• It is a colorless, sublimable hygroscopic solid which occurs mostly in the form of Aluminium tribromide.
• The density of Aluminium Bromide is 3.2 g/cm³ as anhydrous and 2.54 g/cm³ as a hexahydrate.
• The melting point of Aluminium Bromide is 97.5 °C as an anhydrous and 93 °C as a hexahydrate.
• The boiling point of Aluminium Bromide is 255 °C as an anhydrous and 252 °C as a hexahydrate.
• The heat capacity of Aluminium Bromide is 100.6 J J/mol K.
• The standard molar entropy of Aluminium Bromide is 180.2 J J/mol K.
• The standard enthalpy of formation of Aluminium Bromide is −572.5 kJ/mol.
• Its aqueous solution is neutral in nature with a pH of 6 at 14 °C.
• It is slightly soluble in methanol, diethyl ether, acetone but entirely soluble in water.

Chemical Properties of Aluminium Bromide

• At 100 °C, aluminium tribromide combines with carbon tetrachloride to produce carbon tetrabromide.

4 AlBr₃ + 3 CCl₄ → 4 AlCl₃ + 3 CBr₄

• It reacts with phosgene gas to produce carbonyl bromide and aluminium chlorobromide.

AlBr₃ + COCl₂ → COBr₂ + AlCl₂Br

• Water hydrolyzes Al₂Br₆ in line with the properties of Lewis acid. Furthermore, water hydrolysis occurs during the development of HBr and the production of the Al-OH-Br species. It also reacts fast with alcohol and carboxylic acids.

Uses of Aluminium Bromide

• It is used as a catalyst in Friedel–Crafts reactions.
• It is used as a medication in the field of health.
• As it is a more powerful Lewis acid than Al₂Cl₆, it is employed in industries to create various substances.
• It acts as an electron pair acceptor to increase the reactivity of a substrate.
• It has a number of specific chemical uses in synthesis and extractive metallurgy.

Sample Questions

Question 1: What is the process for the preparation of Aluminium Bromide?

Solution:

Aluminium bromide is prepared by the reaction of Hydrogen bromide (HBr) with aluminium metal (Al).

2 Al + 6 HBr → Al₂Br₆ + 3 H₂

The second method to obtain Aluminium Bromide is through bromination.

2 Al + 3 Br₂ → Al₂Br₆

Question 2: How does aluminium bromide exist in nature?

Solution:

In the solid state, solutions in noncoordinating solvents (e.g., CS₂), the melt, and the gas phase, the dimeric form of aluminium tribromide (Al₂Br₆) predominates. These dimers only break down into monomers at high temperatures.

Al₂Br₆ → 2 AlBr₃ (ΔH°diss = 59 kJ/mol)

Question 3: What are the side effects of aluminium bromide?

Solution:

Aluminum Bromide can harm you if inhaled. It is a highly corrosive chemical and direct contact can severely irritate and burn the skin and eyes, perhaps causing eye damage. It may irritate the nose, throat, and lungs, resulting in coughing, wheezing, and/or shortness of breath. Aluminum bromide is a reactive chemical that might cause an explosion.

Question 4: How can one check if he/she is exposed to aluminium bromide?

Solution:

The exposure of hazardous materials such as aluminium bromide should be assessed on a regular basis. This might entail taking personal and regional air samples. A worker’s employer can provide him with copies of the sample findings.

Question 5: Explain some methods of reducing exposure to aluminium bromide.

Solution:

Methods of Reducing Exposure to Aluminium Bromide are:

1. Enclose activities and employ local exhaust ventilation at the place of chemical discharge wherever practicable. Respirators should be worn if no local exhaust ventilation or enclosure is employed.
2. Wear appropriate job clothes.
3. Wash carefully after being exposed to aluminum bromide and at the end of the workshift.
4. Furthermore, as part of a continuing education and training effort, communicate to potentially exposed personnel all information on the health and safety concerns of Aluminum Bromide.

