Sodium aluminium hydride
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Through Chemical formula, Reagent and Organic compound
Structure, preparation, and reactions
Sodium tetrahydroaluminate adopts the structure of (is isostructural with) calcium tungstate. As such, the tetrahedral AlH−
4 centers are linked with eight-coordinate Na+ cations. The compound is prepared from the elements under high pressures of H2 at 200 °C using triethylaluminium catalyst:
- Na + Al + 2 H2 → NaAlH4
As a suspension in diethyl ether, it reacts with lithium chloride to give the popular reagent lithium aluminium hydride:
- LiCl + NaAlH4 → LiAlH4 + NaCl
The compound reacts rapidly, even violently, with protic reagents, such as water, as described in this idealized equation:
- 4 H2O + NaAlH4 → "NaAl(OH)4" + 4 H2
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Applications
Hydrogen storage
Sodium alanate has been explored for hydrogen storage in hydrogen tanks. The relevant reactions are:
- 3 NaAlH4 → Na3AlH6+ 2 Al + 3 H2
- Na3AlH6 → 3 NaH + Al + 3/2 H2
Sodium tetrahydroaluminate can release up to 7.4 wt % of hydrogen when heated at 200 °C (392 °F). Absorption can be slow, with several minutes being required to fill a tank. Both release and uptake are catalysed by titanium.
Reagent in organic chemistry
Sodium aluminium hydride is a strong reducing agent, very similar in reactivity to lithium aluminium hydride (LAH) and, to some extent, Diisobutylaluminium hydride (DIBAL) in organic reactions. It is much more powerful reducing agent than sodium borohydride due to the weaker and more polar Al-H bond compared to the B-H bond. Like LAH, it reduces esters to alcohols.
Safety
Sodium aluminium hydride is highly flammable. It does not react in dry air at room temperature but is very sensitive to moisture. It ignites or explodes on contact with water.