Lithium aluminium hydride

Lithium aluminium hydride, commonly abbreviated to LAH, is an inorganic compound with the chemical formula Template:Chem2 or Template:Chem2. It is a white solid, discovered by Finholt, Bond and Schlesinger in 1947.<ref name="Schlessinger">Template:Cite journal</ref> 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 gaseous hydrogen (H₂). Some related derivatives have been discussed for hydrogen storage.

Properties, structure, preparation

File:Lialh4 sem.png

Scanning Electron Microscope image of LAH powder

LAH is a colourless solid but commercial samples are usually gray due to contamination.<ref name=africa>Template:Cite encyclopedia</ref> 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 can be easily separated from the organic products. The pure powdered material is pyrophoric, but not its large crystals.<ref>Template:Cite book</ref> Some commercial materials contain mineral oil to inhibit reactions with atmospheric moisture, but more commonly it is packed in moisture-proof plastic sacks.<ref>Template:Cite journal</ref>

LAH violently reacts with water, including atmospheric moisture, to liberate hydrogen gas. The reaction proceeds according to the following idealized equation:<ref name="africa" />

Template:Chem2

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 white compounds lithium hydroxide and aluminium hydroxide.<ref name="sittig">Template:Cite book</ref>

Structure

File:Lithium-aluminium-hydride-unit-cell-3D-polyhedra.png

The crystal structure of LAH; Li atoms are purple and Template:Chem2 tetrahedra are tan.

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, Template:Chem2 cations are surrounded by five Template:Chem2 anions, which have tetrahedral molecular geometry. The Template:Chem2 cations are bonded to one hydrogen atom from each of the surrounding tetrahedral Template:Chem2 anion creating a bipyramid arrangement. At high pressures (>2.2 GPa) a phase transition may occur to give β-LAH.<ref name="crystal_structure">Template:Cite journal</ref>

File:Lialh4 xrpd.svg

X-ray powder diffraction pattern of as-received Template:Chem2. The asterisk designates an impurity, possibly LiCl.

Preparation

Template:Chem2 was first prepared from the reaction between lithium hydride (LiH) and aluminium chloride:<ref name="Schlessinger" /><ref name="africa" />

Template:Chem2

In addition to this method, the industrial synthesis entails the initial preparation of sodium aluminium hydride from the elements under high pressure and temperature:<ref name="HollemanAF">Template:Cite book</ref>

Template:Chem2

Template:Chem2 is then prepared by a salt metathesis reaction according to:

Template:Chem2

which proceeds in a high yield. LiCl is removed by filtration from an ethereal solution of LAH, with subsequent precipitation of LAH to yield a product containing around 1 wt% LiCl.<ref name="HollemanAF" />

An alternative preparation starts from LiH, and metallic Al instead of Template:Chem2. Catalyzed by a small quantity of TiCl₃ (0.2%), the reaction proceeds well using dimethylether as solvent. This method avoids the cogeneration of salt.<ref>Template:Cite journal</ref>

Solubility data

Solubility of Template:Chem2 (mol/L)<ref name=sol>Template:Cite journal</ref>

Solvent	Temperature (°C)
	0	25	50	75	100
Diethyl ether	–	5.92	–	–	–
THF	–	2.96	–	–	–
Monoglyme	1.29	1.80	2.57	3.09	3.34
Diglyme	0.26	1.29	1.54	2.06	2.06
Triglyme	0.56	0.77	1.29	1.80	2.06
Tetraglyme	0.77	1.54	2.06	2.06	1.54
Dioxane	–	0.03	–	–	–
Dibutyl ether	–	0.56	–	–	–

LAH is soluble in many ethereal solutions. However, it may spontaneously decompose due to the presence of catalytic impurities, though, it appears to be more stable in tetrahydrofuran (THF). Thus, THF is preferred over, e.g., diethyl ether, despite the lower solubility.<ref name="sol" />

Thermal decomposition

LAH is metastable at room temperature. During prolonged storage it slowly decomposes to Template:Chem2 (lithium hexahydridoaluminate) and LiH.<ref name="Dymova">Template:Cite journal</ref> This process can be accelerated by the presence of catalytic elements, such as titanium, iron or vanadium.

File:Lialh4 dsc.svg

Differential scanning calorimetry of as-received Template:Chem2.

