Lithium aluminium hydride
Encyclopedia
Lithium aluminium hydride, commonly abbreviated to LAH or known as LithAl, is an inorganic compound
with the chemical formula
Li
Al
H4
. It was discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent
in organic synthesis
, especially for the reduction of ester
s, carboxylic acid
s, and amide
s. The solid is dangerously reactive toward water
, releasing gaseous hydrogen
(H2). Some related derivatives have been discussed for the hydrogen storage
.
LAH is a white solid, but commercial samples are usually gray due to contamination. 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 can be easily separated from the organic products. The pure powdered material is pyrophoric, but not its large crystals. 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:
This 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 white compounds lithium hydroxide
and aluminium hydroxide
.
P21/c. The unit cell has the dimensions: a = 4.82, b = 7.81, and c = 7.92 Å, α = γ = 90° and β = 112°. In the structure, Li+ centers are surrounded by five tetrahedra
. The Li+ centers are bonded to one hydrogen
atom from each of the surrounding tetrahedra creating a bipyramid
arrangement. At high pressures (>2.2 GPa) a phase transition may occur to give β-LAH.
(LiH) and aluminium chloride
:
In addition to this method, the industrial synthesis entails the initial preparation of sodium aluminium hydride from the elements under high pressure and temperature:
LAH is then prepared by metathesis reaction according to:
which proceeds in a high yield of LAH. LiCl is removed by filtration
from an ether
eal solution of LAH, with subsequent precipitation of LAH to yield a product containing around 1% w/w LiCl.
al solutions. However, it may spontaneously decompose due to the presence of catalytic impurities, though, it appears to be more stable in tetrahydrofuran
(THF). Thus, THF is preferred over, e.g., diethyl ether
, despite the lower solubility.
data for LAH and reactions involving LAH, in the form of standard
enthalpy
, entropy
and Gibbs free energy
change, respectively.
elements, such as titanium
, iron
or vanadium
.
When heated LAH decomposes in a three-step reaction mechanism
:
R1 is usually initiated by the melting
of LAH in the temperature range 150–170 °C, immediately followed by decomposition into solid Li3AlH6, although R1 is known to proceed below the melting point of LiAlH4 as well. At about 200 °C, Li3AlH6 decomposes into LiH (R2) and Al which subsequently convert into LiAl above 400 °C (R3). Reaction R1 is effectively irreversible. R3 is reversible with an equilibrium pressure of about 0.25 bar at 500 °C. R1 and R2 can occur at room temperature with suitable catalysts.
. It is more powerful than the related reagent
sodium borohydride
due to the weaker Al-H bond compared to the B-H bond. Often as a solution in diethyl ether
and followed by an acid work-up, it will convert ester
s, carboxylic acid
s, aldehydes, and ketone
s into the corresponding alcohol
s (see: carbonyl reduction
). Similarly, it converts amide
, nitro
, nitrile
, imine
, oxime
, and azide
compounds 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 by alkoxy groups
. 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
is more commonly used.
LAH is most commonly used for the reduction of ester
s and carboxylic acid
s to primary alcohols; prior to the advent of LiAlH4, this was a difficult conversion involving sodium
metal in boiling ethanol
(the Bouveault-Blanc reduction
). Aldehyde
s and ketone
s 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. When epoxide
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.
Partial reduction of acid chlorides 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 aluminium tri(t-butoxy)hydride must be used, which reacts significantly faster with the acid chloride than with the aldehyde. For example, when isovaleric acid is treated with thionyl chloride
to give isovaleroyl chloride, it can then be reduced via lithium aluminium tri(t-butoxy)hydride to give isovaleraldehyde in 65% yield.
