Aluminium isopropoxide

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Aluminium isopropoxide
One enantiomorph of Al4(OiPr)12
Identifiers
CAS number 555-31-7 YesY
ChemSpider 10670 YesY
RTECS number BD0975000
Jmol-3D images Image 1
Properties
Molecular formula C9H21O3Al
Molar mass 204.25 g/mol
Appearance white solid
Density 1.035 g/cm³, solid
Melting point Sensitive to purity:
138–142 °C (99.99+%)
118 °C (98+%)[1]
Boiling point @10 torr 135 °C (408 K)
Solubility in water Decomposes
Solubility in isopropanol Soluble
Structure
Crystal structure monoclinic
Hazards
R-phrases R11
S-phrases S8, S16
Main hazards Flammable (F)
NFPA 704
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g., canola oil Health code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g., chloroform Reactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g., phosphorus Special hazards (white): no codeNFPA 704 four-colored diamond
Flash point 16 °C (61 °F; 289 K)
Related compounds
Other cations Titanium isopropoxide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Aluminium isopropoxide is the chemical compound usually described with the formula Al(O-i-Pr)3, where i-Pr is the isopropyl group (CH(CH3)2). This colourless solid is a useful reagent in organic synthesis. The structure of this compound is complex, possibly time-dependent, and may depend on solvent.

Structure[edit]

The structure of the metal alkoxides are often complex and aluminium isopropoxide is no exception. The complexity is also reflected in the disputed melting point for the material which could reflect the presence of trace impurities, such as water, slow oligomerisation ("aging") or both. For aluminium isopropoxide this phenomenon is mainly due to the trimer-tetramer transformation described in detail in the early works by Turova et al. The tetrameric structure of the solid crystalline material was verified by NMR spectroscopy and X-ray crystallography. The species is described by the formula Al[[(μ-O-i-Pr)2Al(O-i-Pr)2]]3.[2][3] The unique central Al is octahedral surrounded by three bidentate "[[Al(O-i-Pr)4]]-" ligands, each featuring tetrahedral Al. The idealised point group symmetry is D3. The tert-butoxide is a dimer with the formula Al2(μ-O-t-Bu)2(O-t-Bu)4[4] It is prepared analogously to the isopropoxide.[5]

Preparation[edit]

This compound is commercially available. Industrially, it is prepared by the reaction between isopropyl alcohol and aluminium metal, or aluminium trichloride:

2 Al + 6 iPrOH → 2 Al(O-i-Pr)3 + 3 H2
AlCl3 + 3 iPrOH → Al(O-i-Pr)3 + 3 HCl

Using aluminium metal, an older process uses a mercury catalyst (see below), whereas a more recent process does not.[6]

In the laboratory, a widely accepted method for preparing aluminium isopropoxide was published in 1936 by Young, Hartung, and Crossley.[7] Their procedure entails heating a mixture of 100 g of aluminium, 1200 mL of isopropyl alcohol, and 5 g of mercuric chloride at reflux. The process occurs via the formation of an amalgam of the aluminium. A catalytic amount of iodine is sometimes added to initiate the reaction, which can be quite vigorous. Young et al. achieved an 85–90% yield, after purification by distillation at 140–150 °C (5 mm Hg).

Reactions[edit]

In a MPV reduction, ketones and aldehydes are reduced to alcohols concomitant with the formation of acetone. This reduction relies on an equilibrium process, hence it produces the thermodynamic product. Conversely, in the Oppenauer Oxidation, secondary alcohols are converted to ketones,[8] and homoallylic alcohols are converted to α,β-unsaturated carbonyls.[8] In these reactions, it is assumed that the tetrameric cluster disagregates.

MPV-reduction.png

Being a basic alkoxide, Al(O-i-Pr)3 has been also investigated as a catalyst for ring opening polymerization of cyclic esters.[9]

History[edit]

Aluminium isopropoxide was first reported in the Dissertation of Alexandre Tischenko in the Annals of the Russian Physico-Chemical society in 1898. This contribution included detailed description of its synthesis, peculiar physico-chemical behavior and catalytic activity in Tishchenko reaction (catalytic transformation of aldehydes into esters). It was later found also to display catalytic activity as a reducing agent by Meerwein and Schmidt in the Meerwein-Ponndorf-Verley reduction ("MPV") in 1925.[10] The reverse of the MPV reaction, oxidation of an alcohol to a ketone, is termed the Oppenauer oxidation. The original Oppenauer oxidation employed aluminium butoxide in place of the isoproxide.[11]

References[edit]

  1. ^ Ishihara, K.; Yamamoto, H. "Aluminum Isopropoxide" in Encyclopedia of Reagents for Organic Synthesis, 2001 John Wiley. doi:10.1002/047084289X.ra084
  2. ^ Folting, K.; Streib, W. E.; Caulton, K. G.; Poncelet, O.; Hubert-Pfalzgraf, L. G. (1991). "Characterization of aluminum isopropoxide and aluminosiloxanes". Polyhedron 10 (14): 1639–46. doi:10.1016/S0277-5387(00)83775-4. 
  3. ^ Turova, N. Y.; Kozunov, V. A.; Yanovskii, A. I.; Bokii, N. G.; Struchkov, Yu T.; Tarnopolskii, B. L. Journal of Inorganic and Nuclear Chemistry 1979, volume 41, p. 5.
  4. ^ Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN 0-12-352651-5. 
  5. ^ Wayne, W.; Adkins. H. (1955), "Aluminum tert-Butoxide", Org. Synth. ; Coll. Vol. 3: 48 
  6. ^ Otto Helmboldt; L. Keith Hudson; Chanakya Misra; Karl Wefers; Wolfgang Heck; Hans Stark; Max Danner; Norbert Rösch (2005), "Aluminum Compounds, Inorganic", Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, doi:10.1002/14356007.a01_527.pub2 
  7. ^ Young, W.; Hartung, W.; Crossley, F. (1936). "Reduction of Aldehydes with Aluminum Isopropoxide". J. Am. Chem. Soc. 58: 100–2. doi:10.1021/ja01292a033. 
  8. ^ a b Eastham, J. F.; Teranishi, R. (1963), "Δ4-Cholesten-3-one", Org. Synth.: 192 ; Coll. Vol. 4 
  9. ^ Tian, D.; Dubois, Ph.; Jérôme, R. (1997). "Macromolecular Engineering of Polylactones and Polylactides. 22. Copolymerization of ε-Caprolactone and 1,4,8-Trioxaspiro[4.6]-9-undecanone Initiated by Aluminum Isopropoxide". Macromolecules 30 (9): 2575–2581. doi:10.1021/ma961567w. 
  10. ^ Meerwein, H.; Schmidt, R. (1925). "Ein neues Verfahren zur Reduktion von Aldehyden und Ketonen". Justus Liebig's Annalen der Chemie 444: 221–238. doi:10.1002/jlac.19254440112. 
  11. ^ Oppenauer, R. V. (1937). "Dehydration of secondary alcohols to ketones. I. Preparation of sterol ketones and sex hormones". Recueil des Travaux Chimiques des Pays-Bas et de la Belgique 56: 137–44.