When heated LAH decomposes in a three-step reaction mechanism:<ref name="Dymova" /><ref>Template:Cite journal</ref><ref name="Blanchard">Template:Cite journal</ref> Template:NumBlk Template:NumBlk Template:NumBlk

Template:EquationNote is usually initiated by the melting of LAH in the temperature range 150–170 °C,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref name="Andreasen">Template:Cite journal</ref> immediately followed by decomposition into solid Template:Chem2, although Template:EquationNote is known to proceed below the melting point of Template:Chem2 as well.<ref>Template:Cite journal</ref> At about 200 °C, Template:Chem2 decomposes into LiH (Template:EquationNote)<ref name="Dymova" /><ref name="Blanchard" /><ref name="Andreasen" /> and Al which subsequently convert into LiAl above 400 °C (Template:EquationNote).<ref name="Blanchard" /> Reaction R1 is effectively irreversible. Template:EquationNote is reversible with an equilibrium pressure of about 0.25 bar at 500 °C. Template:EquationNote and Template:EquationNote can occur at room temperature with suitable catalysts.<ref>Template:Cite journal</ref>

Thermodynamic data

The table summarizes thermodynamic data for LAH and reactions involving LAH,<ref name="InorganicHandbook">Template:Cite book</ref><ref>Template:Cite journal</ref> in the form of standard enthalpy, entropy, and Gibbs free energy change, respectively.

Thermodynamic data for reactions involving Template:Chem2

Reaction	ΔH° (kJ/mol)	ΔS° (J/(mol·K))	ΔG° (kJ/mol)	Comment
Template:Chem2 (s)	−116.3	−240.1	−44.7	Standard formation from the elements.
LiH (s) + Al (s) + Template:Frac H2 (g) → LiAlH4 (s)	−95.6	−180.2	237.6	Using ΔH°f(LiH) = −90.579865, ΔS°f(LiH) = −679.9, and ΔG°f(LiH) = −67.31235744.
Template:Chem2 (l)	22	–	–	Heat of fusion. Value might be unreliable.
LiAlH4 (l) → Template:1/3 Li3AlH6 (s) + Template:2/3 Al (s) + H2 (g)	3.46	104.5	−27.68	ΔS° calculated from reported values of ΔH° and ΔG°.

Applications

Use in organic chemistry

Lithium aluminium hydride (LAH) is widely used in organic chemistry as a reducing agent.<ref name="africa" /> It is more powerful than the related reagent sodium borohydride owing to the weaker Al-H bond compared to the B-H bond.<ref>Template:Cite journal</ref> Often as a solution in diethyl ether and followed by an acid workup, it will convert esters, carboxylic acids, acyl chlorides, aldehydes, and ketones into the corresponding alcohols (see: carbonyl reduction). Similarly, it converts amide,<ref>Template:OrgSynth</ref><ref>Template:OrgSynth</ref> nitro, nitrile, imine, oxime,<ref>Template:OrgSynth</ref> and organic azides into the amines (see: amide reduction). It reduces quaternary ammonium cations into the corresponding tertiary amines. Reactivity can be tuned by replacing hydride groups by alkoxy groups. Due to its pyrophoric nature, instability, toxicity, low shelf life and handling problems associated with its reactivity, it has been replaced in the last decade, both at the small-industrial scale and for large-scale reductions by the more convenient related reagent sodium bis (2-methoxyethoxy)aluminium hydride, which exhibits similar reactivity but with higher safety, easier handling and better economics.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

LAH is most commonly used for the reduction of esters<ref>Template:OrgSynth</ref><ref>Template:OrgSynth</ref> and carboxylic acids<ref>Template:OrgSynth</ref> to primary alcohols; prior to the advent of LAH this was a difficult conversion involving sodium metal in boiling ethanol (the Bouveault-Blanc reduction). Aldehydes and ketones<ref>Template:OrgSynth</ref> can also be reduced to alcohols by LAH, but this is usually done using milder reagents such as [[sodium borohydride|Template:Chem2]]; α, β-unsaturated ketones are reduced to allylic alcohols.<ref>Template:OrgSynth</ref> 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.<ref>Template:Cite journal</ref>

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 tri-tert-butoxyaluminum hydride, which reacts significantly faster with the acid chloride than with the aldehyde, must be used. For example, when isovaleric acid is treated with thionyl chloride to give isovaleroyl chloride, it can then be reduced via lithium tri-tert-butoxyaluminum hydride to give isovaleraldehyde in 65% yield.<ref>Template:Cite book</ref><ref>Template:Cite book</ref>

