aluminium chloride

File:Aluminium-trichloride-3D-structures.png

{{chem2|AlCl3}} adopts three structures, depending on the temperature and the state (solid, liquid, gas). Solid {{chem2|AlCl3}} has a sheet-like layered structure with cubic close-packed chloride ions. In this framework, the Al centres exhibit octahedral coordination geometry.{{cite book | quote = In contrast, {{chem2|AlBr3}} has a more molecular structure, with the {{chem2|Al(3+)}} centers occupying adjacent tetrahedral holes of the close-packed framework of {{chem2|Br−}} ions. | vauthors = Wells AF | date = 1984 | title = Structural Inorganic Chemistry | publisher = Oxford Press | location = Oxford, United Kingdom. | isbn = 0198553706}} Yttrium(III) chloride adopts the same structure, as do a range of other compounds. When aluminium trichloride is in its melted state, it exists as the dimer ({{chem2|Al2Cl6}} point group D_2h), with tetracoordinate aluminium. This change in structure is related to the lower density of the liquid phase (1.78 g/cm³) versus solid aluminium trichloride (2.48 g/cm³). {{chem2|Al2Cl6}} dimers are also found in the vapour phase. At higher temperatures, the {{chem2|Al2Cl6}} dimers dissociate into trigonal planar {{chem2|AlCl3}} monomer (point group D_3h), which is structurally analogous to boron trifluoride. The melt conducts electricity poorly,{{Greenwood&Earnshaw1st}} unlike more ionic halides such as sodium chloride.

The hexahydrate consists of octahedral {{chem2|[Al(H2O)6](3+)}} cation centers and chloride anions ({{chem2|Cl-}}) as counterions. Hydrogen bonds link the cation and anions.{{cite journal| vauthors = Andress KR, Carpenter C |doi=10.1524/zkri.1934.87.1.446|title=Kristallhydrate II. Die Struktur von Chromchlorid- und Aluminiumchloridhexahydrat|journal=Zeitschrift für Kristallographie – Crystalline Materials|volume=87|year=1934|s2cid=263857074 }}

The hydrated form of aluminium chloride has an octahedral molecular geometry, with the central aluminium ion surrounded by six water ligand molecules. Being coordinatively saturated, the hydrate is of little value as a catalyst in Friedel-Crafts alkylation and related reactions.

{{chem2|AlCl3}} is a common Lewis-acid catalyst for Friedel-Crafts reactions, both acylations and alkylations. These types of reactions are the major use for aluminium chloride, for example, in the preparation of anthraquinone (used in the dyestuffs industry) from benzene and phosgene. In the general Friedel-Crafts reaction, an acyl chloride or alkyl halide reacts with an aromatic system as shown:

:File:Benzene Friedel-Crafts alkylation-diagram.svg

The alkylation reaction is more widely used than the acylation reaction, although its practice is more technically demanding. For both reactions, the aluminium chloride, as well as other materials and the equipment, should be dry, although a trace of moisture is necessary for the reaction to proceed.{{cite journal| vauthors = Nenitzescu CD, Cantuniari IP |date=1933|title=Durch Aluminiumchlorid Katalysierte Reaktion, VI. Mitteil.: Die Umlagerung des Cyclohexans in Metyl-cyclopentan |journal=Berichte der Deutschen Chemischen Gesellschaft (A and B Series)|language=en|volume=66|issue=8|pages=1097–1100|doi=10.1002/cber.19330660817|issn=1099-0682}} Detailed procedures are available for alkylation{{cite journal |doi=10.15227/orgsyn.089.0210|title=Synthesis of Trifluoromethyl Ketones from Carboxylic Acids: 4-(3,4-Dibromophenyl)-1,1,1-trifluoro-4-methylpentan-2-one|journal=Organic Syntheses|year=2012|volume=89|page=210| vauthors = Reeves JT, Tan Z, Fandrick DR, Song JJ, Yee NK, Senanayake CH |doi-access=free}} and acylation{{cite journal |doi=10.15227/orgsyn.080.0227|title=Preparation of 9,10-Dimethoxyphenanthrene and 3,6-Diacetyl-9,10-Dimethoxyphenanthrene|journal=Organic Syntheses|year=2003|volume=80|page=227| vauthors = Paruch K, Vyklicky L, Katz TJ }}{{cite journal |doi=10.15227/orgsyn.079.0204|title=3-(4-Bromobenzoyl)propanoic Acid|journal=Organic Syntheses|year=2002|volume=79|page=204| vauthors = Seed AJ, Sonpatki V, Herbert MR }} of arenes.

A general problem with the Friedel-Crafts reaction is that the aluminium chloride catalyst sometimes is required in full stoichiometric quantities, because it complexes strongly with the products. This complication sometimes generates a large amount of corrosive waste. For these and similar reasons, the use of aluminium chloride has often been displaced by zeolites.

