Lithium aluminium hydride

Lithium aluminium hydride (LiAlH₄), commonly abbreviated to LAH, is a reducing agent used in organic synthesis. It is more powerful than the related reagent sodium borohydride due to the weaker Al-H bond compared to the B-H bond. It will convert esters, carboxylic acids, and ketones into the corresponding alcohols; and amide, nitro, and azide compounds into the amines.

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Availability and handling

LAH is a white solid but commercial samples are almost always grey 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 packages in moisture-proof plastic sacks.

LAH violently reacts with water, including atmospheric moisture. The reaction proceeds according the following idealized equation:

LiAlH₄ + 4 H₂O → LiOH + Al(OH)₃ + 4 [H₂

This reaction is 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.

Preparation

LAH was first prepared from the reaction between lithium hydride (LiH) and aluminium chloride:^[1]

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:^[2]

Na + Al + 2 H₂ → NaAlH₄

LAH is then prepared by metathesis reaction according to:

NaAlH₄ + LiCl → LiAlH₄ + NaCl

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.^[2]

Other tetrahydridoaluminiumates

A variety of salts analogous to LAH are known. NaH can be used to efficiently produce sodium aluminium hydride (NaAlH₄) by metathesis in THF:

LiAlH₄ + NaH → NaAlH₄ + LiH

Potassium aluminium hydride (KAlH₄) can be produced similarly in diglyme as a solvent:

LiAlH₄ + KH → KAlH₄ + LiH

The reverse, i.e., production of LAH from either sodium aluminium hydride or potassium aluminium hydride can be obtained by reaction with LiCl in diethyl ether or THF:

NaAlH₄ + LiCl → LiAlH₄ + NaCl

KAlH₄ + LiCl → LiAlH₄ + KCl

"Magnesium alanate" (Mg(AlH₄)₂) arises similarly usingMgBr₂:

2 LiAlH₄ + MgBr₂ → Mg(AlH₄)₂ + 2 LiBr

Use in organic chemistry

Lithium aluminium hydride is widely used in organic chemistry as a reducing agent.^[3] 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, commonly known as Red-Al, is more often used. For such purposes it is usually used in solution in diethyl ether, and an aqueous workup is usually performed after the reduction in order to remove inorganic by-products.

LAH is most commonly used for the reduction of esters^[4][5] and carboxylic acids^[6] 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^[7] 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.^[8] 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.^[9]

Using LAH, amines can be prepared by the reduction of amides,^[10][11], oximes^[12], nitriles, nitro compounds or alkyl azides.

Lithium aluminium hydride also reduces alkyl halides to alkanes, although this reaction is rarely employed.^[13][14] 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, or alkynes are only reduced if an alcohol group is nearby.^[15]

Inorganic chemistry

LAS is widely used to prepare main group and transition metal hydrides from the corresponing metal halides.

Thermal decomposition

At room temperature LAH is metastable. During prolonged storage it can slowly decompose to Li₃AlH₆ and LiH. This process can be accelerated by the presence of catalytic elements e.g. Ti, Fe, V.

When heated LAH decompose in a three step reaction mechanism.

LiAlH₄ → ⅓ Li₃AlH₆ + ⅔ Al + H₂ (R1)

⅓ Li₃AlH₆ → LiH + ⅓ Al + ½ H₂ (R2)

LiH + Al → LiAl + ½ H₂ (R3)

R1 is usually initiated by the melting of LAH around a temperature of 150-170^oC immediately followed by decomposition into solid Li₃AlH₆. From 200-250^oC Li₃AlH₆ decompose into LiH (R2) which subsequently decompose into LiAl above 400^oC (R3). R1 is effectively irreversible, because LiAlH₄ is metastable. The reversibility of R2 has not yet been conclusively established. R3 is reversible with an equilibrium pressure of about 0.25 bar at 500^oC. R1 and R2 can occur at room temperature with suitable catalysts.

According to reactions R1-R3 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).

Solubility data

LAH 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.

Note that lithium aluminium hydride should not be used with water, which reacts violents as in the following equation.

LiAlH₄ + 4 H₂O → Li⁺ + Al³⁺ + 4 OH^- + 4 H₂

Crystal structure

The crystal structure of LAH belongs to the monoclinic crystal system and the space group is P2₁c. The crystal structure of LAH is illustrated to the right. The structure consists of Li atoms surrounded by five AlH₄ tetrahedra. The Li⁺ centers are bonded to one hydrogen atom from each of the surrounding tetrahedra creating a bipyramid arrangement. The unit cell is defined as follows: a = 4.82, b = 7.81, and c = 7.92 Å, α = γ = 90° and β = 112 °. At high pressures (>2.2GPa) a phase transition occurs to give β-LAH.^[16]

Thermodynamic data

The table summarizes thermodynamic data for LAH and reactions involving LAH, in the form of standard enthalpy, entropy and Gibbs free energy change, respectively.

See also

• Sodium borohydride
• Sodium hydride
• Hydride

References

Further reading

• Wiberg, Egon & Amberger, Eberhard (1971). Hydrides of the elements of main groups I-IV. Elsevier. ISBN 0-444-40807-X. 
• Hajos, Andor (1979). Complex Hydrides and Related Reducing Agents in Organic Synthesis. Elsevier. ISBN 0-444-99791-1. 
• Lide (ed.), David R. (1997). Handbook of chemistry and physics. CRC Press. ISBN 0-8493-0478-4. 
• Carey, Francis A. (2002). Organic Chemistry with Online Learning Center and Learning by Model CD-ROM. McGraw-Hill. ISBN 0-07-252170-8.  on-line version
• Chapter 5 in Andreasen, Anders (2005). Hydrogen Storage Materials with Focus on Main Group I-II Elements. Risoe National Laboratory. ISBN 87-550-3498-5.  Full text version