gold(III) bromide
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Gold(III) bromide is a dark-red to black crystalline solid.Macintyre, J. E. (ed.) ''Dictionary of Inorganic Compounds''; Chapman & Hall: London, 1992; vol. 1, pp. 121Greenwood, N.N.; Earnshaw, A. ''Chemistry of the Elements''; Butterworth-Heineman: Oxford,1997; pp. 1183-1185Cotton, F.A.; Wilkinson, G.; Murillo, C.A.; Bochmann, M. ''Advanced Inorganic Chemistry''; John Wiley & Sons: New York, 1999; pp. 1101-1102 It has the
empirical formula In chemistry, the empirical formula of a chemical compound is the simplest whole number ratio of atoms present in a compound. A simple example of this concept is that the empirical formula of sulfur monoxide, or SO, would simply be SO, as is the ...
, but exists primarily as a dimer with the
molecular formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...
in which two gold atoms are bridged by two bromine atoms.Schulz, A.; Hargittai, M. '' Chem. Eur. J.'' 2001, vol. 7, pp. 3657-3670 It is commonly referred to as gold(III) bromide, gold tribromide, and rarely but traditionally auric bromide, and sometimes as digold hexabromide. As is similar with the other gold halides, this compound is unique for being a coordination complex of a group 11 transition metal that is stable in an
oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
of +3 whereas copper or silver complexes persist in oxidation states of +1 or +2.Schwerdtfeger, P. '' J. Am. Chem. Soc.'' 1989, vol. 111, pp. 7261-7262


History

The first mention of any research or study of the gold halides dates back to the early-to-mid-19th century, and there are three primary researchers associated with the extensive investigation of this particular area of chemistry: Thomsen, Schottländer, and Krüss.Lengefield, F. '' J. Am. Chem. Soc.'' 1901, vol. 26, pp. 324Thomsen, J. '' J. prakt. Chem.'' 1876, vol. 13, pp. 337 Schottländer, '' Justus Liebigs Ann. Chem.'', vol. 217, pp. 312Krüss, G. ''
Ber. Dtsch. Chem. Ges. ''Chemische Berichte'' (usually abbreviated as ''Ber.'' or ''Chem. Ber.'') was a German-language scientific journal of all disciplines of chemistry founded in 1868. It was one of the oldest scientific journals in chemistry, until it merged with ' ...
'' 1887, vol. 20, pp. 2634


Structure

The dimer, digold hexabromide, has structural properties similar to those of the other gold trihalide dimeric compounds, such as
gold(III) chloride Gold(III) chloride, traditionally called auric chloride, is a 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. Gold(III) c ...
. The gold centers exhibit square planar coordination with bond angles of roughly 90 degrees. Calculations indicate that in the hypothetical monomeric forms of the gold trihalides, the Jahn-Teller effect causes differences to arise in the structures of the gold halide complexes. For instance, gold(III) bromide contains one long and two short gold-bromine bonds whereas gold(III) chloride and gold(III) fluoride consist of two long and one short gold-halogen bonds. Moreover, gold tribromide does not exhibit the same coordination around the central gold atom as gold trichloride or gold trifluoride. In the latter complexes, the coordination exhibits a T-conformation, but in gold tribromide the coordination exists as more of a dynamic balance between a Y-conformation and a T-conformation. This coordination difference can be attributed to the Jahn-Teller effect but more so to the decrease in π-back bonding of the gold atoms with the bromine ligands compared to the π-back bonding found with fluorine and chlorine ligands. It is also this decrease in π-back bonding which explains why gold tribromide is less stable than its trifluoride and trichloride counterparts. :


Preparation

The most common synthesis method of gold(III) bromide is heating gold and excess liquid
bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table ( halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simi ...
at 140 °C: : Alternatively, the halide-exchange reaction of
gold(III) chloride Gold(III) chloride, traditionally called auric chloride, is a 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. Gold(III) c ...
with
hydrobromic acid Hydrobromic acid is a strong acid formed by dissolving the diatomic molecule hydrogen bromide (HBr) in water. "Constant boiling" hydrobromic acid is an aqueous solution that distills at and contains 47.6% HBr by mass, which is 8.77 mol/L. ...
has also been proven successful in synthesizing gold(III) bromide: : This reaction is driven by the production of the relatively more stable
hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...
compared with
hydrobromic acid Hydrobromic acid is a strong acid formed by dissolving the diatomic molecule hydrogen bromide (HBr) in water. "Constant boiling" hydrobromic acid is an aqueous solution that distills at and contains 47.6% HBr by mass, which is 8.77 mol/L. ...
. Dell'Amico, D.B.; Calderazzo, F.; Morvillo, A.; Pelizzi, G; Robino, P. '' J. Chem. Soc., Dalton Trans.'' 1991, pp. 3009-3016