Image:LAH_rxns.png|
rect 5 12 91 74 alcohol
rect 82 178 170 240 epoxide
rect 121 9 193 69 alcohol2
rect 337 1 414 60 alcohol3
rect 458 55 526 117 alcohol4
rect 170 151 234 210 aldehyde
rect 141 259 207 279 nitrile
rect 135 281 196 300 amide
rect 128 311 204 366 amine1
rect 264 268 339 334 carboxylic acid
rect 457 362 529 413 alcohol5
rect 381 255 433 273 azide
rect 469 244 525 269 amine2
rect 321 193 401 242 ester
rect 261 141 320 203 ketone
desc none
Using LAH, amine
s can be prepared by the reduction of amide
s, oxime
s, nitrile
s, nitro compounds or alkyl azide
s.
Lithium aluminium hydride also reduces alkyl halides to alkane
s, although this reaction is rarely employed. 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, and alkyne
s are only reduced if an alcohol group is nearby.
s. For example, sodium hydride (NaH) can be prepared from sodium chloride (NaCl) through the following reaction:
LAH also reacts with many inorganic ligands to form coordinated alumina complexes associated with lithium ions.
medium for future fuel cell
powered vehicle
s. The high hydrogen content, as well as the discovery of reversible hydrogen storage in Ti-doped NaAlH4, have sparked renewed research into LiAlH4 during the last decade. A substantial research effort has been devoted to accelerating the decomposition kinetics by catalytic doping and by ball mill
ing.
In order to take advantage of the total hydrogen capacity, the intermediate compound LiH
must be dehydrogenated as well. Due to its high thermodynamic stability this requires temperatures in excess of 400 °C which is not considered feasible for transportation purposes. Accepting LiH + Al as the final product, the hydrogen storage capacity is reduced to 7.96 wt%. Another problem related to hydrogen storage is the recycling back to LiAlH4 which, due to its relatively low stability, requires an extremely high hydrogen pressure in excess of 10000 bar. Cycling only reaction R2, that is using Li3AlH6 as starting material, would store 5.6 wt% hydrogen in a single step (vs. two steps for NaAlH4 which stores about the same amount of hydrogen). However, attempts on this have not been successful so far.
can be used to efficiently produce sodium aluminium hydride
(NaAlH4) by metathesis
in THF:
Potassium aluminium hydride (KAlH4) can be produced similarly in diglyme
as a solvent:
The reverse, i.e., production of LAH from either sodium aluminium hydride or potassium aluminium hydride can be achieved by reaction with LiCl
or lithium hydride in diethyl ether
or THF
:
"Magnesium alanate" (Mg(AlH4)2) arises similarly using MgBr2
:
Red-Al
(or SMEAH, NaAlH2(OC2H4OCH3)2) is synthesized by reacting sodium aluminum tetrahydride (NaAlH4) and 2-methoxyethanol
:
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
with the chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
Li
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
Al
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
H4
Hydride
In chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...
. It was discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent
Reducing agent
A reducing agent is the element or compound in a reduction-oxidation reaction that donates an electron to another species; however, since the reducer loses an electron we say it is "oxidized"...
in organic synthesis
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
, especially for the reduction of ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
s, carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
s, and amide
Amide
In chemistry, an amide is an organic compound that contains the functional group consisting of a carbonyl group linked to a nitrogen atom . The term refers both to a class of compounds and a functional group within those compounds. The term amide also refers to deprotonated form of ammonia or an...
s. The solid is dangerously reactive toward water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
, releasing gaseous hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
(H2). Some related derivatives have been discussed for the hydrogen storage
Hydrogen storage
Hydrogen storage describes the methods for storing H2 for subsequent use. The methods span many approaches, including high pressures, cryogenics, and chemical compounds that reversibly release H2 upon heating...
.
Properties, structure, preparation
Diethyl ether
Diethyl ether, also known as ethyl ether, simply ether, or ethoxyethane, is an organic compound in the ether class with the formula . It is a colorless, highly volatile flammable liquid with a characteristic odor...
