Aluminium triacetate

Aluminium triacetate
Names
IUPAC name
Aluminum acetate
Other names
Aluminium(III) acetate
Identifiers
139-12-8
3D model (Jmol) Interactive image
ChemSpider 8427
PubChem 8757
Properties
C6H9AlO6
Molar mass 204.11 g·mol−1
Appearance white solid[1]
soluble
Related compounds
Related compounds
 Basic aluminium diacetate (hydroxyaluminium diacetate), CAS RN 142-03-0, HOAl(CH
3CO
2)
2[1]
Dibasic aluminium monoacetate (dihyrdoxyaluminium acetate), CAS RN 7360-44-3, (HO)
2AlCH
3CO
2
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Aluminium triacetate, formally named aluminium acetate under the IUPAC rules, is a chemical compound with composition Al(CH
3CO
2)
3. Under standard conditions it appears as a white, water-soluble solid[1] that decomposes on heating at around 200 °C.[2] The triacetate hydrolyses to a mixture of basic hydroxyide / acetate salts,[3] and multiple species co-exist in chemical equilibrium, particularly in aqueous solutions of the acetate ion; the name aluminium acetate is commonly used for this mixed system.

It has therapeutic applications for its anti-itching, astringent, and antiseptic properties,[4] and as an over-the-counter preparation like Burow's solution,[5] it is used to treat ear infections[6][7] though less-effectively in cases of fungal infections.[8] Domeboro powder can be dissolved to make a Burow's-like solution[9] to provide symptom relief from the irritation associated with ingrown toenails.[10] Burow's solution preparations have been diluted and modified with amino acids to make them more palatable for use as gargles for conditions like aphthous ulcers of the mouth.[11] In veterinary medicine, aluminium triacetate's astringency property is used for treating Mortellaro disease in hoofed animals such as cattle.[12]

Aluminium triacetate is used as a mordant agent with dyes like alizarin,[13] both alone and in combination. Together with aluminium diacetate[14] or with aluminium sulfacetate[15] it is used with cotton, other cellulose fibres,[16] and silk.[15] It has also been combined with ferrous acetate to produce different colours.[17]

Nomenclature

Under the IUPAC rules for naming inorganic compounds in the "Red Book",[18] the name for Al(CH
3CO
2)
3 is aluminium acetate, though more formal names like aluminium(III) acetate and aluminium ethanoate are acceptable.[3] The use of the "tri" multiplying prefix in the name aluminium triacetate, whilst not technically required, is regularly used to avoid potential ambiguity with related compounds with hydroxo ligands. Basic aluminium diacetate, formally hydroxyaluminium diacetate (CAS RN 142-03-0),[1] has composition HOAl(CH
3CO
2)
2 with one hydroxo ligand in place of an acetate ligand, and dibasic aluminium monoacetate, formally dihyrdoxyaluminium acetate (CAS RN 7360-44-3), has composition (HO)
2AlCH
3CO
2 with only one acetate ligand. These three compounds are distinct in the solid phase but are usually treated as a group and described collectively as aluminium acetate in solution as the triacetate hydrolyses to a mixture which includes the other two forms.[3] The abbreviation as AlAc, along with variants like AlAc2+
 and AlAc+
2, are sometimes used in the discipline of geochemistry,[19] though these are inconsistent with standard practice in mainstream chemistry.[a]

Structure

The structure of aluminium triacetate as an ionic compound
Structure of aluminium triacetate as a covalent molecular compound
Over-simplified representations of Al(CH
3CO
2)
3 with ionic (left) and covalent molecular (right) motifs

The formula Al(CH
3CO
2)
3 indicates the presence of aluminium metal centres in the +3 oxidation state and acetate groups in a ratio of 1:3. Images used to represent this substance, such as those shown at left, represent two highly oversimplified approximations of the solid-state structure: the first is as a purely ionic salt with a single aluminium(III) cation (Al3+) surrounded by and associated electrostatically with three acetate anions (CH
3CO−
2), but this should not be taken to convey information about the crystal structure. By way of example, sodium chloride (NaCl) has a cation-to-anion stoichiometry of 1:1, but it has a cubic structure with each ion surrounded octahedrally by six ions of the opposite charge.[20] The other extreme is a molecular form with the three acetate groups covalently bonded to the metal centre in a trigonal planar geometry and intermolecular interactions holding the molecules together with each other in the crystal structure. It is highly likely that the solid state structure is more complicated and includes both covalent and ionic characteristics and it is possible that multiple aluminium centres and / or bridging acetate groups might be present – both of these have been reported in aluminium acetate solution[21] and aluminium chloride is known to exist as a Al
2Cl
6 dimer.[22]

