Gold(III) chloride, traditionally called auric chloride, is a chemical compound of gold and chlorine. With the molecular formula Au2Cl6, the name gold trichloride is a simplification, referring to the empirical formula. The Roman numerals in the name indicate that the gold has an oxidation state of +3, which is common for gold compounds. There is also another related chloride of gold, gold(I) chloride (AuCl). 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 as well as ethanol. It decomposes above 160 °C or in light.
AuCl3 exists as a chloride-bridged dimer both as a solid and as a vapour, at least at low temperatures. Gold(III) bromide behaves analogously. The structure is similar to that of iodine(III) chloride.
Gold(III) chloride is most often prepared by passing chlorine gas over gold powder at 180 °C:
- 2 Au + 3 Cl2 → 2 AuCl3
Another method of preparation, is the addition reaction, where gold(3+) is reacted with chloride to produce tetrachloridoaurate(1-); after which, the hydrogen tetrachloridoaurate(1-) is heated to eliminate chlorane gas. Gold(3+) is produced by oxidising with nitric acid present in aqua regia:
- Au(s) + 3 NO−
3(aq) + 6 H+(aq) Au3+(aq) + 3 NO2(g) + 3 H2O(l)
- Au3+(aq) + 3 NOCl(g) + 3 NO−
3(aq) → AuCl3(aq) + 6 NO2(g)
- AuCl3(aq) + Cl-(aq) AuCl−
- 2 HAuCl4(s) → Au2Cl6(s) + 2 HCl(g)
On contact with water, AuCl3 forms a series of species, sometimes described as AuCl3·H2O and its conjugate base [AuCl3(OH)]−. Reaction with reducing agents such as hydrogen peroxide or Fe2+ causes elemental gold to be precipitated from solution.
- AuCl3 → AuCl + Cl2 (>160 °C)
- 3 AuCl → AuCl3 + 2 Au (>420 °C)
- HCl + AuCl3 (aq) → H+ + [AuCl4]−
Other chloride sources, such as KCl, also convert AuCl3 into AuCl−
4. 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.
- AuCl3 + 4 KCN → K[Au(CN)4] + 3 KCl
Applications in organic synthesis 
AuCl3 has attracted the interest of organic chemists as a mild acid catalyst for a variety reactions, although no transformations have been commercialized. Gold(III) salts, especially Na[AuCl4] (prepared from AuCl3 + NaCl), provide an alternative to mercury(II) salts as catalysts for reactions involving alkynes. An illustrative reaction is the hydration of terminal alkynes to produce methylketones:
Some alkynes undergo amination in the presence of gold(III) catalysts. Gold catalyses the alkylation of certain aromatic rings and a conversion of furans to phenols. For example, in acetonitrile solution, gold(III) chloride catalyses the alkylation of 2-methylfuran (sylvan) by methyl vinyl ketone at the 5-position:
The efficiency of this organogold reaction is noteworthy because both the furan and the ketone are sensitive to side-reactions such as polymerisation under acidic conditions. In some cases where alkynes are present, phenols sometimes form:
This reaction involves a rearrangement that gives a new aromatic ring.
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