Lithium aluminium hydride
- Lithium tetrahydridoaluminate
- Lithium aluminum hydride
Colorless powder
Fixed
0.92 g · cm -3 ( 20 ° C)
125 ° C ( decomposition)
- With water, violent decomposition
- Soluble in diethyl ether and tetrahydrofuran
- Insoluble in chloroform, benzene and petroleum ether
Risk
-116.3 KJ / mol
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Lithium aluminum hydride ( LAH) is an inorganic reducing agent to the empirical formula LiAlH4.
Synthesis
Lithium aluminum hydride is produced in the laboratory by suspending lithium hydride and aluminum chloride in diethyl ether. After filtration of the lithium chloride, and removing the ether, lithium aluminum hydride is left behind.
Technically, it is also produced by reacting sodium aluminum hydride with lithium chloride. The sodium aluminum hydride required can be obtained from the elements of sodium, aluminum and hydrogen at elevated temperature under pressure.
Responsiveness
Lithium aluminum hydride is a strong reducing agent of organic synthetic chemistry and selectively reduces almost all of the carbon -heteroatom double and triple bonds, such as carbonyls or nitriles, on the other hand it preserves carbon-carbon double bonds and carbon-carbon triple bonds (alkenes / alkynes ), unless these are conjugated to specific activating groups; as is for example the grouping phenyl -CH = CH- NO2 reduced to 2 -phenylethylamine. It reduces nitro compounds, amides, oximes or azides to primary amines, carbonyl compounds to alcohols, carboxylic acids, esters, acid chlorides and acid anhydrides to give primary alcohols. Haloalkanes are reduced to alkanes.
With water it reacts violently and strongly exothermic with the formation of lithium hydroxide, aluminum hydroxide and hydrogen.
Lithium aluminum hydride at room temperature is metastable. It decomposes slowly to Lithiumhexahydridoaluminat Li3AlH6 and lithium hydride, which can be accelerated by catalysts and heating.
The decomposition takes place at a slow heating in several steps.
Takes place only immediately followed by the decomposition Li3AlH6 usually the melting of lithium aluminum hydride. At about 200 ° C. This, in turn, decomposes in aluminum and lithium hydride, the reaction at 400 ° C to LiAl.
Use
Lithium aluminum hydride, as well as sodium borohydride, used in the organic chemistry as the reducing agent. This reducing agent is used as an example of a synthesis method that takes place with low atom economy. In conjunction with chiral reagents, such as TADDOL, it is possible to make enantioselective reduction of ketones. Another application is in the synthesis of sodium - and potassium aluminum hydride, which can be obtained by use of the corresponding hydrides.