[Fe3(μ3–O)(OAc)6(H2O)3]+, the cation found in iron(III) acetate

NMR investigations of the aqueous aluminium(III) / acetate system show the presence of aluminium as a hexaaqua complex, [Al(H
2O)
6]3+
,[23] mononuclear species with different substitutions, and also demonstrate that a significant solution-phase species is an Al
13 tridecamer,[24] a moiety reported in conflicting mechanisms of hydrolysis and polymerisation aluminium solutions.[25] Other trivalent metal cations are known to form polynuclear species: iron(III) acetate, for example, forms a trinuclear structure[26] with a triply-bridged oxo centre[27] with a cation [Fe3(μ3–O)(OAc)6(H2O)3]+.[28] The chromium(III) equivalent is isostructural[27] and a mixed hydroxide / acetate compound chromium acetate hydroxide, Cr3(OH)2(OAc)7, has been described.[29] Analogous ruthenium(III), vanadium(III), rhodium(III), and iridium(III) compounds with trinuclear structures are known.[30] Copper(II) acetate and chromium(II) acetate both have dinuclear dihydrate structures, M2(OAc)4(H2O)2,[31] as does rhodium(II) acetate;[32] each shows significant metal-metal bonding interactions.[31][32]

Chemistry

Preparation

According to the CRC Handbook of Inorganic Compounds, aluminium triacetate is a white, water-soluble solid and is usually prepared from aluminium chloride or directly from aluminium by heating in an acetic acid solution with acetic anhydride.[1]

3 CH
3COOH   +   AlCl
3   →   Al(CH
3CO
2)
3   +   3 HCl
6 CH
3COOH   +   2 Al   →   2 Al(CH
3CO
2)
3   +   3 H
2

Theoretically all of the aluminium / acetate / hydroxide salts can be prepared from aluminium hydroxide or sodium aluminate and acetic acid, but formation of the triacetate only occurs in the absence of water.[3] In solutions, the diacetate is the major product formed, and is also produced when aluminium chloride is treated with a sodium acetate solution in basic conditions.[33] The equations for these processes are:

2 CH
3CO
2Na   +   Al(OH)
3   →   Al(CH
3CO
2)
2OH   +   2 NaOH
2 CH
3CO
2Na   +   AlCl
3   +   NaOH   →   Al(CH
3CO
2)
2OH   +   3 NaCl
2 CH
3CO
2Na   +   NaAlO
2   +   2 H
2O   →   Al(CH
3CO
2)
2OH   +   3 NaOH

An improved process using a combination of aluminium chloride and sodium aluminate with sodium acetate prepared in situ has been patented:[34]

29 NaAlO
2   +   10 NaOH   + 84 CH
3COOH   +   13 AlCl
3   →   42 Al(CH
3CO
2)
2OH   +   39 NaCl   +   26 H
2O

The mordants aluminium triacetate and aluminium sulfacetate can be prepared from aluminium sulfate, the product formed being determined by the amount of lead(II) acetate used:[15]

Al
2(SO
4)
3   +   3 Pb(CH
3CO
2)
2   →   2 Al(CH
3CO
2)
3   +   3 PbSO
4
Al
2(SO
4)
3   +   2 Pb(CH
3CO
2)
2   →   Al
2SO
4(CH
3CO
2)
4   +   2 PbSO
4

Decomposition

On heating, aluminium triacetate decomposes above 200 °C in a process similar to that of aluminium formate.[2] The process begins with loss of acetic anhydride (Ac
2O) between 120 and 140 °C[1] to form the a mixture of the basic oxide acetates such as Al
2O(CH
3CO
2)
4 and Al
3O(CH
3CO
2)
7,[33] which are ultimately transformed to Al
2O
3 (alumina), first as an amorphous anhydrous solid and then through other solid phases (γ-, δ-, and θ- crystal forms) to ultimately become polymorphic α-Al
2O
3:[2]

2 Al(CH
3CO
2)
3   →   Al
2O(CH
3CO
2)
4   +   CH
3CO(O)COCH
3   →   Al
2O
3   +   3 CH
3CO(O)COCH
3
2 Al(CH
3CO
2)
2OH   →   Al
2O
3   +   2 CH
3COOH   +   CH
3CO(O)COCH
3