. Large-scale purifications employ a Soxhlet extractor
Soxhlet extractor
A Soxhlet extractor is a piece of laboratory apparatusinvented in 1879 by Franz von Soxhlet. It was originally designed for the extraction of a lipid from a solid material. However, a Soxhlet extractor is not limited to the extraction of lipids...
. Commonly, the impure gray material is used in synthesis, since the impurities are innocuous and can be easily separated from the organic products. The pure powdered material is pyrophoric, but not its large crystals. Some commercial materials contain mineral oil
Mineral oil
A mineral oil is any of various colorless, odorless, light mixtures of alkanes in the C15 to C40 range from a non-vegetable source, particularly a distillate of petroleum....
to inhibit reactions with atmospheric moisture, but more commonly it is packed in moisture-proof plastic sacks.
LAH violently reacts with water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
, including atmospheric moisture. The reaction proceeds according the following idealized equation:
- LiAlH4 + 4 H2O → LiOH + Al(OH)3 + 4 H2
This 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 white compounds lithium hydroxide
Lithium hydroxide
Lithium hydroxide is an inorganic compound with the formula LiOH. It is a white hygroscopic crystalline material. It is soluble in water and slightly soluble in ethanol...
and aluminium hydroxide
Aluminium hydroxide
Aluminium hydroxide, Al3, ATH, sometimes erroneously called Hydrate of alumina, is found in nature as the mineral gibbsite and its three, much more rare forms, polymorphs: bayerite, doyleite and nordstrandite. Closely related are aluminium oxide hydroxide, AlO, and aluminium oxide, Al2O3,...
.
Structure
LAH crystallizes in the monoclinic space groupSpace group
In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...
P21/c. The unit cell has the dimensions: a = 4.82, b = 7.81, and c = 7.92 Å, α = γ = 90° and β = 112°. In the structure, Li+ centers are surrounded by five tetrahedra
Tetrahedron
In geometry, a tetrahedron is a polyhedron composed of four triangular faces, three of which meet at each vertex. A regular tetrahedron is one in which the four triangles are regular, or "equilateral", and is one of the Platonic solids...
. The Li+ centers are bonded to one hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
atom from each of the surrounding tetrahedra creating a bipyramid
Bipyramid
An n-gonal bipyramid or dipyramid is a polyhedron formed by joining an n-gonal pyramid and its mirror image base-to-base.The referenced n-gon in the name of the bipyramids is not an external face but an internal one, existing on the primary symmetry plane which connects the two pyramid halves.The...
arrangement. At high pressures (>2.2 GPa) a phase transition may occur to give β-LAH.
Preparation
LAH was first prepared from the reaction between lithium hydrideLithium hydride
Lithium hydride is the inorganic compound with the formula LiH. It is a colorless solid, although commercial samples are gray. Characteristic of a salt-like, or ionic, hydride, it has a high melting point and is not soluble in any solvent with which it does not react...
(LiH) and aluminium chloride
Aluminium chloride
Aluminium chloride is the main compound of aluminium and chlorine. It is white, but samples are often contaminated with iron trichloride, giving it a yellow colour. The solid has a low melting and boiling point. It is mainly produced and consumed in the production of aluminium metal, but large...
:
- 4 LiH + AlCl3 → LiAlH4 + 3 LiCl
In addition to this method, the industrial synthesis entails the initial preparation of sodium aluminium hydride from the elements under high pressure and temperature:
- 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
Filtration
Filtration is commonly the mechanical or physical operation which is used for the separation of solids from fluids by interposing a medium through which only the fluid can pass...
from an ether
Ether
Ethers are a class of organic compounds that contain an ether group — an oxygen atom connected to two alkyl or aryl groups — of general formula R–O–R'. A typical example is the solvent and anesthetic diethyl ether, commonly referred to simply as "ether"...
eal solution of LAH, with subsequent precipitation of LAH to yield a product containing around 1% w/w LiCl.