<imagemap> File:LAH rxns.png| rect 5 12 91 74 alcohol rect 82 178 170 240 epoxide rect 121 9 193 69 alcohol2 rect 337 1 414 60 alcohol3 rect 458 55 526 117 alcohol4 rect 170 151 234 210 aldehyde rect 141 259 207 279 nitrile rect 135 281 196 300 amide rect 128 311 204 366 amine1 rect 264 268 339 334 carboxylic acid rect 457 362 529 413 alcohol5 rect 381 255 433 273 azide rect 469 244 525 269 amine2 rect 321 193 401 242 ester rect 261 141 320 203 ketone desc none

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Lithium aluminium hydride also reduces alkyl halides to alkanes.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> 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.<ref>Template:Cite book</ref>

Lithium aluminium hydride does not reduce simple alkenes or arenes. Alkynes are reduced only if an alcohol group is nearby,<ref>Template:OrgSynth</ref> and alkenes are reduced in the presence of catalytic TiCl₄.<ref>Brendel, G. (May 11, 1981) "Hydride reducing agents" (letter to the editor) in Chemical and Engineering News. {{#invoke:doi|main}}</ref> It was observed that the Template:Chem2 reduces the double bond in the N-allylamides.<ref>Template:Cite journal</ref>

Inorganic chemistry

LAH is widely used to prepare main group and transition metal hydrides from the corresponding metal halides.

LAH also reacts with many inorganic ligands to form coordinated alumina complexes associated with lithium ions.<ref name="InorganicHandbook" />

LiAlH₄ + 4NH₃ → Li[Al(NH₂)₄] + 4H₂

Hydrogen storage

File:Volvsgrav.png

Volumetric and gravimetric hydrogen storage densities of different hydrogen storage methods. Metal hydrides are represented with squares and complex hydrides with triangles (including LiAlH₄). Reported values for hydrides are excluding tank weight. DOE FreedomCAR targets are including tank weight.

LiAlH₄ contains 10.6 wt% hydrogen, thereby making LAH a potential hydrogen storage medium for future fuel cell-powered vehicles. The high hydrogen content, as well as the discovery of reversible hydrogen storage in Ti-doped NaAlH₄,<ref>Template:Cite journal</ref> have sparked renewed research into LiAlH₄ during the last decade. A substantial research effort has been devoted to accelerating the decomposition kinetics by catalytic doping and by ball milling.<ref name="varin">Template:Cite book</ref> In order to take advantage of the total hydrogen capacity, the intermediate compound LiH must be dehydrogenated as well. Due to its high thermodynamic stability this requires temperatures in excess of 400 °C, which is not considered feasible for transportation purposes. Accepting LiH + Al as the final product, the hydrogen storage capacity is reduced to 7.96 wt%. Another problem related to hydrogen storage is the recycling back to LiAlH₄ which, owing to its relatively low stability, requires an extremely high hydrogen pressure in excess of 10000 bar.<ref name="varin" /> Cycling only reaction R2 — that is, using Li₃AlH₆ as starting material — would store 5.6 wt% hydrogen in a single step (vs. two steps for NaAlH₄ which stores about the same amount of hydrogen). However, attempts at this process have not been successful so far.Template:Citation needed

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:<ref name=react>Template:Cite journal</ref>

LiAlH₄ + KH → KAlH₄ + LiH

The reverse, i.e., production of LAH from either sodium aluminium hydride or potassium aluminium hydride can be achieved by reaction with LiCl or lithium hydride in diethyl ether or THF:<ref name="react" />

NaAlH₄ + LiCl → LiAlH₄ + NaCl

KAlH₄ + LiCl → LiAlH₄ + KCl

"Magnesium alanate" (Mg(AlH₄)₂) arises similarly using MgBr₂:<ref>Template:Cite book</ref>

2 LiAlH₄ + MgBr₂ → Mg(AlH₄)₂ + 2 LiBr

Red-Al (or SMEAH, NaAlH₂(OC₂H₄OCH₃)₂) is synthesized by reacting sodium aluminum tetrahydride (NaAlH₄) and 2-methoxyethanol:<ref>Template:Cite journal</ref>

See also

• Hydride
• Sodium borohydride
• Sodium hydride

Further reading

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External links

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