Aluminium chloride can also be used to introduce aldehyde groups onto aromatic rings, for example via the Gattermann-Koch reaction which uses carbon monoxide, hydrogen chloride and a copper(I) chloride co-catalyst.{{cite book | vauthors = Wade LG | date = 2003 | title = Organic Chemistry | edition = 5th | publisher = Prentice Hall | location = Upper Saddle River, New Jersey | isbn = 013033832X }}

:File:AlCl3 formylation.gif

Aluminium chloride finds a wide variety of other applications in organic chemistry.{{cite book | vauthors = Galatsis P | chapter = Aluminum Chloride | date = 1999 | series = Handbook of Reagents for Organic Synthesis | title = Acidic and Basic Reagents | veditors = Reich HJ, Rigby JH | publisher = Wiley | location = New York City | pages = 12–15 | isbn = 978-0-471-97925-8 }} For example, it can catalyse the ene reaction, such as the addition of 3-buten-2-one (methyl vinyl ketone) to carvone:{{cite journal | vauthors = Snider BB | title = Lewis-acid catalyzed ene reactions | journal = Acc. Chem. Res. | volume = 13 | page = 426 | year = 1980 | doi = 10.1021/ar50155a007 | issue = 11}}

:File:AlCl3 ene rxn.gif

It is used to induce a variety of hydrocarbon couplings and rearrangements.{{cite journal |doi=10.15227/orgsyn.059.0085|title=Highly Reactive Magnesium for the Preparation of Grignard Reagents: 1-Norbornanecarboxylic Acid|journal=Organic Syntheses|year=1979|volume=59|page=85| vauthors = Rieke RD, Bales SE, Hudnall PM, Burns TP, Poindexter GS }}{{cite journal |doi=10.15227/orgsyn.061.0062|title=Hexamethyl Dewar Benzene|journal=Organic Syntheses|year=1983|volume=61|page=62| vauthors = Shama SA, Wamser CC }}

Aluminium chloride combined with aluminium in the presence of an arene can be used to synthesize bis(arene) metal complexes, e.g. bis(benzene)chromium, from certain metal halides via the Fischer–Hafner synthesis. Dichlorophenylphosphine is prepared by reaction of benzene and phosphorus trichloride catalyzed by aluminium chloride.{{cite journal | vauthors = Buchner B, Lockhart Jr LB | title = Phenyldichlorophosphine |journal=Organic Syntheses | doi = 10.15227/orgsyn.031.0088 | volume = 31 | page = 88 | year = 1951}}

Topical aluminum chloride hexahydrate is used for the treatment of hyperhidrosis (excessive sweating).{{cite journal | vauthors = McConaghy JR, Fosselman D | title = Hyperhidrosis: Management Options | journal = American Family Physician | volume = 97 | issue = 11 | pages = 729–734 | date = June 2018 | pmid = 30215934 | doi = }}{{cite journal | vauthors = Nawrocki S, Cha J | title = The etiology, diagnosis, and management of hyperhidrosis: A comprehensive review: Therapeutic options | journal = Journal of the American Academy of Dermatology | volume = 81 | issue = 3 | pages = 669–680 | date = September 2019 | pmid = 30710603 | doi = 10.1016/j.jaad.2018.11.066 }}{{cite web | title = Aluminum Chloride (Topical) (Monograph) | url = https://www.drugs.com/monograph/aluminum-chloride-topical.html | work = American Society of Health System Pharmacists (ASHP) | publisher = drugs.com }}

Anhydrous aluminium chloride is a powerful Lewis acid, capable of forming Lewis acid-base adducts with even weak Lewis bases such as benzophenone and mesitylene.{{cite book | veditors = Olah GA |year=1963 |title=Friedel-Crafts and Related Reactions |volume=1 |publisher=Interscience |place=New York City}} It forms tetrachloroaluminate ({{chem2|[AlCl4]−}}) in the presence of chloride ions.

Aluminium chloride reacts with calcium and magnesium hydrides in tetrahydrofuran forming tetrahydroaluminates.{{cn|date=September 2023}}

Anhydrous aluminium chloride is hygroscopic, having a very pronounced affinity for water. It fumes in moist air and hisses when mixed with liquid water as the Cl⁻ ligands are displaced with H₂O molecules to form the hexahydrate {{chem2|[Al(H2O)6]Cl3}}. The anhydrous phase cannot be regained on heating the hexahydrate. Instead HCl is lost leaving aluminium hydroxide or alumina (aluminium oxide):

:{{chem2|[Al(H2O)6]Cl3 → Al(OH)3 + 3 HCl + 3 H2O}}

Like metal aquo complexes, aqueous {{chem2|AlCl3}} is acidic owing to the ionization of the aquo ligands:

:{{chem2|[Al(H2O)6](3+) ⇌ [Al(OH)(H2O)5](2+) + H+}}

Aqueous solutions behave similarly to other aluminium salts containing hydrated {{chem2|Al(3+)}} ions, giving a gelatinous precipitate of aluminium hydroxide upon reaction with dilute sodium hydroxide:

:{{chem2|AlCl3 + 3 NaOH → Al(OH)3 + 3 NaCl}}

Aluminium chloride is manufactured on a large scale by the exothermic reaction of aluminium metal with chlorine or hydrogen chloride at temperatures between {{convert|650|and|750|C}}.

