Lithium aluminium hydride

Chembox new
Name = Lithium aluminium hydride
ImageFile = lialh4_sem.png ImageName = Lithium aluminium hydride
ImageFile1 = lithium-aluminium-hydride.png ImageName1 = Structure of lithium aluminium hydride
ImageFile2 = Lithium-aluminium-hydride-layer-3D-balls.png IUPACName = Lithium aluminium hydride
OtherNames = LAH, lithium alanate,
lithium tetrahydridoaluminate
Lithal (UK slang)
Section1 = Chembox Identifiers
CASNo = 16853-85-3
RTECS = BD0100000
Section2 = Chembox Properties
Formula = LiAlH₄
MolarMass = 37.95 g/mol
Appearance = white crystals (pure samples)
grey powder (commercial material)
Density = 0.917 g/cm³, solid
Solubility = reactive
MeltingPt = 150 °C (423 K), decomposing
BoilingPt =
Section3 = Chembox Structure
Coordination =
CrystalStruct = monoclinic
Dipole =
Section7 = Chembox Hazards
ExternalMSDS = | [http://msds.ehs.cornell.edu/msds/MSDSDOD/A441/M220131.htm External MSDS]
MainHazards = highly flammable
NFPA-H = 3
NFPA-R = 2
NFPA-F = 2
NFPA-O = W
FlashPt =
RSPhrases = R: 15 S: 7/8, 24/25, 43
Section8 = Chembox Related
Function = hydride
OtherFunctn = aluminium hydride
sodium borohydride
sodium 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. Often supported by water, It will convert esters, carboxylic acids, and ketones into the corresponding alcohols; and amide, nitro, nitrile, imine, oxime, and azide compounds into the amines.

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 packed 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 H₂This 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.

Preparation

LAH was first prepared from the reaction between lithium hydride (LiH) and aluminium chloride: [A. E. Finholt, A. C. Bond, and H. I. Schlesinger "Lithium Aluminum Hydride, Aluminum Hydride and Lithium Gallium Hydride, and Some of their Applications in Organic and Inorganic Chemistry" Journal of the American Chemical Society 1947, volume 69, pp 1199 - 1203; DOI: 10.1021/ja01197a061] :4 LiH + AlCl₃ → LiAlH₄ + 3 LiClIn addition to this method, the industrial synthesis entails the initial preparation of sodium aluminium hydride from the elements under high pressure and temperature:cite book | author=Holleman, A. F., Wiberg, E., Wiberg, N. | title=Lehrbuch der Anorganischen Chemie, 102nd ed. | publisher=de Gruyter | year=2007 | id=ISBN 978-3-11-017770-1] :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.

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 + LiHPotassium 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. [Brown, H. C. "Org. React." 1951, "6", 469. (Review)] 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 [Reetz, M. T.; Drewes, M. W.; Schwickardi, R. "Organic Syntheses", Coll. Vol. 10, p.256 (2004); Vol. 76, p.110 (1999). ( [http://www.orgsyn.org/orgsyn/prep.asp?prep=v76p0110 Article] )] [Oi, R.; Sharpless, K. B. "Organic Syntheses", Coll. Vol. 9, p.251 (1998); Vol. 73, p.1 (1996). ( [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv9p0251 Article] )] and carboxylic acids [Koppenhoefer, B.; Schurig, V. "Organic Syntheses", Coll. Vol. 8, p.434 (1993); Vol. 66, p.160 (1988). ( [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv8p0434 Article] )] 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 [Barnier, J. P.; Champion, J.; Conia, J. M. "Organic Syntheses", Coll. Vol. 7, p.129 (1990); Vol. 60, p.25 (1981). ( [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv7p0129 Article] )] 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. [Elphimoff-Felkin, I.; Sarda, P. "Organic Syntheses", Coll. Vol. 6, p.769 (1988); Vol. 56, p.101 (1977). ( [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv6p0769 Article] )] 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. [Rickborn, B.; Quartucci, J. "J. Org. Chem." 1984, "29", 3185.]

