Lithium borohydride: Difference between revisions

| ImageFile=Tetrahydridoboritan lithný.png

| ImageFile1=[[File:Li+.svg]][[File:Borhydrid-.svg|150px]]

| ImageFile2=File:Lithiumborhydrid.png

| ImageSize2 = 250px

| ImageCaption2 = Unit cell of lithium borohydride at room temperature

| ImageAlt2 = Unit cell of lithium borohydride at room temperature

| Watchedfields = changed

| verifiedrevid = 421092672

| UNII_Ref = {{fdacite|correct|FDA}}

| UNII = 8L87X4S4KP

| Density = 0.666 g/cm³<ref name="Sigma-Aldrich">[http://www.sigmaaldrich.com/catalog/search/ProductDetail/ALDRICH/222356 Sigma-Aldrich Product Detail Page].</ref>

| MeltingPtC = 268

| BoilingPtC = 380

| BoilingPt_notes = decomposes

| Solvent = [[diethyl ether|ether]]

| Section3 = {{Chembox Structure

| Structure_ref =<ref>{{cite journal |author=J-Ph. Soulie, G. Renaudin, R. Cerny, K. Yvon |title=Lithium boro-hydride LiBH₄: I. Crystal structure |doi=10.1016/S0925-8388(02)00521-2 |journal=[[Journal of Alloys and Compounds]] |volume=346 |issue=1–2 |date=2002-11-18 |pages=200–205}}</ref>

| CrystalStruct = orthorhombic

| SpaceGroup =

| PointGroup = Pnma

| LattConst_a = 7.17858(4)

| LattConst_b = 4.43686(2)

| LattConst_c = 6.80321(4)

| LattConst_alpha =

| LattConst_beta =

| LattConst_gamma =

| LattConst_ref =

| LattConst_Comment =

| UnitCellVolume = 216.685(3) A³

| UnitCellFormulas = 4

| Coordination = [4]B

| MolShape =

| OrbitalHybridisation =

| Dipole =

}}

| Section4 = {{Chembox Thermochemistry

| DeltaHf = −198.83 kJ/mol

| Entropy = 75.7 J/(mol⋅K)

| HeatCapacity = 82.6 J/(mol⋅K)

| Section5 = {{Chembox Hazards

| AutoignitionPt = >

| AutoignitionPtC = 180

'''Lithium borohydride''' (LiBH₄) is a [[borohydride]] and known in [[organic synthesis]] as a [[reducing agent]] for [[ester]]s. Although less common than the related [[sodium borohydride]], the lithium salt offers some advantages, being a stronger reducing agent and highly soluble in ethers, whilst remaining safer to handle than [[lithium aluminium hydride]].<ref name=eros>Luca Banfi, Enrica Narisano, Renata Riva, Ellen W. Baxter, "Lithium Borohydride" e-EROS Encyclopedia of Reagents for Organic Synthesis, 2001, John Wiley & Sons. {{doi|10.1002/047084289X.rl061.pub2}}.</ref>

Lithium borohydride may be prepared by the [[Salt metathesis reaction|metathesis reaction]], which occurs upon ball-milling the more commonly available [[sodium borohydride]] and [[lithium bromide]]:<ref>Peter Rittmeyer, Ulrich Wietelmann, "Hydrides" in Ullmann's Encyclopedia of Industrial Chemistry, 2002, Wiley-VCH, Weinheim. {{doi|10.1002/14356007.a13_199}}.</ref>

: NaBH₄ + LiBr → NaBr + LiBH₄

Alternatively, it may be synthesized by treating [[boron trifluoride]] with [[lithium hydride]] in [[diethyl ether]]:<ref name="Georg">{{cite book |last=Brauer |first=Georg |title=Handbook of Preparative Inorganic Chemistry |volume=1 |edition=2nd |date=1963 |publisher=Academic Press |location=New York |isbn=978-0-12-126601-1 |page=775 |url=https://books.google.com/books?id=TLYatwAACAAJ&q=Handbook+of+Preparative+Inorganic+Chemistry}}</ref>

: BF₃ + 4 LiH → LiBH₄ + 3 LiF

Lithium borohydride is useful as a source of [[hydride]] (H^–). It can react with a range of [[carbonyl]] substrates and other polarized carbon structures to form a hydrogen–carbon bond. It can also react with [[Brønsted–Lowry]]-acidic substances (sources of H⁺) to form [[hydrogen]] gas.

===Reduction reactions===

As a [[hydride]] reducing agent, lithium borohydride is stronger than sodium borohydride<ref>{{cite book |editor1-last=Trost |editor1-first=Barry |editor2-last=Fleming |editor2-first=Ian |editor3-last=Schreiber |editor3-first=Stuart |last1=Barrett |first1=Anthony G.&nbsp;M. |chapter=Reduction of Carboxylic Acid Derivatives to Alcohols, Ethers and Amines |title=Reduction: Selectivity, Strategy & Efficiency in Modern Organic Chemistry |date=1991 |publisher=Pergamon Press |location=New York |isbn=978-0-08-040599-5 |page=244 |edition=1st |doi=10.1016/B978-0-08-052349-1.00226-2}}</ref><ref name=chemoselective>{{cite journal |last1=Ookawa |first1=Atsuhiro |last2=Soai |first2=Kenso |title=Mixed solvents containing methanol as useful reaction media for unique chemoselective reductions within lithium borohydride |journal=The Journal of Organic Chemistry |date=1986 |volume=51 |issue=21 |pages=4000–4005 |doi=10.1021/jo00371a017}}</ref> but weaker than lithium aluminium hydride.<ref name=chemoselective /> Unlike the sodium analog, it can reduce esters to alcohols, [[nitrile]]s and [[Primary (chemistry)|primary]] [[amide]]s to [[amine]]s, and can open [[epoxide]]s. The enhanced reactivity in many of these cases is attributed to the polarization of the carbonyl substrate by complexation to the lithium cation.<ref name=eros/> Unlike the aluminium analog, it does not react with [[nitro group]]s, [[carbamic acid]]s, [[alkyl halides]], or [[Secondary (chemistry)|secondary]] and [[Tertiary (chemistry)|tertiary]] amides.

