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Lithium aluminium hydride
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=== 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>{{cite journal | author = Brown, H. C. | title = Reductions by Lithium Aluminum Hydride | journal = Organic Reactions | year = 1951 | volume = 6 | page = 469 | doi = 10.1002/0471264180.or006.10 | isbn = 0-471-26418-0 }}</ref> Often as a solution in [[diethyl ether]] and followed by an acid workup, it will convert [[ester]]s, [[carboxylic acid]]s, [[acyl chloride]]s, [[aldehydes]], and [[ketone]]s into the corresponding [[Alcohol (chemistry)|alcohols]] (see: [[carbonyl reduction]]). Similarly, it converts [[amide]],<ref>{{OrgSynth |author1=Seebach, D.|author2=Kalinowski, H.-O.|author3=Langer, W.|author4=Crass, G.|author5=Wilka, E.-M. | title = Chiral Media for Asymmetric Solvent Inductions. (S,S)-(+)-1,4-bis(Dimethylamino)-2,3-Dimethoxybutane from (R,R)-(+)-Diethyl Tartrate | collvol = 7 | collvolpages = 41 | year = 1991 | prep = cv7p0041 }}</ref><ref>{{OrgSynth |author1=Park, C. H.|author2=Simmons, H. E. | title = Macrocyclic Diimines: 1,10-Diazacyclooctadecane | collvol = 6 | collvolpages = 382 | volume = 54 | pages = 88 | year = 1974 | prep = cv6p0382 }}</ref> [[Nitro compound|nitro]], [[nitrile]], [[imine]], [[oxime]],<ref>{{OrgSynth |author1=Chen, Y. K.|author2=Jeon, S.-J.|author3=Walsh, P. J.|author4=Nugent, W. A. | title = (2S)-(β)-3-exo-(Morpholino)Isoborneol | volume = 82 | pages = 87 | year = 2005 | prep = v82p0087 }}</ref> and [[organic azide]]s into the [[amine]]s (see: [[amide reduction]]). It reduces [[quaternary ammonium cation]]s into the corresponding tertiary amines. Reactivity can be tuned by replacing hydride groups [[reductions with metal alkoxyaluminum hydrides|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 [[Red-Al|sodium bis (2-methoxyethoxy)aluminium hydride]], which exhibits similar reactivity but with higher safety, easier handling and better economics.<ref>{{Cite web | url = https://www.organic-chemistry.org/chemicals/reductions/sodiumbis(2-methoxyethoxy)aluminumhydride-red-al.shtm | title = Red-Al, Sodium bis(2-methoxyethoxy)aluminumhydride | publisher = Organic Chemistry Portal }}</ref> LAH is most commonly used for the reduction of [[ester]]s<ref>{{OrgSynth |author1=Reetz, M. T.|author2=Drewes, M. W.|author3=Schwickardi, R. | title = Preparation of Enantiomerically Pure Ξ±-N,N-Dibenzylamino Aldehydes: S-2-(N,N-Dibenzylamino)-3-Phenylpropanal | collvol = 10 | collvolpages = 256 | volume = 76 | pages = 110 | year = 1999 | prep = v76p0110 }}</ref><ref>{{OrgSynth |author1=Oi, R.|author2=Sharpless, K. B. | title = 3-<nowiki>[</nowiki>(1S)-1,2-Dihydroxyethyl<nowiki>]</nowiki>-1,5-Dihydro-3H-2,4-Benzodioxepine | collvol = 9 | collvolpages = 251 | volume = 73 | pages = 1 | year = 1996 | prep = cv9p0251 }}</ref> and [[carboxylic acid]]s<ref>{{OrgSynth |author1=Koppenhoefer, B.|author2=Schurig, V. | title = (R)-Alkyloxiranes of High Enantiomeric Purity from (S)-2-Chloroalkanoic Acids via (S)-2-Chloro-1-Alkanols: (R)-Methyloxirane | collvol = 8 | collvolpages = 434 | volume = 66 | pages = 160 | year = 1988 | prep = cv8p0434 }}</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]]). [[Aldehyde]]s and [[ketone]]s<ref>{{OrgSynth |author1=Barnier, J. P.|author2=Champion, J.|author3=Conia, J. M. | title = Cyclopropanecarboxaldehyde | collvol = 7 | collvolpages = 129 | volume = 60 | pages = 25 | year = 1981 | prep = cv7p0129 }}</ref> can also be reduced to alcohols by LAH, but this is usually done using milder reagents such as [[sodium borohydride|{{chem2|Na[BH4]}}]]; Ξ±, Ξ²-unsaturated ketones are reduced to allylic alcohols.<ref>{{OrgSynth |author1=Elphimoff-Felkin, I.|author2=Sarda, P. | title = Reductive Cleavage of Allylic Alcohols, Ethers, or Acetates to Olefins: 3-Methylcyclohexene | collvol = 6 | collvolpages = 769 | volume = 56 | pages = 101 | year = 1977 | prep = cv6p0769 }}</ref> When [[epoxide]]s are reduced using LAH, the reagent attacks the less [[steric effects|hindered]] end of the epoxide, usually producing a secondary or tertiary alcohol. [[Epoxycyclohexane]]s are reduced to give axial alcohols preferentially.