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Lithium aluminum hydride is an inorganic compound with the formula LiAlH4. It is a complex hydride, a compound in which hydride ions (H-) are bonded coordinately to metal ions. It is technically known as lithium tetrahydridoaluminate(III) and trivially as "lith-al".

Lith-al is a white crystalline solid prepared by the reaction of aluminium chloride with lithium hydride in a diethyl ether solvent. It reacts violently with water, so the solvent must be anhydrous - completely dry.

It is a powerful reducing agent and is used to reduce various classes of organic chemicals. Aldehydes, ketones, carboxylic acids, acyl chlorides and esters can all be reduced to alcohols using lith-al. For example propanone (acetone) would be reduced to propan-2-ol, while the ester methyl ethanoate would be reduced to its constituent alcohols methanol and ethanol.

Meanwhile nitriles (which contain a carbon-nitrogen triple bond) and nitro compounds (containing the NO2 functional group) can be reduced to amines, which contain the functional group NH2.

Lith-al can also be used to prepare the hydrides of other elements. For example, it will react with silicon tetrachloride to produce silane, SiH4, the hydride of silicon, with lithium chloride and aluminium chloride as the other products.

Lith-al must be kept in anhydrous conditions at all times to prevent reaction with water. Anhydrous diethyl ether is required as a solvent for its reactions as well as its formation.

Lithium aluminum hydride gets its extreme reducing characteristic from the negative hydride ions, H-, which act as nucleophiles, attacking centres of positive charge.


Source: Michael Freemantle, Chemistry in Action, 1987

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