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Gold(III) chloride, traditionally called '''auric chloride''', is one of the most common Compound s of Gold . It has the formula Au Cl 3. The Roman numerals in the name indicate that the gold has an Oxidation State of +3, which is the most stable form for gold in its compounds. Gold also forms another Chloride , gold(I) chloride (AuCl), which is less stable than AuCl3. Also, chlorauric acid (HAuCl4), the product formed when gold dissolves in Aqua Regia , is sometimes referred to rather loosely 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, and it forms a variety of Complexes with many Ligands . STRUCTURE AuCl3 exists as a both as a solid and as a vapour; the bromide AuBr3 follows the same pattern. Each Au center is square planar. This structure is reminiscent of the bi Tetrahedral structures adopted by AlCl3 and FeCl3 . The bonding in AuCl3 is mainly Covalent , reflecting the high oxidation state and relatively high Electronegativity (for a metal) of gold. CHEMICAL PROPERTIES Anhydrous AuCl3 begins to decompose to AuCl at around 160 °C; however, this will in turn undergo Disproportionation at higher temperatures to give gold metal and AuCl3. AuCl3 → AuCl + Cl2 (>160 °C) 3 AuCl → AuCl3 + 2 (>420 °C) AuCl3 is a Lewis Acid which readily forms complexes. For example with Hydrochloric Acid , chlorauric acid (HAuCl4) is formed: HCl ( Aq ) + AuCl3(aq) → H+AuCl4−(aq) Ionic chlorides such as KCl will also form the AuCl4− ion with AuCl3. Aqueous solutions of AuCl3 react with alkalis such as Sodium Hydroxide to form a Precipitate of impure 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, 1960H. Nechamkin, ''The Chemistry of the Elements'', McGraw-Hill, New York, 1968A. F. Wells, ''Structural Inorganic Chemistry'', 5th ed., Oxford University Press, Oxford, UK, 1984G. Dyker, ''An Eldorado for Homogeneous Catalysis?'', in ''Organic Synthesis Highlights V'', H.-G. Schmaltz, T. Wirth (eds.), pp 48-55, Wiley-VCH, Weinheim, 2003 PREPARATION Gold(III) chloride is most often prepared by direct Chlorination of the metal at high temperatures: 2 Au + 3 Cl2 → 2 AuCl3 USES Gold(III) chloride is one of the most common gold compounds and it is therefore used as the starting point for the synthesis of many other gold compounds, for example the water-soluble Cyanide Complex KAu(CN)4: AuCl3 + 4 KCN → KAu(CN)4 + 3 KCl Gold(III) 56, 3729-3731 (1991) Ketones are generally formed in over 90% yield under these conditions. Also useful is the related Amination of alkynes which can use 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 Aromatic s and a conversion of Furan s to Phenol s (see below). Such reactions find use in Organic Synthesis and in the Pharmaceutical industry. For example, 2-methylfuran (sylvan) undergoes smooth alkylation by Methyl Vinyl Ketone at the 5-position: The reaction gives a 91% yield in only 40 minutes at room temperature, using only 1 mole% of AuCl3 in 122, 11553-11554 (2000) The reaction undergoes a complex rearrangement that leads to formation of the new aromatic ringHashmi, A. S. K.; Rudolph, M.; Weyrauch, J. P.; Wölfle, M.; Frey, W.; Bats, J. W. Angew. Chem. Int. Ed. 2005, 44, 2798-2801 PRECAUTIONS Gold(III) chloride should be handled wearing gloves and goggles; direct contact with the material should be avoided. REFERENCES EXTERNAL LINKS |
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