- 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 inorganic synthesis . It is more powerful than the relatedreagent sodium borohydride due to the weaker Al-H bond compared to the B-H bond. Often supported by water, It will convertester s,carboxylic acid s, andketone s into the correspondingalcohol s; andamide ,nitro ,nitrile ,imine ,oxime , andazide compounds into theamine s.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 fromdiethyl ether . Large-scale purifications employ aSoxhlet 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 ispyrophoric . Some commercial materials containmineral 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 compoundslithium hydroxide andaluminium hydroxide .Preparation
LAH was first prepared from the reaction between
lithium hydride (LiH) andaluminium 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 → NaAlH4LAH is then prepared by
metathesis reaction according to::NaAlH4 + LiCl → LiAlH4 + NaClwhich proceeds in a high yield of LAH. LiCl is removed by
filtration from anether eal 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 inTHF ::LiAlH4 + NaH → NaAlH4 + LiHPotassium aluminium hydride (KAlH4) can be produced similarly indiglyme as a solvent::LiAlH4 + KH → KAlH4 + LiHThe 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 orTHF ::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 asRed-Al , is more often used. For such purposes it is usually used in solution indiethyl ether , and anaqueous workup is usually performed after the reduction in order to remove inorganic by-products.LAH is most commonly used for the reduction of
ester s [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] )] andcarboxylic acid s [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 involvingsodium metal in boilingethanol (theBouveault-Blanc reduction ).Aldehyde s andketone s [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] )] Whenepoxide s 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.]rect 5 12 91 74
alcohol rect 82 178 170 240epoxide rect 121 9 193 69 alcohol2rect 337 1 414 60 alcohol3rect 458 55 526 117 alcohol4rect 170 151 234 210aldehyde rect 141 259 207 279nitrile rect 135 281 196 300amide rect 128 311 204 366 amine1rect 264 268 339 334carboxylic acid rect 457 362 529 413 alcohol5rect 381 255 433 273 aziderect 469 244 525 269 amine2rect 321 193 401 242ester rect 261 141 320 203ketone desc none
#Notes:
#Details on the new coding for clickable images is here:
# [http://tools.wikimedia.de/~dapete/ImageMapEdit/ImageMapEdit.html?en This image editor] was used.Using LAH,
amine s can be prepared by the reduction ofamide s,, [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] )]oxime s, [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] )]nitrile s, nitro compounds or alkylazide s.Lithium aluminium hydride also reduces
alkyl halide s toalkane s, 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
alkene s,arene s, andalkyne s are only reduced if analcohol 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
halide s.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 150-170oC immediately followed by decomposition into solid Li3AlH6. At about 200oC Li3AlH6 decomposes into LiH (R2) and Al which subsequently decompose into LiAl above 400oC (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 500oC. 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 futurefuel cell poweredvehicle s. 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
ether al solutions. However, it may spontaneously decompose due to the presence of catalytic impurities, though, it appears to be more stable inTHF . 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 themonoclinic crystal system and thespace group is P21c. The crystal structure of LAH is illustrated to the right. The structure consists ofLi atom s surrounded by five AlH4 tetrahedra. The Li+ centers are bonded to onehydrogen atom from each of the surrounding tetrahedra creating abipyramid arrangement. Theunit 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 andGibbs 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]
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