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Lithium aluminium hydride, commonly abbreviated to LAH, is an inorganic compound with the chemical formula LiAlH4. This compound is used as a reducing agent in organic synthesis, especially for the reduction of esters, carboxylic acids, and amides. The solid is dangerously reactive toward water, releasing H2, and some related derivatives have been discussed for the storage of H2.
[edit] Properties, structure, preparationLAH is a white solid, but commercial samples are almost always gray due to contamination with traces of aluminium metal. This material can be purified by recrystallization from diethyl ether. Large-scale purifications employ a Soxhlet extractor. Commonly, the impure gray material is used in synthesis, since the impurities are innocuous and easily separated from the organic products. The pure material is pyrophoric. Some commercial materials contain mineral oil to inhibit reactions with atmospheric moisture, but more commonly it is packed in moisture-proof plastic sacks. LAH violently reacts with water,[1] including atmospheric moisture. The reaction proceeds according the following idealized equation:
This reaction provides a useful method to generate hydrogen in the laboratory. Aged, air-exposed samples often appear white because they have absorbed enough moisture to generate a mixture of the colorless compounds lithium hydroxide and aluminium hydroxide. [edit] StructureLAH crystallizes in the monoclinic space group P21c. The unit cell is defined as follows: a = 4.82, b = 7.81, and c = 7.92 Å, α = γ = 90° and β = 112°. The solid consists of Li+ centers surrounded by five AlH−4 tetrahedra. The Li+ centers are bonded to one hydrogen atom from each of the surrounding tetrahedra creating a bipyramid arrangement. At high pressures (>2.2GPa) a phase transition may occur to give β-LAH.[2]  [edit] PreparationLAH was first prepared from the reaction between lithium hydride (LiH) and aluminium chloride:[3]
In addition to this method, the industrial synthesis entails the initial preparation of sodium aluminium hydride from the elements under high pressure and temperature:[4]
LAH is then prepared by metathesis reaction according to:
which proceeds in a high yield of LAH. LiCl is removed by filtration from an ethereal solution of LAH, with subsequent precipitation of LAH to yield a product containing around 1% w/w LiCl.[4] [edit] Applications[edit] Use in organic chemistryLithium aluminium hydride is widely used in organic chemistry as a reducing agent. It is more powerful than the related reagent sodium borohydride due to the weaker Al-H bond compared to the B-H bond.[5] Often as a solution in diethyl ether and followed by an acid work-up, it will convert esters, carboxylic acids, aldehydes, and ketones into the corresponding alcohols. Similarly, it converts amide, nitro, nitrile, imine, oxime, and azide compounds into the amines. It reduces quaternary ammonium cations into the corresponding tertiary amines. Despite handling problems associated with its reactivity, it is even used at the small-industrial scale, although for large scale reductions, the related reagent sodium bis(2-methoxyethoxy)aluminium hydride is more commonly used. LAH is most commonly used for the reduction of esters[6][7] and carboxylic acids[8] to primary alcohols; prior to the advent of LiAlH4 this was a difficult conversion involving sodium metal in boiling ethanol (the Bouveault-Blanc reduction). Aldehydes and ketones[9] can also be reduced to alcohols by LAH, but this is usually done using milder reagents such as NaBH4. α,β-Unsaturated ketones are reduced to allylic alcohols.[10] When epoxides are reduced using LAH, the reagent attacks the less hindered end of the epoxide, usually producing a secondary or tertiary alcohol. Epoxycyclohexanes are reduced to give axial alcohols preferentially.[11] Partial reduction of acid chlorides to give the corresponding aldehyde product cannot proceed via LAH, since the latter reduces all the way to the primary alcohol. Instead, the milder lithium aluminium tri(t-butoxy)hydride must be used, which reacts significantly faster with the acid chloride than with the aldehyde. For example, when isovaleric acid is treated with thionyl chloride to give isovaleroyl chloride, it can then be reduced via lithium aluminium tri(t-butoxy)hydride to give isovaleraldehyde in 65% yield[12].  Using LAH, amines can be prepared by the reduction of amides,[13][14] oximes,[15] nitriles, nitro compounds or alkyl azides. Lithium aluminium hydride also reduces alkyl halides to alkanes, although this reaction is rarely employed.[16][17] Alkyl iodides react the fastest, followed by alkyl bromides and then alkyl chlorides. Primary halides are the most reactive followed by secondary halides. Tertiary halides react only in certain cases. Lithium aluminium hydride does not reduce simple alkenes, arenes, and alkynes are only reduced if an alcohol group is nearby.[18] [edit] Inorganic chemistryLAH is widely used to prepare main group and transition metal hydrides from the corresponding metal halides. For example, sodium hydride (NaH) can be prepared from sodium chloride (NaCl) through the following reaction:[19]
LAH also reacts with many inorganic ligands to form coordinated alumina complexes associated with lithium ions. [19]
[edit] Other tetrahydridoaluminiumatesA variety of salts analogous to LAH are known. NaH can be used to efficiently produce sodium aluminium hydride (NaAlH4) by metathesis in THF:
Potassium aluminium hydride (KAlH4) can be produced similarly in diglyme as a solvent:
The reverse, i.e., production of LAH from either sodium aluminium hydride or potassium aluminium hydride can be achieved by reaction with LiCl in diethyl ether or THF:
"Magnesium alanate" (Mg(AlH4)2) arises similarly using MgBr2:
[edit] Thermal decompositionAt room temperature LAH is metastable. During prolonged storage it slowly decomposes to Li3AlH6 and LiH. This process can be accelerated by the presence of catalytic elements e.g. Ti, Fe, V. When heated LAH decomposes in a three step reaction mechanism.
R1 is usually initiated by the melting of LAH in the range 150-170 °C immediately followed by decomposition into solid Li3AlH6. At about 200 °C Li3AlH6 decomposes into LiH (R2) and Al which subsequently decompose into LiAl above 400 °C (R3). R1 is effectively irreversible. R3 is reversible with an equilibrium pressure of about 0.25 bar at 500 °C. R1 and R2 can occur at room temperature with suitable catalysts. LiAlH4 contains 10.6 wt% hydrogen thereby making LAH a potential hydrogen storage medium for future fuel cell powered vehicles. Cycling only R2 would store 5.6 wt% in the material in a single step (comparable to the two steps of NaAlH4). However, attempts on this have not been successful. [edit] Solubility dataLAH is soluble in many etheral solutions. However, it may spontaneously decompose due to the presence of catalytic impurities, though, it appears to be more stable in THF. Thus, THF is preferred over e.g. diethyl ether even despite the lower solubility.
[edit] Thermodynamic dataThe table summarizes thermodynamic data for LAH and reactions involving LAH, in the form of standard enthalpy, entropy and Gibbs free energy change, respectively.
[edit] SafetyLAH reacts violently with water and related protic compounds and is combustable in air. See MSDS. [edit] See also
[edit] References
[edit] Further reading
[edit] External links
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