Gold(III) chloride, traditionally called auric chloride, is an inorganic compound of gold and chlorine with the molecular formula . The "III" in the name indicates that the gold has an oxidation state of +3, typical for many gold compounds. It has two forms, the monohydrate (AuCl<sub>3</sub>·H<sub>2</sub>O) and the anhydrous form, which are both hygroscopic and light-sensitive solids. This compound is a dimer of . This compound has a few uses, such as an oxidizing agent and for catalyzing various organic reactions.

Structure

exists as a chloride-bridged dimer both as a solid and vapour, at least at low temperatures. Gold(III) bromide behaves analogously.

:

This method is the most common method of preparing gold(III) chloride. It can also be prepared by reacting gold powder with iodine monochloride:

Another method of preparation is via chloroauric acid, which is obtained by first dissolving the gold powder in aqua regia to give chloroauric acid:

:

The resulting chloroauric acid is subsequently heated in an inert atmosphere at around 100&nbsp;°C to give :

Gold(III) chloride is more stable in a chlorine atmosphere and can sublime at around 200&nbsp;°C without any decomposition. In a chlorine atmosphere, AuCl<sub>3</sub> decomposes at 254&nbsp;°C yielding AuCl which in turn decomposes at 282&nbsp;°C to elemental gold. This fact that no gold chlorides can exist above 400&nbsp;°C is used in the Miller process.

Other reactions

is a Lewis acid and readily forms complexes. For example, it reacts with hydrochloric acid to form chloroauric acid ():

Other chloride sources, such as KCl, also convert into . Aqueous solutions of react with an aqueous base such as sodium hydroxide to form a precipitate of , which will dissolve in excess NaOH to form sodium aurate (). If gently heated, decomposes to gold(III) oxide, , and then to gold metal.

Gold(III) chloride is the starting point for the chemical synthesis of many other gold compounds. For example, the reaction with potassium cyanide produces the water-soluble complex, :

:

Gold(III) fluoride can be also produced from gold(III) chloride by reacting it with bromine trifluoride.

:

Gold(III) chloride reacts with carbon monoxide in a variety of ways. For example, the reaction of anhydrous AuCl<sub>3</sub> and carbon monoxide under SOCl<sub>2</sub> produces gold(I,III) chloride with Au(CO)Cl as an intermediate:

:2 AuCl<sub>3</sub> + 2 CO → Au<sub>4</sub>Cl<sub>8</sub> + 2 COCl<sub>2</sub>

If carbon monoxide is in excess, Au(CO)Cl is produced instead.

Applications

Gold(III) chloride has many uses in the laboratory, and primarily thrives in this environment. although no transformations have been commercialised. Gold(III) salts, especially sodium tetrachloroaurate|, provide an alternative to mercury(II) salts as catalysts for reactions involving alkynes. An illustrative reaction is the hydration of terminal alkynes to produce acetyl compounds.

:[[File:Example of gold-catalyzed alkyne hydration reaction.svg|class=skin-invert-image|

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Gold catalyses the alkylation of certain aromatic rings and the conversion of furans to phenols. Some alkynes undergo amination in the presence of gold(III) catalysts. For example, a mixture of acetonitrile and gold(III) chloride catalyses the alkylation of 2-methylfuran by methyl vinyl ketone at the 5-position:

:[[File:AuCl3 phenol synthesis.svg|class=skin-invert-image|

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This reaction involves a rearrangement that gives a new aromatic ring.

Another example of an AuCl<sub>3</sub> catalyzed reaction is a hydroarylation, which is basically a Friedel-Crafts reaction using metal-alkyne complexes. Example, the reaction of mesitylene with phenylacetylene:

:[[File:Hydroarylation_reetz.png|class=skin-invert-image|

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Gold(III) chloride can be used for the direct oxidation of primary amines into ketones, such as the oxidation of cyclohexylamine to cyclohexanone.

Gold nanoparticles

Gold(III) chloride is used in the synthesis of gold nanoparticles, which are extensively studied for their unique size-dependent properties and applications in fields such as electronics, optics, and biomedicine. Gold nanoparticles can be prepared by reducing gold(III) chloride with a reducing agent such as sodium tetrafluoroborate, followed by stabilization with a capping agent.

Photography

Gold(III) chloride has been used historically in the photography industry as a sensitizer in the production of photographic films and papers. However, with the advent of digital photography, its use in this field has diminished.

Natural occurrence

This compound does not occur naturally; however, a similar compound with the formula AuO(OH,Cl)·nH<sub>2</sub>O is known as a product of natural gold oxidation.

References