Gold(III) chloride

Chembox new
Name = Gold(III) chloride
ImageFile =
ImageName = Gold(III) chloride solution
IUPACName = Gold(III) chloride
OtherNames = Auric chloride
Gold trichloride
Section1 = Chembox Identifiers
CASNo = 13453-07-1
RTECS = MD5420000 (anhydrous)
Section2 = Chembox Properties
Formula = AuCl₃
(exists as Au₂Cl₆)
MolarMass = a poop
Appearance = Red crystalline solid
Density = 3.9 g/cm³ (solid)
Solubility = 68 g/100 ml (cold)
MeltingPt = 254 °C (527 K)
(decomposes)
Section4 = Chembox Structure
Coordination = Square planar
CrystalStruct = monoclinic
Section7 = Chembox Hazards
MainHazards = Irritant
RPhrases = R36/37/38
SPhrases = S26-36
Section8 = Chembox Related
OtherAnions = Gold(III) fluoride
Gold(III) bromide
OtherCations = Gold(I) chloride
Silver(I) chloride
Platinum(II) chloride
Mercury(II) chloride

Gold(III) chloride, traditionally called auric chloride, is the chemical compound with the formula AuCl₃. The Roman numerals in the name indicate that the gold has an oxidation state of +3, which is common for gold in its compounds. The other chloride of gold, gold(I) chloride (AuCl), is less stable than AuCl₃. Chloroauric acid, HAuCl₄, the product formed when gold dissolves in aqua regia, is sometimes referred to as "gold chloride", "acid gold trichloride" or even "gold(III) chloride trihydrate."

Gold(III) chloride is very hygroscopic and highly soluble in water and ethanol. It decomposes above 160 °C or in light.

tructure

AuCl₃ exists as a both as a solid and as a vapour at low temperatures; the bromide AuBr₃ follows the same pattern. Each gold center is square planar, a similar structure to iodine(III) chloride. The bonding in AuCl₃ is mainly covalent, reflecting the high oxidation state and relatively high electronegativity (for a metal) of gold.

Properties and inorganic chemistry

Anhydrous AuCl₃ begins to decompose to AuCl at around 160 °C; however, this in turn undergoes disproportionation at higher temperatures to give gold metal and AuCl₃.

:AuCl₃ → AuCl + Cl₂ (>160 °C)

:3 AuCl → AuCl₃ + 2 Au (>420 °C)

AuCl₃ is Lewis acidic and readily forms complexes. For example with hydrochloric acid, chloroauric acid (H [AuCl₄] ) is formed:

:HCl + AuCl₃(aq) → H⁺ [AuCl₄] ⁻

Other chloride sources, such as KCl, also convert AuCl₃ into [AuCl₄] ⁻. Aqueous solutions of AuCl₃ react with aqueous base such as sodium hydroxide to form a precipitate of Au(OH)₃, which will dissolve in excess NaOH to form sodium aurate (NaAuO₂). If gently heated, Au(OH)₃ decomposes to gold(III) oxide, Au₂O₃, and then to gold metal. [N. N. Greenwood, A. Earnshaw, "Chemistry of the Elements", 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997] ["Handbook of Chemistry and Physics", 71st edition, CRC Press, Ann Arbor, Michigan, 1990] ["The Merck Index", 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960] [H. Nechamkin, "The Chemistry of the Elements", McGraw-Hill, New York, 1968] [A. F. Wells, "Structural Inorganic Chemistry", 5th ed., Oxford University Press, Oxford, UK, 1984] [G. Dyker, "An Eldorado for Homogeneous Catalysis?", in "Organic Synthesis Highlights V", H.-G. Schmaltz, T. Wirth (eds.), pp 48-55, Wiley-VCH, Weinheim, 2003]

Gold(III) chloride is the starting point for the synthesis of many other gold compounds, for example the water-soluble cyanide complex K [Au(CN)₄] ::AuCl₃ + 4 KCN → K [Au(CN)₄] + 3 KCl

Preparation

Gold(III) chloride is most often prepared by direct chlorination of the metal at high temperatures:Fact|date=February 2008

:2 Au + 3 Cl₂ → 2 AuCl₃

Another method of preparation is the reaction in which solid gold is placed in a solution of aqua regia.

Applications in organic synthesis

Gold(III) salts, especially Na [AuCl₄] (prepared from AuCl₃ + NaCl), provide a non-toxic alternative to mercury(II) salts as catalysts for alkyne reactions. An illustrative reaction is the hydration of terminal alkynes to produce methyl ketones: [cite journal | author = Y. Fukuda and K. Utimoto | title = Effective transformation of unactivated alkynes into ketones or acetals with a gold(III) catalyst | journal = J. Org. Chem. | year = 1991 | volume = 56 | issue = 11 | doi = 10.1021/jo00011a058 | pages = 3729]

Also, alkynes undergo amination in the presence of gold(III) catalysis. In recent years AuCl₃ has begun to attract the interest of organic chemists as a mild acid catalyst for other reactions such as alkylation of aromatics and a conversion of furans to phenols (see below). Such reactions are of potential value in organic synthesis, for example for the preparation of pharmaceuticals. For example, in acetonitrile, 2-methylfuran (sylvan) undergoes smooth alkylation by methyl vinyl ketone at the 5-position:

The efficiency of this reaction is noteworthy because both the furan and the ketone are normally very sensitive to side-reactions such as polymerisation under acidic conditions. In some cases where alkynes are present, a phenol may be formed: [cite journal | author = A. S. K. Hashmi, T. M. Frost and J. W. Bats | title = Highly Selective Gold-Catalyzed Arene Synthesis | journal = J. Am. Chem. Soc. | year = 2000 | volume = 122 | issue = 46 | doi = 10.1021/ja005570d | pages = 11553]

The reaction undergoes a complex rearrangement that leading to a new aromatic ring. [cite journal | author = A. Stephen, K. Hashmi, M. Rudolph, J. P. Weyrauch, M. Wölfle, W. Frey and J. W. Bats | title = Gold Catalysis: Proof of Arene Oxides as Intermediates in the Phenol Synthesis | journal = Angewandte Chemie International Edition| year = 2005 | volume = 44 | issue = 18 | doi = 10.1002/anie.200462672 | pages = 2798]

References