What is Lithium Aluminum Hydride?

Basic Informations

Lithium aluminum hydride is a white to gray powder. A combustible solid. monoclinic crystals; grey in the presence of aluminum impurity; stable below 120°C in dry air; turns grey on standing; hygroscopic; density 0.917 g/cm3; melts at 190°C (decomposes); reacts with water and alcohols; soluble in diethylether and tetrahydrofuran (about 30 and 13 g/100g, respectively at 25°C; also soluble in dimethylcellosolve; sparingly soluble in dibutylether; slightly soluble in dioxane (1g/L) and practically insoluble in hydrocarbons; can be solubilized in benzene by crown ether.

Lithium aluminum hydride is used as a powerful reducing agent inorganic synthesis. Except for olefinic doublebonds, almost all organic functional groupsare reduced by lithium aluminum hydride(Sullivan and Wade 1980). It is used extensivelyin pharmaceutical synthesis and in catalytichydrogenation.

Structure

LAH crystallizes in the monoclinic space group P21/c. The unit cell has the dimensions: a = 4.82, b = 7.81, and c = 7.92 Å, α = γ = 90° and β = 112°. In the structure, Li+ centers are surrounded by five AlH₄⁻ tetrahedra. The Li+ centers are bonded to one hydrogen atom from each of the surrounding tetrahedra creating a bipyramid arrangement. At high pressures (>2.2 GPa) a phase transition may occur to give β-LAH.

Stability

Stable in dry air at room temperature. It decomposes above 125' forming Al, H2, and lithium hydride. Very powerful reducer. Can ignite if pulverized even in a dry box. Reacts violently with air, acids, alcohols, benzoyl peroxide, boron trifluoride etherate, (2 chloromethyl furan + ethyl acetate), diethylene glycol dimethyl ether, diethyl ether, 1,2-dimethoxyethane, dimethyl ether, methyl ethyl ether, (nitriles + H2O), perfluorosuccinamide, (perfluorosuccinamide + H20), tetrahydrofuran, water. To fight fire, use dry chemical, includmg special formulations of dry chemicals as recommended by the supplier of the lithium aluminum hydride. Do not use water, fog, spray, or mist. Incompatible with bis (2-methoxyethyl)ether, CO2, BF3, diethyl etherate, dibenzoyl peroxide, 3,5 dbromocyclopentene, 172-dimethoxy ethane, ethyl acetate, fluoro amides, pyridine, tetrahydrofuran. Used as a reducing agent in the preparation of pharmaceuticals. 

Uses

Lithium aluminum hydride (LiAlH4) is an effective reducing agent that can be used in chemical synthesis to reduce esters, carboxylic acids, acyl chlorides, aldehydes, epoxides, and ketones into the corresponding alcohols. In addition, amide, nitro, nitrile, imine, oxime, and azide compounds are converted into amines.

LiAlH4 is a promising substance for hydrogen storage applications. Its properties include high gravimetric and volumetric hydrogen densities . It can also be used as a reducing agent in the preparation of reduced graphene oxide (rGO).    

Lithium aluminum hydride is among the most important industrial reducingagents. It is used extensively in organic syntheses and also in catalytichydrogenation. Reactant or reagent for:

1. The preparation of thermoplastic polyester polyamides from oleic acid

2. Lithium-polymer batteries

3. Hydrodefluorination of gem-difluoromethylene derivatives

4. Asymmetric aldol reactions

5. Synthesis of Li-Al-N-H composites with hydrogen absorption / desorption properties

6. LAH is a powerful reducing agent for many different reduction reactions such as that of ketones to alcohols

Preparation

LiAlH4 was first prepared from the reaction between lithium hydride (LiH) and aluminium chloride:

4 LiH + AlCl3 → LiAlH4 + 3 LiCl

In addition to this method, the industrial synthesis entails the initial preparation of sodium aluminium hydride from the elements under high pressure and temperature:

Na + Al + 2 H2 → NaAlH4

LiAlH4 is then prepared by a salt metathesis reaction according to:

NaAlH4 + LiCl → LiAlH4 + NaCl

which proceeds in a high yield. LiCl is removed by filtration from an ethereal solution of LAH, with subsequent precipitation of LiAlH4 to yield a product containing around 1% w/w LiCl.

An alternative preparation starts from LiH, and metallic Al instead of AlCl3. Catalyzed by a small quantity of TiCl3 (0.2%), the reaction proceeds well using dimethylether as solvent. This method avoids the cogeneration of salt.

Health Hazard

Lithium aluminum hydride is highly corrosive to the skin, eyes, and mucous membranes. Contact with moisture forms lithium hydroxide, which can cause severe burns. Powdered LAH forms dusts that can pose an inhalation hazard. Ingestion of this substance may cause aching muscles, nausea, vomiting, dizziness, and unconsciousness and may be fatal. Ingestion can result in gas embolism due to the formation of hydrogen.

No chronic effects of lithium aluminum hydride have been identified.