Solubility data
LAH is soluble in many etherEther
Ethers are a class of organic compounds that contain an ether group — an oxygen atom connected to two alkyl or aryl groups — of general formula R–O–R'. A typical example is the solvent and anesthetic diethyl ether, commonly referred to simply as "ether"...
al solutions. However, it may spontaneously decompose due to the presence of catalytic impurities, though, it appears to be more stable in tetrahydrofuran
Tetrahydrofuran
Tetrahydrofuran is a colorless, water-miscible organic liquid with low viscosity at standard temperature and pressure. This heterocyclic compound has the chemical formula 4O. As one of the most polar ethers with a wide liquid range, it is a useful solvent. Its main use, however, is as a precursor...
(THF). Thus, THF is preferred over, e.g., diethyl ether
Diethyl ether
Diethyl ether, also known as ethyl ether, simply ether, or ethoxyethane, is an organic compound in the ether class with the formula . It is a colorless, highly volatile flammable liquid with a characteristic odor...
, despite the lower solubility.
Thermodynamic data
The table summarizes thermodynamicThermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...
data for LAH and reactions involving LAH, in the form of standard
Standard state
In chemistry, the standard state of a material is a reference point used to calculate its properties under different conditions. In principle, the choice of standard state is arbitrary, although the International Union of Pure and Applied Chemistry recommends a conventional set of standard states...
enthalpy
Enthalpy
Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Enthalpy is a...
, entropy
Entropy
Entropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when...
and Gibbs free energy
Gibbs free energy
In thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from a thermodynamic system at a constant temperature and pressure...
change, respectively.
| Reaction | ΔH° (kJ/mol) |
ΔS° (J/(mol·K)) |
ΔG° (kJ/mol) |
Comment |
|---|---|---|---|---|
| Li (s) + Al (s) + 2 H2(g) → LiAlH4 (s) | −116.3 | −240.1 | −44.7 | Standard formation from the elements. |
| LiH (s) + Al (s) + 3/2 H2 (g) → LiAlH4 (s) | −25.6 | −170.2 | 23.6 | Using ΔH°f(LiH) = −90.5, ΔS°f(LiH) = −69.9, and ΔG°f(LiH) = −68.3. |
| LiAlH4 (s) → LiAlH4 (l) | 22 | – | – | Heat of fusion. Value might be unreliable. |
| LiAlH4 (l) → ⅓ Li3AlH6 (s) + ⅔ Al (s) + H2 (g) | 3.46 | 104.5 | −27.68 | ΔS° calculated from reported values of ΔH° and ΔG°. |
Thermal decomposition
LAH is metastable at room temperature. During prolonged storage it slowly decomposes to Li3AlH6 and LiH. This process can be accelerated by the presence of catalyticCatalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
elements, such as titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
, iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
or vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
.
When heated LAH decomposes in a three-step reaction mechanism
Reaction mechanism
In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in...
:
- 3 LiAlH4 → Li3AlH6 + 2 Al + 3 H2 (R1)
- 2 Li3AlH6 → 6 LiH + 2 Al + 3 H2 (R2)
- 2 LiH + 2 Al → 2 LiAl + H2 (R3)
R1 is usually initiated by the melting
Melting
Melting, or fusion, is a physical process that results in the phase change of a substance from a solid to a liquid. The internal energy of a substance is increased, typically by the application of heat or pressure, resulting in a rise of its temperature to the melting point, at which the rigid...
of LAH in the temperature range 150–170 °C, immediately followed by decomposition into solid Li3AlH6, although R1 is known to proceed below the melting point of LiAlH4 as well. At about 200 °C, Li3AlH6 decomposes into LiH (R2) and Al which subsequently convert into LiAl above 400 °C (R3). Reaction R1 is effectively irreversible. R3 is reversible with an equilibrium pressure of about 0.25 bar at 500 °C. R1 and R2 can occur at room temperature with suitable catalysts.
Use in organic chemistry
Lithium aluminium hydride is widely used in organic chemistry as a reducing agentReducing agent
A reducing agent is the element or compound in a reduction-oxidation reaction that donates an electron to another species; however, since the reducer loses an electron we say it is "oxidized"...