From Wikipedia, the free encyclopedia

Names
Preferred IUPAC name Aluminium bromide
Other names Aluminic bromide Aluminium(III) bromide Aluminium tribromide
Identifiers
CAS Number	7727-15-3  7784-11-4 hexahydrate
3D model (JSmol)	Interactive image dimer: Interactive image
ChemSpider	22818  9040513 hemi(acetyl bromide)  9499890 ethanethiol
ECHA InfoCard	100.028.891
EC Number	231-779-7
PubChem CID	24409 11062022 tertikis(tetrachloromethane) 10865226 hemi(acetyl bromide) 11324936 ethanethiol 6093832 tris(pyridine)
RTECS number	BD0350000
UNII	IY20HBK5LK
UN number	1725
CompTox Dashboard (EPA)	DTXSID2064783
InChI InChI=1S/Al.3BrH/h;3*1H/q+3;;;/p-3  Key: PQLAYKMGZDUDLQ-UHFFFAOYSA-K  InChI=1/Al.3BrH/h;3*1H/q+3;;;/p-3 Key: PQLAYKMGZDUDLQ-DFZHHIFOAT
SMILES Br[Al](Br)Br dimer: Br[Al-]1(Br)[Br+][Al-]([Br+]1)(Br)Br
Properties
Chemical formula	AlBr3 Al2Br6 AlBr3·6H2O (hexahydrate)
Molar mass	266.694 g/mol (anhydrous) 374.785 g/mol (hexahydrate)[1]
Appearance	white to pale yellow powder[1]
Odor	pungent
Density	3.2 g/cm3 (anhydrous) 2.54 g/cm3 (hexahydrate)[1]
Melting point	97.5 °C (anhydrous) 93 °C (hexahydrate)[1]
Boiling point	255 (anhydrous)[1]
Solubility in water	very soluble, partially hydrolyses indicated by a fuming solution and an optional appearance of white precipitate
Solubility	slightly soluble in methanol, diethyl ether, acetone
Structure[2]
Crystal structure	Monoclinic, mP16 (anhydrous)
Space group	P21/c, No. 14
Lattice constant	a = 0.7512 nm, b = 0.7091 nm, c = 1.0289 nm α = 90°, β = 96.44°, γ = 90°
Formula units (Z)	4
Thermochemistry[1]
Heat capacity (C)	100.6 J/(mol·K)
Std molar entropy (S⦵298)	180.2 J/(mol·K)
Std enthalpy of formation (ΔfH⦵298)	-572.5 kJ/mol
Hazards
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H302, H314
Precautionary statements	P260, P264, P270, P280, P301+P312, P301+P330+P331, P303+P361+P353, P304+P340, P305+P351+P338, P310, P321, P330, P363, P405, P501
NFPA 704 (fire diamond)	3 1 1
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).  verify (what is  ?) Infobox references

Aluminium bromide is any chemical compound with the empirical formula AlBr_x. Aluminium tribromide is the most common form of aluminium bromide.^[3] It is a colorless, sublimable hygroscopic solid; hence old samples tend to be hydrated, mostly as aluminium tribromide hexahydrate (AlBr₃·6H₂O).

Structure[edit]

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

Al₂Br₆ → 2 AlBr₃ ΔH°_diss = 59 kJ/mol

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

6/n «[AlBr]_n» → Al₂Br₆ + 4 Al

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.^[4]

Al₂Br₆ consists of two AlBr₄ tetrahedra that share a common edge. The molecular symmetry is D_2h.

The monomer AlBr₃, observed only in the vapor, can be described as trigonal planar, D_3h point group. The atomic hybridization of aluminium is often described as sp². The Br-Al-Br bond angles are 120 °.

Synthesis[edit]

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:

2 Al + 6 HBr → Al₂Br₆ + 3 H₂

Alternatively, the direct bromination occurs also:

2 Al + 3 Br₂ → Al₂Br₆

Reactions[edit]

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 is hydrolyzed by 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 AlBr₃L.

Aluminium tribromide reacts with carbon tetrachloride at 100 °C to form carbon tetrabromide:

4 AlBr₃ + 3 CCl₄ → 4 AlCl₃ + 3 CBr₄

and with phosgene yields carbonyl bromide and aluminium chlorobromide:^{[citation needed]}

AlBr₃ + COCl₂ → COBr₂ + AlCl₂Br

Al₂Br₆ is used as a catalyst for the Friedel-Crafts alkylation reaction.^[3] Related Lewis acid-promoted reactions include as epoxide ring openings and decomplexation of dienes from iron carbonyls. It is a stronger Lewis acid than the more common Al₂Cl₆.