Hydrolysis

Aluminium triacetate hydrolyses to produce both the mono- and di-basic hydoxide acetates in solution or by hygroscopy:[3]

Al(CH
3CO
2)
3   +   H
2O   →   Al(CH
3CO
2)
2OH   +   CH
3COOH
Al(CH
3CO
2)
3   +   2 H
2O   →   Al(CH
3CO
2)(OH)
2   +   2 CH
3COOH

Uses

According to the National Cancer Institute, the aluminium acetates are used topically in humans as antiseptic agents which also cause body tissues to shrink.[4] Its astringency property is also used for treating Mortellaro disease in hoofed animals such as cattle.[12] Aluminium acetate promotes healing of infected skin and also assists with inflammation, itching, and stinging.[4] The Food and Drug Administration has approved it for use for "temporary relief of minor skin irritations due to ... 'poison ivy,' 'poison oak,' 'poison sumac,' 'insect bites,' 'athlete's foot,' or 'rashes caused by soaps, detergents, cosmetics, or jewelry.'"[35] For these applications, over-the-counter preparations such as Burow's solution are typically used[5] while diluted forms are used as gargles for conditions like aphthous ulcers of the mouth, including with amino acid additives to improve palatability and taste.[11] The most common use of Burow's solution is in treating ear infections[6][7] including otomycosis, though it is generally not as effective as clotrimazole in these fungal infections.[8] Topical astringent powder Domeboro contains aluminium sulfate tetradecahydrate, [Al(H
2O)
6]
2(SO
4)
3•2H
2O, and calcium acetate monohydrate, Ca(CH
3CO
2)
2•H
2O, and forms an aluminium acetate solution similar to Burow's solution when dissolved.[9] Domeboro solutions in warm water can be used in cases of ingrown toenails,[10] to reduce irritation and contain any infection which might be present.

Mordant

Kiel and Heertjes' structure for the anion of Ca[Al(H
2O)(OH)Az
2]•2H
2O,[36] which alizarin forms with an aluminium mordant

A mordant is a substance used to set dyes on fabrics or tissue sections by forming a coordination complex with the dye which then attaches to the fabric or tissue.[37] A mordant often contains a polyvalent metal ion, commonly aluminium or iron,[38] as is the case with mixtures of aluminium triacetate with aluminium sulfacetate[15] or with basic aluminium diacetate.[14] Aluminium triacetate mordants have been used with cotton, other cellulose-based fibres,[16] and silk.[15] They have also been combined with ferrous acetate to produce different colours.[17]

Wunderlich and Bergerhoff's structure for alizarin bound to an aluminium mordant as CaAz
2Al(μ-OH)
2AlAz
2Ca[13]

In the case of the dye alizarin (1,2-dihydroxyanthraquinone, H
2Az), mordanting was hypothesised to involve the formation of a dianion of alizarin which formed a five-coordinate aluminium complex, CaAl(OH)Az
2,[39] which can take up water to form a hydate with a six-coordinate aluminium-centred dianion, Ca[Al(H
2O)(OH)Az
2]•2H
2O.[40] Kiel and Heertjes's proposal was based on Inrared spectroscopic data, and was subsequently challenged by work suggesting a structure with two bridging hydroxyl ligands connecting a dinuclear core, Az
2Al(μ-OH)
2AlAz4−
2, with two alizarin moieties each chelating to each aluminium centre.[36] The structure was proposed by Soubayrol et al. based on 27Al NMR spectroscopy and electrospray ionisation mass spectrometry evidence.[41] They reported that the degree of hydration was dependent on the identity of the counter-ion, with the sodium salt being a stable tetrahydrate but a monohydrate being formed from potassium hydroxide, and that these were distinguishable based on their chemical shifts, suggesting the waters are associating with the aluminium centres or the alizarin moieties, and not behaving as is typical for waters of crystallisation.[41] A related structure with calcium ions was reported in 1994, though in it the alizarins chelate to the calcium ions to form AzCaAz bridges between the aluminium centres, which are also bridged by hydroxo groups, and the aluminium centres binding the deprotonated phenol residues of the dye;[13] in the Soubayrol model, each alizarin is associated with a single aluminium cation.[41] As with the structure of aluminium acetate itself, the forms it takes in applications has not been resolved.

Notes

1 This "Ac" is not referring to the element actinium. Used in this way, the convention in organic chemistry is for Ac to refer to the acetyl group, the radical form of which is CH
3CO,[42] and OAc or AcO would be used for the acetate radical, CH
3CO
2,[43] sometimes also called "acetoxy." The acetate ion would then be AcO− and acetic acid would be AcOH or HOAc. Publications in geochemistry, however, are using Ac to refer to acetate rather than acetyl.