. It is more powerful than the related reagent
Reagent
A reagent is a "substance or compound that is added to a system in order to bring about a chemical reaction, or added to see if a reaction occurs." Although the terms reactant and reagent are often used interchangeably, a reactant is less specifically a "substance that is consumed in the course of...
sodium borohydride
Sodium borohydride
Sodium borohydride, also known as sodium tetrahydridoborate, is an inorganic compound with the formula NaBH4. This white solid, usually encountered as a powder, is a versatile reducing agent that finds wide application in chemistry, both in the laboratory and on a technical scale. Large amounts are...
due to the weaker Al-H bond compared to the B-H bond. Often as a solution in diethyl ether
Diethyl ether
Diethyl ether, also known as ethyl ether, simply ether, or ethoxyethane, is an organic compound in the ether class with the formula . It is a colorless, highly volatile flammable liquid with a characteristic odor...
and followed by an acid work-up, it will convert ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
s, carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
s, aldehydes, and ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
s into the corresponding alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
s (see: carbonyl reduction
Carbonyl reduction
Carbonyl reduction in organic chemistry is the organic reduction of any carbonyl group containing compound by a reducing agent. Typical carbonyl compounds are ketones , aldehydes, carboxylic acids and esters. A carbonyl group can be reduced to the alcohol or the oxygen atom can be removed...
). Similarly, it converts amide
Amide
In chemistry, an amide is an organic compound that contains the functional group consisting of a carbonyl group linked to a nitrogen atom . The term refers both to a class of compounds and a functional group within those compounds. The term amide also refers to deprotonated form of ammonia or an...
, nitro
Nitro compound
Nitro compounds are organic compounds that contain one or more nitro functional groups . They are often highly explosive, especially when the compound contains more than one nitro group and is impure. The nitro group is one of the most common explosophores used globally...
, nitrile
Nitrile
A nitrile is any organic compound that has a -C≡N functional group. The prefix cyano- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds, one example being super glue .Inorganic compounds containing the -C≡N group are not called...
, imine
Imine
An imine is a functional group or chemical compound containing a carbon–nitrogen double bond, with the nitrogen attached to a hydrogen atom or an organic group. If this group is not a hydrogen atom, then the compound is known as a Schiff base...
, oxime
Oxime
An oxime is a chemical compound belonging to the imines, with the general formula R1R2C=NOH, where R1 is an organic side chain and R2 may be hydrogen, forming an aldoxime, or another organic group, forming a ketoxime. O-substituted oximes form a closely related family of compounds...
, and azide
Azide
Azide is the anion with the formula N3−. It is the conjugate base of hydrazoic acid. N3− is a linear anion that is isoelectronic with CO2 and N2O. Per valence bond theory, azide can be described by several resonance structures, an important one being N−=N+=N−...
compounds into the amine
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
s (see: amide reduction
Amide reduction
Amide reduction in chemistry is the organic reduction of amides . The main reaction product in this functional group interconversion is an amine...
). It reduces quaternary ammonium cation
Quaternary ammonium cation
Quaternary ammonium cations, also known as quats, are positively charged polyatomic ions of the structure NR4+, R being an alkyl group or an aryl group. Unlike the ammonium ion and the primary, secondary, or tertiary ammonium cations, the quaternary ammonium cations are permanently charged,...
s into the corresponding tertiary amines. Reactivity can be tuned by replacing hydride groups by alkoxy groups
Reductions with metal alkoxyaluminum hydrides
Reductions with metal alkoxyaluminium hydrides are chemical reactions that involve either the net hydrogenation of an unsaturated compound or the replacement of a reducible functional group with hydrogen by metal alkoxyaluminium hydride reagents.-Introduction:...