Safety[edit]

Aluminium tribromide is a highly reactive material.^[5]

References[edit]

From Wikipedia, the free encyclopedia

Names
Preferred IUPAC name Aluminium bromide
Other names Aluminic bromide Aluminium(III) bromide Aluminium tribromide
Identifiers
CAS Number	7727-15-3  7784-11-4 hexahydrate
3D model (JSmol)	Interactive image dimer: Interactive image
ChemSpider	22818  9040513 hemi(acetyl bromide)  9499890 ethanethiol
ECHA InfoCard	100.028.891
EC Number	231-779-7
PubChem CID	24409 11062022 tertikis(tetrachloromethane) 10865226 hemi(acetyl bromide) 11324936 ethanethiol 6093832 tris(pyridine)
RTECS number	BD0350000
UNII	IY20HBK5LK
UN number	1725
CompTox Dashboard (EPA)	DTXSID2064783
InChI InChI=1S/Al.3BrH/h;3*1H/q+3;;;/p-3  Key: PQLAYKMGZDUDLQ-UHFFFAOYSA-K  InChI=1/Al.3BrH/h;3*1H/q+3;;;/p-3 Key: PQLAYKMGZDUDLQ-DFZHHIFOAT
SMILES Br[Al](Br)Br dimer: Br[Al-]1(Br)[Br+][Al-]([Br+]1)(Br)Br
Properties
Chemical formula	AlBr3 Al2Br6 AlBr3·6H2O (hexahydrate)
Molar mass	266.694 g/mol (anhydrous) 374.785 g/mol (hexahydrate)[1]
Appearance	white to pale yellow powder[1]
Odor	pungent
Density	3.2 g/cm3 (anhydrous) 2.54 g/cm3 (hexahydrate)[1]
Melting point	97.5 °C (anhydrous) 93 °C (hexahydrate)[1]
Boiling point	255 (anhydrous)[1]
Solubility in water	very soluble, partially hydrolyses indicated by a fuming solution and an optional appearance of white precipitate
Solubility	slightly soluble in methanol, diethyl ether, acetone
Structure[2]
Crystal structure	Monoclinic, mP16 (anhydrous)
Space group	P21/c, No. 14
Lattice constant	a = 0.7512 nm, b = 0.7091 nm, c = 1.0289 nm α = 90°, β = 96.44°, γ = 90°
Formula units (Z)	4
Thermochemistry[1]
Heat capacity (C)	100.6 J/(mol·K)
Std molar entropy (S⦵298)	180.2 J/(mol·K)
Std enthalpy of formation (ΔfH⦵298)	-572.5 kJ/mol
Hazards
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H302, H314
Precautionary statements	P260, P264, P270, P280, P301+P312, P301+P330+P331, P303+P361+P353, P304+P340, P305+P351+P338, P310, P321, P330, P363, P405, P501
NFPA 704 (fire diamond)	3 1 1
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).  verify (what is  ?) Infobox references

Aluminium bromide is any chemical compound with the empirical formula AlBr_x. Aluminium tribromide is the most common form of aluminium bromide.^[3] It is a colorless, sublimable hygroscopic solid; hence old samples tend to be hydrated, mostly as aluminium tribromide hexahydrate (AlBr₃·6H₂O).

Structure[edit]

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

Al₂Br₆ → 2 AlBr₃ ΔH°_diss = 59 kJ/mol

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

6/n «[AlBr]_n» → Al₂Br₆ + 4 Al

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.^[4]

Al₂Br₆ consists of two AlBr₄ tetrahedra that share a common edge. The molecular symmetry is D_2h.

The monomer AlBr₃, observed only in the vapor, can be described as trigonal planar, D_3h point group. The atomic hybridization of aluminium is often described as sp². The Br-Al-Br bond angles are 120 °.

Synthesis[edit]

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:

2 Al + 6 HBr → Al₂Br₆ + 3 H₂

Alternatively, the direct bromination occurs also:

2 Al + 3 Br₂ → Al₂Br₆

Reactions[edit]

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 is hydrolyzed by 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 AlBr₃L.

Aluminium tribromide reacts with carbon tetrachloride at 100 °C to form carbon tetrabromide:

4 AlBr₃ + 3 CCl₄ → 4 AlCl₃ + 3 CBr₄

and with phosgene yields carbonyl bromide and aluminium chlorobromide:^{[citation needed]}

AlBr₃ + COCl₂ → COBr₂ + AlCl₂Br

Al₂Br₆ is used as a catalyst for the Friedel-Crafts alkylation reaction.^[3] Related Lewis acid-promoted reactions include as epoxide ring openings and decomplexation of dienes from iron carbonyls. It is a stronger Lewis acid than the more common Al₂Cl₆.

Safety[edit]

Aluminium tribromide is a highly reactive material.^[5]

References[edit]

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