Description
Lithium aluminum hydride (LiAlH4) is a promising compound for hydrogen storage, with a high gravimetric and volumetric hydrogen density and a low decomposition temperature. Similar to other metastable hydrides, LiAlH4 does not form by direct hydrogenation at reasonable hydrogen pressures; therefore, there is considerable interest in developing new routes to regenerate the material from the dehydrogenated products LiH and Al. It can also be used as a reducing agent in the preparation of reduced graphene oxide (rGO).
Chemical Properties
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.
Physical properties
White crystalline powder when pure; 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/cm
3; 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.
Uses
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
Uses
It 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.
Preparation
Lithium aluminum hydride is prepared by reaction of lithium hydride with aluminum chloride in diethylether:
4LiH + AlCl3 →(C2H5)2O→LiAlH4+3LiCl
Application
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).
General Description
A white powder that turns gray on standing. If spread out over a large flat combustible surface, friction can cause ignition. Used to make other chemicals, as a polymerization catalyst, as a hydrogen source, and as a propellant.
Air & Water Reactions
Reacts with water vigorously attaining incandescence and ignition of evolved hydrogen [Kelen, Cahiers, 1977, (86), 100]. Reactions with water or moist air (or heated air) are violent and may be explosive [Schmidt, D.L., et al. Inorg. Synth. 1973. p. 14, 51].
Reactivity Profile
Lithium aluminium hydride is a powerful reducing agent. React violently on contact with many oxidizing agents. Ignites by friction, especially if powdered. Reacts vigorously with hydroxy compounds such as water, alcohols, carboxylic acids [Mellor 2 Supp. 2:142. 1961]. Caused a violent explosion when used to dry diethylene glycol dimethyl ether: Ignition may have been caused by heat from reaction with impurity water or perhaps decomposition of peroxides in the ether. About 75% of the ether had been removed when the explosion occurred [MCA Case History 1494. 1968]. Reduces carbon dioxide or sodium hydrogen carbonate to methane and ethane at elevated temperatures. These flammable or explosive gases can form when CO2 extinguishers are used to fight hydride fires. Forms explosive complexes with ether, dimethylamine and various tetrazoles. Tetrazoles include, 2-methyl, 2-ethyl, 5-ethyl, 2-methyl-5-vinyl, 5-amino-2-ethyl [US Pat. 3 396 170, 1968].
Hazard
Lithium aluminum hydride is a flammable substance. It ignites spontaneously on grinding and reacts violently with water and many organic substances. Diethyl ether, tetrahydrofuran or another suitable solvent should be used in its synthetic applications. Dry or powdered limestone is an appropriate fire extinguishing agent.
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
Flammability and Explosibility
Lithium aluminum hydride is a highly flammable solid and may ignite in moist or
heated air. Exposure to water results in the release of hydrogen, which can be ignited
by the heat from the exothermic reaction. Lithium aluminum hydride should not be
used as a drying agent for solvents because fires can easily result (LAH decomposes
at about 125° C, a temperature easily reached at a flask's surface in a heating
mantle). The decomposition products of LAH can be quite explosive, and the
products of its reaction with carbon dioxide have been reported to be explosive. Use
dry chemical powder or sand to extinguish fires involving lithium aluminum
hydride. Never use water or carbon dioxide extinguishers on an LAH fire.
reaction suitability
reagent type: reductant
Safety Profile
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. See also ALUMINUM,
LITHIUM COMPOUNDS, and
HYDRIDES.
Potential Exposure
This material is used as a catalyst and
as a specialty reducing agent in organic synthesis.
First aid
If this chemical gets into the eyes, remove anycontact lenses at once and irrigate immediately for at least15 min, occasionally lifting upper and lower lids. Seek medical attention immediately. If this chemical contacts theskin, remove contaminated clothing and wash immediatelywith soap and water. Seek medical attention immediately. Ifthis chemical has been inhaled, remove from exposure,begin rescue breathing (using universal precautions, including resuscitation mask) if breathing has stopped and CPR ifheart action has stopped. Transfer promptly to a medicalfacility. When this chemical has been swallowed, get medical attention. Give large quantities of water and inducevomiting. Do not make an unconscious person vomit.Medical observation is recommended for 24-48 h afterbreathing overexposure, as pulmonary edema may bedelayed. As first aid for pulmonary edema, a doctor orauthorized paramedic may consider administering a corticosteroid spray.
storage
LAH should be handled in areas free of ignition sources
under an inert atmosphere. Safety glasses, impermeable gloves, and a fire-retardant
laboratory coat are required. A dry powder fire extinguisher or pail of sand (and
shovel) must be available in areas where LAH is to be handled or stored. Work with
large quantities of powdered LAH should be conducted in a fume hood under an
inert gas such as nitrogen or argon. Lithium aluminum hydride should be stored in
tightly sealed containers in a cool, dry area separate from combustible materials. Dry
LAH powder should never be exposed to water or moist air. Lithium aluminum
hydride can be a finely powdered reagent that produces a reactive dust on handling.
The older practice of grinding lithium aluminum hydride prior to use can cause
explosions and should not be employed.
Shipping
UN1410 Lithium aluminum hydride (dry),
Hazard Class: 4.3; Labels: 4.3-Dangerous when wet material. UN1411 Lithium aluminum hydride, ethereal, Hazard
Class: 4.3; Labels: 4.3-Dangerous when wet material, 3-
Flammable liquid.
Purification Methods
Extract it with Et2O, and, after filtering, the solvent is removed under vacuum. The residue is dried at 60o for 3hours, under high vacuum [Ruff J Am Chem Soc 83 1788 1961]. It is a strong reducing agent. It IGNITES in the presence of a small amount of water and reacts with it EXPLOSIVELY. [Becher in Handbook of Preparative Inorganic Chemistry (Ed. Brauer) Academic Press Vol I p 805 1963.]
Incompatibilities
Combustible solid. Can ignite spontaneously in moist air or heat. Decomposes on heating @
125C forming aluminum, lithium hydride and flammable
hydrogen gas. A strong reducing agent; violent reaction
with oxidizers. Violent reaction with water, alcohols, acids,
dimethylether, ethers, tetrahydrofuran, benzoyl peroxide;
boron trifluoride etherate. Reduces carbon dioxide or
sodium hydrogen carbonate to methane and ethane at elevated temperatures. These flammable or explosive gases
can form when CO2 extinguishers are used to fight hydride
fires. Forms explosive complexes with ether, dimethylamine and various tetrazoles. Tetrazoles include, 2-methyl,
2-ethyl, 5-ethyl, 2-methyl-5-vinyl, 5-amino-2-ethyl .
Waste Disposal
Small amounts of excess LAH can be destroyed by forming a suspension or solution in an inert solvent
such as diethyl ether or hexane, cooling in an ice bath, and slowly and carefully adding ethyl acetate
dropwise with stirring. This is followed by the addition of a saturated aqueous solution of ammonium
chloride.
Excess lithium aluminum hydride and the products of the treatment described above should be placed in an
appropriate container, clearly labeled, and handled according to your institution's waste disposal guidelines.
For more information on disposal procedures, see Chapter 7 .