. 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
Red-Al
Red-Al, or sodium bisaluminumhydride, is an organoaluminium compound with the formula NaAlH22. The name which is a trademark of Sigma-Aldrich refers to its being a Reducing aluminium compound. It is used almost exclusively as a solution in toluene, which is used as a reagent in organic synthesis...
is more commonly used.
LAH is most commonly used for the reduction of ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
s and carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
s to primary alcohols; prior to the advent of LiAlH4, this was a difficult conversion involving sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
metal in boiling ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
(the Bouveault-Blanc reduction
Bouveault-Blanc reduction
The Bouveault-Blanc reduction is a chemical reaction in which an ester is reduced to primary alcohols using absolute ethanol and sodium metal.This reaction is an inexpensive and large-scale alternative to lithium aluminium hydride reduction of esters....
). Aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
s and ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
s can also be reduced to alcohols by LAH, but this is usually done using milder reagents such as NaBH4
Sodium borohydride
Sodium borohydride, also known as sodium tetrahydridoborate, is an inorganic compound with the formula NaBH4. This white solid, usually encountered as a powder, is a versatile reducing agent that finds wide application in chemistry, both in the laboratory and on a technical scale. Large amounts are...
; α,β-unsaturated ketones are reduced to allylic alcohols. When epoxide
Epoxide
An epoxide is a cyclic ether with three ring atoms. This ring approximately defines an equilateral triangle, which makes it highly strained. The strained ring makes epoxides more reactive than other ethers. Simple epoxides are named from the parent compound ethylene oxide or oxirane, such as in...
s are reduced using LAH, the reagent attacks the less hindered
Steric effects
Steric effects arise from the fact that each atom within a molecule occupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds , and this may affect the molecule's preferred shape and reactivity.-Steric...
end of the epoxide, usually producing a secondary or tertiary alcohol. Epoxycyclohexanes are reduced to give axial alcohols preferentially.
Partial reduction of acid chlorides 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 aluminium tri(t-butoxy)hydride must be used, which reacts significantly faster with the acid chloride than with the aldehyde. For example, when isovaleric acid is treated with thionyl chloride
Thionyl chloride
Thionyl chloride is an inorganic compound with the formula SOCl2. It is a reactive chemical reagent used in chlorination reactions. It is a colorless, distillable liquid at room temperature and pressure that decomposes above 140 °C. Thionyl chloride is sometimes confused with sulfuryl...
to give isovaleroyl chloride, it can then be reduced via lithium aluminium tri(t-butoxy)hydride to give isovaleraldehyde in 65% yield.
Image:LAH_rxns.png|
rect 5 12 91 74 alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
rect 82 178 170 240 epoxide
Epoxide
An epoxide is a cyclic ether with three ring atoms. This ring approximately defines an equilateral triangle, which makes it highly strained. The strained ring makes epoxides more reactive than other ethers. Simple epoxides are named from the parent compound ethylene oxide or oxirane, such as in...
rect 121 9 193 69 alcohol2
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
rect 337 1 414 60 alcohol3
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
rect 458 55 526 117 alcohol4
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
rect 170 151 234 210 aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
rect 141 259 207 279 nitrile
Nitrile
A nitrile is any organic compound that has a -C≡N functional group. The prefix cyano- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds, one example being super glue .Inorganic compounds containing the -C≡N group are not called...
rect 135 281 196 300 amide
Amide
In chemistry, an amide is an organic compound that contains the functional group consisting of a carbonyl group linked to a nitrogen atom . The term refers both to a class of compounds and a functional group within those compounds. The term amide also refers to deprotonated form of ammonia or an...
rect 128 311 204 366 amine1
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
rect 264 268 339 334 carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
rect 457 362 529 413 alcohol5
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
rect 381 255 433 273 azide
rect 469 244 525 269 amine2
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
rect 321 193 401 242 ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
rect 261 141 320 203 ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
desc none
- Notes:
- Details on the new coding for clickable images is here: mw:Extension:ImageMap
- This image editor was used.
