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Lithium aluminium hydride

2007 Schools Wikipedia Selection. Related subjects: Chemical compounds

                                                 Lithium aluminium hydride
          Lithium aluminium hydride Structure of lithium aluminium hydride
                                                                   General
                                 Systematic name Lithium aluminium hydride
                                         Other names LAH, lithium alanate,
                                             lithium tetrahydridoaluminate
                                                         Lithal (UK slang)
                                                Molecular formula LiAlH[4]
                                                    Molar mass 37.95 g/mol
                                  Appearance white crystals (pure samples)
                                         grey powder (commercial material)
                                                   CAS number [16853-85-3]
                                                                Properties
                                     Density and phase 0.917 g/cm^3, solid
                                              Solubility in water reactive
                                 in diethyl ether, THF 5.92 and 2.96 mol/L
                                Melting point 150 °C (423 K), decomposing
                                                         Boiling point n/a
                                                                 Structure
                                                  Coordination geometry  ?
                                              Crystal structure monoclinic
                                                        Dipole moment  ? D
                                                                   Hazards
                                                        MSDS External MSDS
                                             Main hazards highly flammable
                                                                  NFPA 704

   2
   3
   2
   [DEL: W :DEL]

                                                           Flash point n/a
                                     R/S statement R: 15 S: 7/8, 24/25, 43
                                                    RTECS number BD0100000
                                                   Supplementary data page
                                     Structure & properties n, ε[r], etc.
                                        Thermodynamic data Phase behaviour
                                                        Solid, liquid, gas
                                             Spectral data UV, IR, NMR, MS
                                                         Related compounds
                                        Related hydride sodium borohydride
                                                            sodium hydride
                          Except where noted otherwise, data are given for
                     materials in their standard state (at 25°C, 100 kPa)
                                         Infobox disclaimer and references

   Lithium aluminium hydride (LiAl H[4]), commonly abbreviated to LAH, is
   a powerful reducing agent used in organic chemistry. It is more
   powerful than the related reducing agent sodium borohydride due to the
   weaker Al-H bond compared to the B-H bond. It will convert esters,
   carboxylic acids and ketones to alcohols; and nitro compounds into
   amines.

   LAH violently reacts with water, including atmospheric moisture, and
   the pure material is pyrophoric. Commercial material is inhibited with
   mineral oil to allow handling in air.

   Pure, recrystallized (from diethyl ether) LAH is a white solid.
   Commercial samples are almost always grey due to trace contamination
   with aluminium metal. White air-exposed commercial samples of LAH have
   absorbed enough moisture to become a mixture of lithium hydroxide and
   aluminium hydroxide.

Preparation

   LAH is usually produced by the reaction between lithium hydride (LiH)
   and aluminium chloride (AlCl[3])

          4LiH + AlCl[3] \rightarrow LiAlH[4] + 3LiCl

   which proceeds with a high yield of LAH (97 % w/w). LiCl is removed by
   filtration from an ethereal solution of LAH, with subsequent
   precipitation of LAH to yield a product with around 1 % w/w LiCl.

Inorganic reactions

   LAH and NaH can be used to produce sodium aluminium hydride (NaAlH[4])
   by metathesis in THF with a yield of 90.7 % w/w

          LiAlH[4] + NaH \rightarrow NaAlH[4] + LiH

   Potassium aluminium hydride (KAlH[4]) can be produced with a yield of
   90 % w/w by reaction in diglyme in a similar way

          LiAlH[4] + KH \rightarrow KAlH[4] + 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 and THF with a yield of 93.5 and 91 % w/w,
   respectively.

          NaAlH[4] + LiCl \rightarrow LiAlH[4] + NaCl
          KAlH[4] + LiCl \rightarrow LiAlH[4] + KCl

   Magnesium alanate (Mg(AlH[4])[2]) can be synthezised from LAH and
   MgBr[2]

          2LiAlH[4] + MgBr[2] \rightarrow Mg(AlH[4])[2] + 2LiBr

Use in organic chemistry

   Lithium aluminium hydride is widely used in organic chemistry as a very
   powerful reducing agent. 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 or Red-Al is more usual. 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. It is most commonly used for the
   reduction of esters^ ^and carboxylic acids^ to primary alcohols; prior
   to the advent of LiAlH[4] this was a difficult conversion involving
   sodium metal in boiling ethanol (the Bouveault-Blanc reduction).
   Aldehydes and ketones^ can also be reduced to alcohols by LAH, but this
   is usually done using milder reagents such as NaBH[4]. α,β-Unsaturated
   ketones are reduced to allylic alcohols.^ When epoxides are reduced
   using LAH, the reagent attacks the less hindered end of the epoxide,
   usually producing a secondary or tertiary alcohol. It reduces by
   progressive breakup of the complex AlH[4]^− and transfer of hydride
   ions to the positive centre in an organic compound which may have a low
   density of electrons due to inductive or mesomeric effects.

