Reduces LiAlH4 alkynes

Lithium aluminum hydride

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Lithium aluminum hydride (LAH) is an inorganic reducing agent with the empirical formula LiAlH4.

synthesis

In the laboratory, lithium aluminum hydride is obtained by suspending lithium hydride and aluminum chloride in diethyl ether.[6] After filtering off the lithium chloride and removing the ether, lithium aluminum hydride remains.

$ \ mathrm {4 \ LiH + \ AlCl_3 \ longrightarrow \ LiAlH_4 + 3 \ LiCl} $
Synthesis of lithium aluminum hydride from lithium hydride and aluminum chloride

Technically, it is also produced by reacting sodium aluminum hydride with lithium chloride. The required sodium aluminum hydride can be obtained from the elements sodium, aluminum and hydrogen at elevated temperature under pressure.[7]

$ \ mathrm {Na \ + \ Al \ + \ 2 \ H_2 \ longrightarrow \ NaAlH_4} $
$ \ mathrm {NaAlH_4 + \ LiCl \ longrightarrow \ LiAlH_4 + NaCl} $

Responsiveness

Lithium aluminum hydride is a strong reducing agent in organic-synthetic chemistry and selectively reduces almost all carbon-heteroatom double and triple bonds such as carbonyls or nitriles; on the other hand, it is gentle on CC double bonds and CC triple bonds (alkenes / alkynes), unless these are conjugated to certain activating groups; for example, the grouping is phenyl-CH = CH-NO2 reduced to 2-phenylethylamine. It reduces nitro compounds, amides[8][9], Azides or oximes[10] to primary amines, carbonyl compounds to alcohols[11], Carboxylic acids[12], Ester[13][14], Acid chlorides and acid anhydrides to primary alcohols. Haloalkanes are reduced to alkanes.

Reductions with lithium aluminum hydride

It reacts violently and strongly exothermically with water to form lithium hydroxide, aluminum hydroxide and hydrogen.

$ \ mathrm {LiAlH_4 + 4 \ H_2O \ rightarrow \ LiOH + \ Al (OH) _3 + 4 \ H_2} $

Lithium aluminum hydride is metastable at room temperature. It slowly decomposes to lithium hexahydridoaluminate Li3AlH6 and lithium hydride, which can be accelerated by catalysts and heating.

The decomposition takes place in several steps with slow heating.

$ \ mathrm {3 \ LiAlH_4 \ rightarrow Li_3AlH_6 + 2 \ Al + 3 \ H_2} $
$ \ mathrm {2 \ Li_3AlH_6 \ rightarrow 6 \ LiH + 2 \ Al + 3 \ H_2} $
$ \ mathrm {2 \ LiH + 2 \ Al \ rightarrow 2 \ LiAl + H_2} $

As a rule, lithium aluminum hydride is first melted, immediately followed by decomposition to Li3AlH6. At over 200 ° C, this in turn breaks down into aluminum and lithium hydride, which react to form LiAl at 400 ° C.

use

Like sodium borohydride, lithium aluminum hydride is used as a reducing agent in organic chemistry. This use as a reducing agent is an example of a synthesis method that proceeds with low atom economy. In connection with chiral reagents, e.g. TADDOL, it is possible to carry out enantioselective reductions of ketones.
Another application is the synthesis of sodium and potassium aluminum hydride, which can be obtained by using the corresponding hydrides.

$ \ mathrm {LiAlH_4 + \ KH \ longrightarrow \ KAlH_4 + \ LiH} $

Individual evidence

  1. 1,01,11,2 Thieme Chemistry (Ed.): RÖMPP Online - Version 3.5. Georg Thieme Verlag KG, Stuttgart 2009.
  2. 2,02,12,2data sheet Lithium aluminum hydride at Merck, accessed January 19, 2011.
  3. 3,03,1Entry from the CLP regulation too CAS no. 16853-85-3 in the GESTIS substance database of the IFA (JavaScript required)
  4. ↑ entry to CAS no. 16853-85-3 in the GESTIS substance database of the IFA, accessed on April 7, 2011 (JavaScript required).
  5. ↑ Since December 1, 2012, only GHS hazardous substance labeling has been permitted for substances. Until June 1, 2015, the R-phrases of this substance can still be used to classify preparations, after which the EU hazardous substance labeling is of purely historical interest.
  6. ↑ A. E. Finholt, A. C. Bond, H. I. Schlesinger: Lithium Aluminum Hydride, Aluminum Hydride and Lithium Gallium Hydride, and Some of their Applications in Organic and Inorganic Chemistry, in: J. Am. Chem. Soc.1947, 69, 1199–1203.
  7. ↑ A. F. Holleman, E. Wiberg, N. Wiberg, Inorganic Chemistry Textbook2007, 102nd edition, de Gruyter. ISBN 978-3-11-017770-1.
  8. ↑ D. Seebach, H.-O. Kalinowski, W. Langer, G. Crass, E.-M. Wilka, Organic Syntheses1983, 61, 24.
  9. ↑ C. H. Park, H. E. Simmons, Organic Syntheses1974, 54, 88.
  10. ↑ Y. K. Chen, S.-J. Jeon, P. J. Walsh, W. A. ​​Nugent, Organic Syntheses2005, 82, 87.
  11. ↑ J. P. Barnier, J. Champion, J. M. Conia, Organic Syntheses1981, 60, 25.
  12. ↑ B. Koppenhöfer, V. Schurig, Organic Syntheses1988, 66, 160.
  13. ↑ M. T. Reetz, M. W. Drewes, R. Schwickardi, Organic Syntheses1999, 76, 110.
  14. ↑ R. Oi, K.B. Sharpless, Organic Syntheses1996, 73, 1.

literature

  • Arnold F. Holleman, Nils Wiberg: Inorganic Chemistry Textbook, 102nd edition, de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1.
  • Reinhard Brückner: Reaction mechanisms. 3rd edition, Spektrum Akademischer Verlag, Munich 2004, ISBN 3-8274-1579-9.
  • Hans Beyer and Wolfgang Walter: Organic Chemistry Textbook, 19th edition, S. Hirzel Verlag, Stuttgart 1981, ISBN 3-7776-0356-2.

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