Using LAH, amine
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
s can be prepared by the reduction of amide
Amide
In chemistry, an amide is an organic compound that contains the functional group consisting of a carbonyl group linked to a nitrogen atom . The term refers both to a class of compounds and a functional group within those compounds. The term amide also refers to deprotonated form of ammonia or an...
s, oxime
Oxime
An oxime is a chemical compound belonging to the imines, with the general formula R1R2C=NOH, where R1 is an organic side chain and R2 may be hydrogen, forming an aldoxime, or another organic group, forming a ketoxime. O-substituted oximes form a closely related family of compounds...
s, nitrile
Nitrile
A nitrile is any organic compound that has a -C≡N functional group. The prefix cyano- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds, one example being super glue .Inorganic compounds containing the -C≡N group are not called...
s, nitro compounds or alkyl azide
Azide
Azide is the anion with the formula N3−. It is the conjugate base of hydrazoic acid. N3− is a linear anion that is isoelectronic with CO2 and N2O. Per valence bond theory, azide can be described by several resonance structures, an important one being N−=N+=N−...
s.
Lithium aluminium hydride also reduces alkyl halides to alkane
Alkane
Alkanes are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds without any cycles...
s, although this reaction is rarely employed. 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
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
s, arene
Arene
Arene or Arênê or Arène may refer to:*an aromatic hydrocarbon*Arene , a genus of marine snails in the family Areneidae*Arene , the wife of Aphareus and mother of Idas and Lynceus in Greek mythology...
s, and alkyne
Alkyne
Alkynes are hydrocarbons that have a triple bond between two carbon atoms, with the formula CnH2n-2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC nomenclature...
s are only reduced if an alcohol group is nearby.
Inorganic chemistry
LAH is widely used to prepare main group and transition metal hydrides from the corresponding metal halideHalide
A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...
s. For example, sodium hydride (NaH) can be prepared from sodium chloride (NaCl) through the following reaction:
- LiAlH4 + 4 NaCl → 4 NaH + LiCl + AlCl3
LAH also reacts with many inorganic ligands to form coordinated alumina complexes associated with lithium ions.
- LiAlH4 + NH3 → Li[Al(NH2)4]
Hydrogen storage
LiAlH4 contains 10.6 wt% hydrogen thereby making LAH a potential hydrogen storageHydrogen storage
Hydrogen storage describes the methods for storing H2 for subsequent use. The methods span many approaches, including high pressures, cryogenics, and chemical compounds that reversibly release H2 upon heating...
medium for future fuel cell
Fuel cell
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used...
powered vehicle
Vehicle
A vehicle is a device that is designed or used to transport people or cargo. Most often vehicles are manufactured, such as bicycles, cars, motorcycles, trains, ships, boats, and aircraft....
s. The high hydrogen content, as well as the discovery of reversible hydrogen storage in Ti-doped NaAlH4, have sparked renewed research into LiAlH4 during the last decade. A substantial research effort has been devoted to accelerating the decomposition kinetics by catalytic doping and by ball mill
Ball mill
A ball mill is a type of grinder used to grind materials into extremely fine powder for use in mineral dressing processes, paints, pyrotechnics, and ceramics.-Description:...
ing.
In order to take advantage of the total hydrogen capacity, the intermediate compound LiH
LIH
LIH may refer to:* Andrew Lih, American author of The Wikipedia Revolution* Lihue Airport, an airport on the island of Kauai, Hawaii, United States of America* Lithium hydride, or LiH, a Lithium compound in chemistry...
must be dehydrogenated as well. Due to its high thermodynamic stability this requires temperatures in excess of 400 °C which is not considered feasible for transportation purposes. Accepting LiH + Al as the final product, the hydrogen storage capacity is reduced to 7.96 wt%. Another problem related to hydrogen storage is the recycling back to LiAlH4 which, due to its relatively low stability, requires an extremely high hydrogen pressure in excess of 10000 bar. Cycling only reaction R2, that is using Li3AlH6 as starting material, would store 5.6 wt% hydrogen in a single step (vs. two steps for NaAlH4 which stores about the same amount of hydrogen). However, attempts on this have not been successful so far.