   Organic reactions of lithium aluminium hydride

   Amines can be prepared by the LAH reduction of amides^ ^, oximes^ ,
   nitriles, nitro compounds or alkyl azides. LAH is also able to reduce
   primary alkyl halides to alkanes.

   Lithium aluminium hydride is not able to reduce simple alkenes or
   benzene rings, and alkynes are only reduced if an alcohol group is
   nearby.^

Thermal decomposition

   At room temperature LAH is metastable. During prolonged storage it may
   slowly decompose to Li[3]AlH[6] and LiH. This process can be
   accelerated by the presence of catalytic elements e.g. Ti, Fe, V.

   When heated LAH decompose in a three step reaction mechanism.

          LiAlH[4] \rightarrow 1/3Li[3]AlH[6] + 2/3Al + H[2] (R1)
          1/3Li[3]AlH[6] \rightarrow LiH + 1/3Al + 1/2H[2] (R2)
          LiH + Al \rightarrow LiAl + 1/2H[2] (R3)

   R1 is usually initiated by the melting of LAH around a temperature of
   150-170^oC immediately followed by decomposition into solid
   Li[3]AlH[6]. From 200-250^oC Li[3]AlH[6] decompose into LiH (R2) which
   subsequently decompose into LiAl above 400^oC (R3). R1 is effectively
   irreversible, because LiAlH[4] is metastable. The reversibility of R2
   has not yet been conclusively established. 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.

   According to reactions R1-R3 LiAlH[4] 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 NaAlH[4]).

Solubility 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.

   CAPTION: Solubility data for LiAlH[4] (mol/l)

                                     Temperature (^oC)
                      Solvent     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  --   --   --

   Note that water should not be used as a solvent for lithium aluminium
   hydride, which would react as in the following equation.

          LiAlH[4] + 4H[2]O \rightarrow Li^+ + Al^3+ + 4OH^- + 4H[2]

Crystal structure

   The crystal structure of LAH. Li atoms are blue and AlH4 tetrahedra are
   red. The unit cell border is marked by a black line.
   Enlarge
   The crystal structure of LAH. Li atoms are blue and AlH[4] tetrahedra
   are red. The unit cell border is marked by a black line.

   The crystal structure of LAH belongs to the monoclinic crystal system
   and the space group is P2[1]c. The crystal structure of LAH is
   illustrated to the right. The structure consists of Li atoms surrounded
   by five AlH[4] tetrahedra. The Li atoms are bonded to one hydrogen atom
   from each of the surrounding tetrahedra creating a bipyramid
   arrangement. The side lengths of the unit cell are approx. a=4.82,
   b=7.81 and c=7.92, and the β angle is approx. 112 °. At high pressures
   (>2.2GPa) a phase transition into β-LAH occurs.

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.

   CAPTION: Thermodynamic data for reactions involving LiAlH[4]

      Reaction ΔH^o (kJ/mol) ΔS^o (J/(mol K)) ΔG^o (kJ/mol) Comment
    Li(s) + Al(s) + 2H[2](g) \rightarrow LiAlH[4](s) -116.3 -240.1 -44.7
                    Standard formation from the elements.
    LiH(s) + Al(s) + 3/2H[2](g) \rightarrow LiAlH[4](s) -25.6 -170.2 23.6
   Using ΔH^o[f](LiH) = -90.5, ΔS^o[f](LiH) = -69.9, and ΔG^o[f](LiH) =
                                   -68.3.
    LiAlH[4](s) \rightarrow LiAlH[4](l) 22 -- -- Heat of fusion. Value is
                            probably unreliable.
     LiAlH[4](l) \rightarrow 1/3Li[3]AlH[6](s) + 2/3Al(s) + H[2](g) 3.46
   104.5 -27.68 ΔS^o calculated from reported values of ΔH^o and ΔG^o.

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