Chemical properties

The neutral monomer , as well as the other neutral gold trihalide species, has not been isolated in the gas phase which indicates the
coordination number In chemistry, crystallography, and materials science, the coordination number, also called ligancy, of a central atom in a molecule or crystal is the number of atoms, molecules or ions bonded to it. The ion/molecule/atom surrounding the central io ...
three is not favored.Schwerdtfeger, P.; Boyd, P.D.W.; Brienne, S.; Burrell, K. '' Inorg. Chem.'' 1992, vol. 31, pp. 3411-3422 Predominantly, gold(III) displays square planar coordination corresponding to a preferred coordination number of four. Specifically, in solution gold(III) trihalides have the tendency to add a fourth ligand to form the more preferred four-coordinate complex. With respect to gold tribromide, it is common to purchase gold(III) bromide hydrate, , where the central gold atom exhibits a coordination number of four, rather than the anhydrous form of the compound, which exhibits a coordination number of three. Alternatively, if there is no addition of a fourth ligand, gold tribromide will oligomerize to form the halogen-bridged dimer complex mentioned previously. : Furthermore, like
gold(III) chloride Gold(III) chloride, traditionally called auric chloride, is a 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. Gold(III) c ...
, gold tribromide is a Lewis acid and can form several complexes. For example, in the presence of
hydrobromic acid Hydrobromic acid is a strong acid formed by dissolving the diatomic molecule hydrogen bromide (HBr) in water. "Constant boiling" hydrobromic acid is an aqueous solution that distills at and contains 47.6% HBr by mass, which is 8.77 mol/L. ...
, the dimer dissolves and bromoauric acid is formed. : The dimer also undergoes
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolys ...
rapidly in moist air.


Uses


Catalytic chemistry

Gold(III) bromide is used as a catalyst in a variety of reactions, but one of its most interesting uses is found in the Diels-Alder reaction. Specifically, the compound catalyzes the reaction between an enynal unit and
carbonyl In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containi ...
compounds to form a six-membered cyclic compound.Asao, N.; Aikawa, H.; Yamamoto, Y. '' J. Am. Chem. Soc.'' 2004, vol. 126, pp. 7458-7459 : Another catalytic use of gold tribromide is in the
nucleophilic substitution In chemistry, a nucleophilic substitution is a class of chemical reactions in which an electron-rich chemical species (known as a nucleophile) replaces a functional group within another electron-deficient molecule (known as the electrophile). The ...
reaction of propargylic alcohols. In this reaction, the gold complex acts as an alcohol-activating agent to facilitate the substitution.Georgy, M.; Boucard, V.; Campagne, J.'' J. Am. Chem. Soc.'' 2005, vol. 127, pp. 14180-14181 :


Ketamine Detection

Gold(III) bromide can be used as a testing reagent for the presence of
ketamine Ketamine is a dissociative anesthetic used medically for induction and maintenance of anesthesia. It is also used as a recreational drug. It is one of the safest anesthetics, as, in contrast with opiates, ether, and propofol, it suppresses ...
. 0.25% 0.1M NaOH is prepared to give a brownish-yellow solution. Two drops of this are added to a spotting plate and a small amount of ketamine is added. The mixture gives a deep purple color within approximately one minute, which turns to a dark, blackish-purple color within approximately two minutes. Acetaminophen, ascorbic acid, heroin, lactose,
mannitol Mannitol is a type of sugar alcohol used as a sweetener and medication. It is used as a low calorie sweetener as it is poorly absorbed by the intestines. As a medication, it is used to decrease pressure in the eyes, as in glaucoma, and to lo ...
,
morphine Morphine is a strong opiate that is found naturally in opium, a dark brown resin in poppies (''Papaver somniferum''). It is mainly used as a pain medication, and is also commonly used recreationally, or to make other illicit opioids. T ...
, and sucrose all cause an instant colour change to purple, as do other compounds with phenol and hydroxyl groups. Nothing commonly found in conjunction with ketamine gave the same colour change in the same time. "The initial purple color may be due to the formation of a complex between the gold and the ketamine. The cause for the change of color from purple to dark blackish-purple is unknown; however, it may be due to a redox reaction that produces a small amount of colloidal gold."


References

{{DEFAULTSORT:Gold(Iii) Bromide Bromides Metal halides Gold(III) compounds Drug testing reagents Gold–halogen compounds