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rect 5 12 91 74 alcoholrect 82 178 170 240 epoxiderect 121 9 193 69 alcohol2rect 337 1 414 60 alcohol3rect 458 55 526 117 alcohol4rect 170 151 234 210 aldehyderect 141 259 207 279 nitrilerect 135 281 196 300 amiderect 128 311 204 366 amine1rect 264 268 339 334 carboxylic acidrect 457 362 529 413 alcohol5rect 381 255 433 273 aziderect 469 244 525 269 amine2rect 321 193 401 242 esterrect 261 141 320 203 ketone

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Using LAH, amines can be prepared by the reduction of amides,, [Seebach, D.; Kalinowski, H.-O.; Langer, W.; Crass, G.; Wilka, E.-M. "Organic Syntheses", Coll. Vol. 7, p.41 (1990); Vol. 61, p.24 (1983). ( [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv7p0041 Article] )] [Park, C. H.; Simmons, H. E. "Organic Syntheses", Coll. Vol. 6, p.382 (1988); Vol. 54, p.88 (1974). ( [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv6p0382 Article] )] oximes, [Chen, Y. K.; Jeon, S.-J.; Walsh, P. J.; Nugent, W. A. "Organic Syntheses", Vol. 82, p.87 (2005). ( [http://www.orgsyn.org/orgsyn/prep.asp?prep=v82p0087 Article] )] nitriles, nitro compounds or alkyl azides.

Lithium aluminium hydride also reduces alkyl halides to alkanes, although this reaction is rarely employed. [Johnson, J. E.; Blizzard, R. H.; Carhart, H. W. "J. Am. Chem. Soc." 1948, "70", 3664.] [Krishnamurthy, S.; Brown, H. C. "J. Org. Chem." 1982, "47", 276.] 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. [Wender, P. A.; Holt, D. A.; Sieburth, S. Mc N. "Organic Syntheses", Coll. Vol. 7, p.456 (1990); Vol. 64, p.10 (1986). ( [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv7p0456 Article] )]

Inorganic chemistry

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

Thermal decomposition

At 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.
: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. At about 200^oC Li₃AlH₆ decomposes into LiH (R2) and Al which subsequently decompose into LiAl above 400^oC (R3). R1 is effectively irreversible. The reversibility of R2 has not been proven. 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.

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.

olubility 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 violently as described by 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 may occur to give β-LAH.Løvvik, O.M.; Opalka, S.M.; Brinks, H.W.; Hauback, B.C. " [http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PRBMDO000069000013134117000001&idtype=cvips&prog=normal Crystal structure and thermodynamic stability of the lithium alanates LiAlH4 and Li3AlH6] ." "Physical Review B." Vol. 69, 2004, 134117.]

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.

ee also

*Sodium borohydride
*Sodium hydride
*Hydride

References

Further reading

*cite book | author=Wiberg, Egon & Amberger, Eberhard | title=Hydrides of the elements of main groups I-IV | publisher=Elsevier | year=1971 | id=ISBN 0-444-40807-X
*cite book | author=Hajos, Andor | title=Complex Hydrides and Related Reducing Agents in Organic Synthesis | publisher=Elsevier | year=1979 | id=ISBN 0-444-99791-1
*cite book | author=Lide (ed.), David R. | title=Handbook of chemistry and physics | publisher=CRC Press | year=1997 | id=ISBN 0-8493-0478-4
*cite book | author=Carey, Francis A. | title=Organic Chemistry with Online Learning Center and Learning by Model CD-ROM | publisher= McGraw-Hill | year=2002 | id=ISBN 0-07-252170-8 [http://www.chem.ucalgary.ca/courses/351/Carey5th/Carey.html on-line version]
*Chapter 5 in cite book | author=Andreasen, Anders | title=Hydrogen Storage Materials with Focus on Main Group I-II Elements | publisher=Risoe National Laboratory | year=2005 | id=ISBN 87-550-3498-5 [http://www.risoe.dk/rispubl/AFM/ris-phd-21.htm Full text version]

External links

*Usage of LiAlH₄ in [http://www.orgsyn.org/orgsyn/chemname.asp?nameID=36257 Organic Syntheses]
* [http://webbook.nist.gov/cgi/cbook.cgi?Formula=LiAlH4&NoIon=on&Units=SI Condensed phase thermochemistry data from Nist webbook]
* [http://msds.ehs.cornell.edu/msds/MSDSDOD/A441/M220131.htm Materials Safety Data Sheet from Cornell University]
* [http://hydpark.ca.sandia.gov/ Sandia National Laboratory - Hydride information center]
* [http://designer-drugs.com/pte/12.162.180.114/dcd/chemistry/lah.synthesis.html Synthesis of LAH]
* [http://www.chem2.bham.ac.uk/labs/cox/Teaching/4th_Year/II/Reduction_Reactions.pdf Reduction reactions, University of Birmingham, Teaching Resources - 4th Year]
* [http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=28112 PubChem LiAlH₄ summary]