References

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  2. 1 2 3 Sato, Taichi; Ikoma, Shuji; Ozawa, Fusaji (1984). "Thermal decomposition of organic basic aluminium salts—formate and acetate". Thermochim. Acta. 75 (1–2): 129–137. doi:10.1016/0040-6031(84)85013-3.
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  24. ↑ Thomas, Fabien; Masion, Armand; Bottero, Jean Yves; Rouiller, James; Genevrier, Francine; Boudot, Denis (1991). "Aluminum(III) Speciation with Acetate and Oxalate. A Potentiometric and 27Al NMR Study". Environ. Sci. Technol. 25 (9): 1553–1559. doi:10.1021/es00021a004.
  25. ↑ Bi, Shuping; Wang, Chenyi; Cao, Qing; Zhang, Caihua (2004). "Studies on the mechanism of hydrolysis and polymerization of aluminum salts in aqueous solution: correlations between the "Core-links" model and "Cage-like" Keggin-Al13 model". Coord. Chem. Rev. 248 (5–6): 441–455. doi:10.1016/j.ccr.2003.11.001.
  26. ↑ Weinland, R.; Dinkelacker, P. (1909). "Über Salze einer Hexaacetato(formiato)-trichrombase. II". Ber. Dtsch. Chem. Ges. (in German). 42 (3): 2997–3018. doi:10.1002/cber.19090420318.
  27. 1 2 Figgis, B. N.; Robertson, G. B. (1965). "Crystal-Molecular Structure and Magnetic Properties of Cr3(CH3.COO)6OCl.5H2O". Nature. 205 (4972): 694–695. doi:10.1038/205694a0.
  28. ↑ Burgess, J.; Twigg, M. V. (2005). King, R. Bruce, ed. Encyclopedia of Inorganic Chemistry (10th ed.). Wiley. ISBN 9780470860786.
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  30. ↑ Catterick, Janet; Thornton, Peter (1977). "Structures and Physical Properties of Polynuclear Carboxylates". In Emeléus, H. J.; Sharpe, A. G. Advances in Inorganic Chemistry and Radiochemistry. 20. Academic Press. pp. 291–362. ISBN 9780080578699.
  31. 1 2 Van Niekerk, J. N.; Schoening, F. R. L. (1953). "X-Ray Evidence for Metal-to-Metal Bonds in Cupric and Chromous Acetate". Nature. 171 (4340): 36–37. doi:10.1038/171036a0.
  32. 1 2 Cotton, F. A.; Deboer, B. G.; Laprade, M. D.; Pipal, J. R.; Ucko, D. A. (1971). "The crystal and molecular structures of dichromium tetraacetate dihydrate and dirhodium tetraacetate dihydrate". Acta Crystallogr. B. 27 (8): 1664. doi:10.1107/S0567740871004527.
  33. 1 2 Wade, K.; Banister, A. J. (1973). "The Chemistry of Aluminium, Gallium, Indium and Thallium". In Bailar, J. C.; Emeléus, H. J.; Nyholm, R. Comprehensive Inorganic Chemistry. Elsevier. p. 1047. ISBN 9781483153223.
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  38. ↑ Llewellyn, Bryan D. (May 2005). "Stain Theory – How mordants work". Archived from the original on 14 August 2007.
  39. ↑ Kiel, E. G.; Heertjes, P. M. (1965). "Metal Complexes of Alizarin V—Investigations of Alizarin–dyed Cotton Fabrics". Coloration Technology. doi:10.1111/j.1478-4408.1965.tb02647.x.
  40. ↑ Kiel, E. G.; Heertjes, P. M. (1963). "Metal Complexes of Alizarin I—The Structure of the Calcium–Aluminium Lake of Alizarin". Coloration Technology. doi:10.1111/j.1478-4408.1963.tb02507.x.
  41. 1 2 3 Soubayrol, Patrick; Dana, Gilbert; Man, Pascal P. (1996). "Aluminium-27 Solid-State NMR Study of Aluminium Coordination Complexes of Alizarin". Magnetic Resonance in Chemistry. 34 (8): 638–645. doi:10.1002/(SICI)1097-458X(199608)34:8<638::AID-OMR926>3.0.CO;2-5.
  42. ↑ Hanson, James Ralph (2001). Functional group chemistry. Royal Society of Chemistry. p. 11. ISBN 0854046275.
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