<ref>{{cite journal | last1 = Rickborn | first1 = B. | last2 = Quartucci | first2 = J. | title = Stereochemistry and Mechanism of Lithium Aluminum Hydride and Mixed Hydride Reduction of 4-''t''-Butylcyclohexene Oxide | journal = The Journal of Organic Chemistry | year = 1964 | volume = 29 | issue = 11 | pages = 3185β3188 | doi = 10.1021/jo01034a015 }}</ref> Partial reduction of [[acid chloride]]s 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|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>{{cite book | author = Wade, L. G. Jr. | title = Organic Chemistry | edition = 6th | publisher = Pearson Prentice Hall | year = 2006 | isbn = 0-13-147871-0 }}</ref><ref>{{cite book |last1=Wade |first1=L. G. |title=Organic chemistry |date=2013 |publisher=Pearson |location=Boston |isbn=978-0-321-81139-4 |pages=835 |edition=8th}}</ref> <imagemap> File:LAH rxns.png| rect 5 12 91 74 [[Alcohol (chemistry)|alcohol]] rect 82 178 170 240 [[epoxide]] rect 121 9 193 69 [[Alcohol (chemistry)|alcohol2]] rect 337 1 414 60 [[Alcohol (chemistry)|alcohol3]] rect 458 55 526 117 [[Alcohol (chemistry)|alcohol4]] rect 170 151 234 210 [[aldehyde]] rect 141 259 207 279 [[nitrile]] rect 135 281 196 300 [[amide]] rect 128 311 204 366 [[amine]]1 rect 264 268 339 334 [[carboxylic acid]] rect 457 362 529 413 [[Alcohol (chemistry)|alcohol5]] rect 381 255 433 273 [[Azide#Organic azides|azide]] rect 469 244 525 269 [[amine]]2 rect 321 193 401 242 [[ester]] rect 261 141 320 203 [[ketone]] desc none #Notes: #Details on the new coding for clickable images is here: [[mw:Extension:ImageMap]] #[https://web.archive.org/web/20080327003154/http://tools.wikimedia.de/~dapete/ImageMapEdit/ImageMapEdit.html?en This image editor] was used. </imagemap> Lithium aluminium hydride also reduces [[alkyl halide]]s to [[alkane]]s.<ref>{{cite journal | last1 = Johnson | first1 = J. E. | last2 = Blizzard | first2 = R. H. | last3 = Carhart | first3 = H. W. | title = Hydrogenolysis of Alkyl Halides by Lithium Aluminum Hydride | journal = Journal of the American Chemical Society | year = 1948 | volume = 70 | issue = 11 | pages = 3664β3665 | pmid = 18121883 | doi = 10.1021/ja01191a035 }}</ref><ref>{{cite journal | last1 = Krishnamurthy | first1 = S. | last2 = Brown | first2 = H. C. | title = Selective Reductions. 28. The Fast Reaction of Lithium Aluminum Hydride with Alkyl Halides in THF. A Reappraisal of the Scope of the Reaction | journal = The Journal of Organic Chemistry | year = 1982 | volume = 47 | issue = 2 | pages = 276β280 | doi = 10.1021/jo00341a018 }}</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>{{cite book | author = Carruthers, W. | title = Some Modern Methods of Organic Synthesis | publisher = Cambridge University Press | year = 2004 | page = 470 | isbn = 0-521-31117-9 | url = https://books.google.com/books?id=ti7yMYYW7CMC&pg=PA470 }}</ref> Lithium aluminium hydride does not reduce simple [[alkene]]s or [[arene]]s. [[Alkyne]]s are reduced only if an alcohol group is nearby,<ref>{{OrgSynth |author1=Wender, P. A.|author2=Holt, D. A.|author3=Sieburth, S. Mc N.|author3-link=Scott Sieburth | title = 2-Alkenyl Carbinols from 2-Halo Ketones: 2-E-Propenylcyclohexanol | collvol = 7 | collvolpages = 456 | volume = 64 | pages = 10 | year = 1986 | prep = cv7p0456 }}</ref> and alkenes are reduced in the presence of catalytic [[titanium tetrachloride|TiCl<sub>4</sub>]].<ref>Brendel, G. (May 11, 1981) "Hydride reducing agents" (letter to the editor) in ''Chemical and Engineering News''. {{doi|10.1021/cen-v059n019.p002|doi-access=free}}</ref> It was observed that the {{chem2|LiAlH4}} reduces the double bond in the ''N''-allylamides.<ref>{{Cite journal|title=Reduction of N-allylamides by LiAlH<sub>4</sub>: Unexpected Attack of the Double Bond With Mechanistic Studies of Product and Byproduct Formation|year = 2014|pmid = 25347383|last1 = Thiedemann|first1 = B.|last2 = Schmitz|first2 = C. M.|last3 = Staubitz|first3 = A.|journal = The Journal of Organic Chemistry|volume = 79|issue = 21|pages = 10284β95|doi = 10.1021/jo501907v}}</ref>
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