===Hydrogen generation===

Lithium borohydride reacts with water to produce hydrogen. This reaction can be used for hydrogen generation.<ref>{{cite journal |first1= Yoshitsugu |last1= Kojima |first2= Yasuaki |last2= Kawai |first3= Masahiko |last3= Kimbara |first4= Haruyuki |last4= Nakanishi |first5= Shinichi |last5= Matsumoto |title= Hydrogen Generation by Hydrolysis Reaction of Lithium Borohydride |journal= [[International Journal of Hydrogen Energy]] |volume= 29 |issue= 12 |pages= 1213–1217 |date= August 2004 |doi= 10.1016/j.ijhydene.2003.12.009}}</ref>

Although this reaction is usually spontaneous and violent, somewhat-stable [[aqueous solution]]s of lithium borohydride can be prepared at low temperature if [[Degassing|degassed]], [[distilled water]] is used and exposure to [[oxygen]] is carefully avoided.<ref>{{cite journal |title= Preparation of Quaternary Ammonium Borohydrides from Sodium and Lithium Borohydrides |first1= M. Douglas |last1= Banus |first2= Robert W. |last2= Bragdon |first3= Thomas R. P. Jr |last3= Gibb |journal= J. Am. Chem. Soc. |year= 1952 |volume= 74 |issue= 9 |pages= 2346–2348 |doi= 10.1021/ja01129a048 }}</ref>

[[File:Volvsgrav.png|thumb|left|Volumetric vs gravimetric [[energy density]]]]

[[Image:4002726 schematics.png|thumb|left|Schematic of lithium borohydride recycling. Inputs are lithium borate and hydrogen.]]

Lithium borohydride is renowned as one of the highest-[[energy density|energy-density]] chemical [[energy carrier]]s. Although presently of no practical importance, the solid liberates 65&nbsp;[[megajoule|MJ]]/[[kg]] heat upon treatment with atmospheric oxygen. Since it has a density of 0.67&nbsp;[[g/cm3|g/cm³]], oxidation of liquid lithium borohydride gives 43&nbsp;[[MJ/L]]. In comparison, gasoline gives 44&nbsp;MJ/kg (or 35&nbsp;MJ/L), while liquid hydrogen gives 120&nbsp;MJ/kg (or 8.0&nbsp;MJ/L).<ref name="LH2" group="nb">The greater ratio of energy density to specific energy for hydrogen is because of the very low mass density (0.071&nbsp;g/cm³).</ref> The high specific energy density of lithium borohydride has made it an attractive candidate to propose for automobile and rocket fuel, but despite the research and advocacy, it has not been used widely. As with all chemical-hydride-based energy carriers, lithium borohydride is very complex to recycle (i.e. recharge) and therefore suffers from a low [[energy conversion efficiency]]. While batteries such as [[Lithium-ion battery|lithium-ion]] carry an energy density of up to 0.72&nbsp;MJ/kg and 2.0&nbsp;MJ/L, their [[Direct current|DC]]-to-DC conversion efficiency can be as high as 90%.<ref>Valøen, Lars Ole and Shoesmith, Mark I. (2007). The effect of PHEV and HEV duty cycles on battery and battery pack performance (PDF). 2007 Plug-in Highway Electric Vehicle Conference: Proceedings. Retrieved 11 June 2010.</ref> In view of the complexity of recycling mechanisms for metal hydrides,<ref>{{US Patent|4002726}} (1977) lithium borohydride recycling from lithium borate via a methyl borate intermediate.</ref> such high energy-conversion efficiencies are not practical with present technology.

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|+ Comparison of physical properties

! Substance !! [[Specific energy]],<br/> [[megajoule|MJ]]/[[kg]] !! [[Density]],,<br/> [[g/cm3|g/cm³]] !! [[Energy density]],<br/> [[MJ/L]]

! LiBH₄

| {{0}}65.2{{0}} <!-- from text -->

| 0.666{{0}} <!-- from text, matches back-calculation -->

! Regular [[gasoline]]

| {{0}}44{{0|.00}} <!-- from [[Gasoline#Energy_content]] -->

| 0.72{{0|00}} <!-- value from [[Gasoline#Density]]. Back-calculated from text gives 0.659. Conversely, back-calculated from these values give 0.79 -->

! [[Liquid Hydrogen|Liquid hydrogen]]

| 120{{0|.00}} <!-- from text -->

! [[Lithium-ion battery]]

| {{0|00}}0.72 <!-- from text -->

| 2.8{{0|000}} <!-- back-calculated from text-->

[[Category:Lithium salts]]