:{{chem2|2 Al + 3 Cl2 → 2 AlCl3}}

:{{chem2|2 Al + 6 HCl → 2 AlCl3 + 3 H2}}

Aluminium chloride may be formed via a single displacement reaction between copper(II) chloride and aluminium.

:{{chem2|2 Al + 3 CuCl2 → 2 AlCl3 + 3 Cu}}

In the US in 1993, approximately 21,000 tons were produced, not counting the amounts consumed in the production of aluminium.

Hydrated aluminium trichloride is prepared by dissolving aluminium oxides in hydrochloric acid. Metallic aluminium also readily dissolves in hydrochloric acid ─ releasing hydrogen gas and generating considerable heat. Heating this solid does not produce anhydrous aluminium trichloride, the hexahydrate decomposes to aluminium hydroxide when heated:

:{{chem2|[Al(H2O)6]Cl3 → Al(OH)3 + 3 HCl + 3 H2O}}

Aluminium also forms a lower chloride, aluminium(I) chloride (AlCl), but this is very unstable and only known in the vapour phase.

Anhydrous aluminium chloride is not found as a mineral. The hexahydrate, however, is known as the rare mineral chloraluminite.{{cite web |url=https://www.ima-mineralogy.org/Minlist.htm |title=List of Minerals|date=March 21, 2011 |website=www.ima-mineralogy.org |publisher=International Mineralogical Association}} A more complex, basic and hydrated aluminium chloride mineral is cadwaladerite.{{cite web |url=https://www.mindat.org/min-845.html|title=Cadwaladerite|website=www.mindat.org}}

Aluminium chlorides were known in the 18th century as muriate of alumina, marine alum, argillaceous marine salt,{{Cite book | vauthors = de Fourcroy AF |url=https://books.google.com/books?id=wD9XsU46qSsC&pg=RA1-PA54 |title=Elements of natural history, and of chemistry: being the second edition of the elementary lectures on those sciences ... enlarged and improved by the author ... Translated into English, with ... notes; and an historical preface by the translator W. Nicholson |date=1790 |language=en}} muriated clay.{{Cite book | vauthors = Berthollet CL |url= https://books.google.com/books?id=TpCzhu7JkEIC&pg=PA371 |title=Elements of the Art of Dyeing ... Translated ... by William Hamilton |date=1791 |publisher=Stephen Couchman; sold by J. Johnson |language=en}} It was first chemically studied in the 1830s.{{Cite book | vauthors = Mather WW | chapter = Chloride of Aluminium and its Analysis | chapter-url=https://books.google.com/books?id=qqdGAQAAMAAJ&pg=PA249 | pages = 241–253 (249) | veditors = Silliman B |title=The American Journal of Science | volume = 27 |date=1835 |publisher=Kline Geology Laboratory, Yale University. |language=en}}

Anhydrous {{chem2|AlCl3}} is strongly corrosive and releases hydrochloric acid in contact with water.

• Aluminium monochloride

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{{reflist}}

{{Commons category|Aluminium chloride}}

• [http://www.inchem.org/documents/icsc/icsc/eics1125.htm International Chemical Safety Card 1125]
• [http://www.orgsyn.org/orgsyn/chemname.asp?nameID=33169 Index of Organic Synthesis procedures that utilize AlCl₃]
• [http://www.chemguide.co.uk/inorganic/period3/chlorides.html The period 3 chlorides]
• [http://www.solvaychemicals.us/static/wma/pdf/5/1/1/8/ALCL.pdf MSDS] {{Webarchive|url=https://web.archive.org/web/20110722155335/http://www.solvaychemicals.us/static/wma/pdf/5/1/1/8/ALCL.pdf |date=2011-07-22}}
• [http://www.chemicalsubstanceschimiques.gc.ca/fact-fait/alum-salts-sels-eng.php Government of Canada Fact Sheets and Frequently Asked Questions: Aluminum Salts]

{{Aluminium compounds}}

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Category:Chlorides

Category:Metal halides

Category:Aluminium compounds

Category:Inorganic compounds

Category:Reagents for organic chemistry

Category:Deliquescent materials

Category:Acid catalysts