Other tetrahydridoaluminiumates
A variety of salts analogous to LAH are known. NaHSodium hydride
Sodium hydride is the chemical compound with the empirical formula NaH. It is primarily used as a strong base in organic synthesis. NaH is representative of the saline hydrides, meaning it is a salt-like hydride, composed of Na+ and H− ions, in contrast to the more molecular hydrides such as...
can be used to efficiently produce sodium aluminium hydride
Sodium aluminium hydride
Sodium aluminium hydride or sodium alanate is a chemical compound used as a reducing agent. It is similar to lithium aluminium hydride....
(NaAlH4) by metathesis
Metathesis (chemistry)
Salt metathesis is a molecular process involving the exchange of bonds between the two reacting chemical species, which results in the creation of products with similar or identical bonding affiliations. This is represented by the general reaction scheme:These chemical species can either be ionic...
in THF:
- LiAlH4 + NaH → NaAlH4 + LiH
Potassium aluminium hydride (KAlH4) can be produced similarly in diglyme
Diglyme
Diglyme, or bis ether, is a solvent with a high boiling point. It is an organic compound which is the dimethyl ether of diethylene glycol. It is a clear, colorless liquid with a slight ether-like odor...
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 achieved by reaction with LiCl
Lithium chloride
Lithium chloride is a chemical compound with the formula LiCl. The salt is a typical ionic compound, although the small size of the Li+ ion gives rise to properties not seen for other alkali metal chlorides, such as extraordinary solubility in polar solvents and its hygroscopic...
or lithium hydride in diethyl ether
Diethyl ether
Diethyl ether, also known as ethyl ether, simply ether, or ethoxyethane, is an organic compound in the ether class with the formula . It is a colorless, highly volatile flammable liquid with a characteristic odor...
or THF
ThF
Follicular B helper T cells , are antigen-experienced CD4+ T cells found in the B cell follicles of secondary lymphoid organs such as lymph nodes, spleens and Peyer's patches, and are identified by their constitutive expression of the B cell follicle homing receptor CXCR5...
:
- NaAlH4 + LiCl → LiAlH4 + NaCl
- KAlH4 + LiCl → LiAlH4 + KCl
"Magnesium alanate" (Mg(AlH4)2) arises similarly using MgBr2
Magnesium bromide
Magnesium bromide is a chemical compound of magnesium and bromine that is white and deliquescent. It is often used as a mild sedative and as an anticonvulsant for treatment of nervous disorders. It is water soluble and somewhat soluble in alcohol...
:
- 2 LiAlH4 + MgBr2 → Mg(AlH4)2 + 2 LiBr
Red-Al
Red-Al
Red-Al, or sodium bisaluminumhydride, is an organoaluminium compound with the formula NaAlH22. The name which is a trademark of Sigma-Aldrich refers to its being a Reducing aluminium compound. It is used almost exclusively as a solution in toluene, which is used as a reagent in organic synthesis...
(or SMEAH, NaAlH2(OC2H4OCH3)2) is synthesized by reacting sodium aluminum tetrahydride (NaAlH4) and 2-methoxyethanol
2-Methoxyethanol
2-Methoxyethanol, or methyl cellosolve, is an organic compound that is used mainly as a solvent. It is a clear, colorless liquid with an ether-like odor. It is in a class of solvents known as glycol ethers which are notable for their ability to dissolve a variety of different types of chemical...
:
See also
- HydrideHydrideIn chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...
- Sodium borohydrideSodium borohydrideSodium borohydride, also known as sodium tetrahydridoborate, is an inorganic compound with the formula NaBH4. This white solid, usually encountered as a powder, is a versatile reducing agent that finds wide application in chemistry, both in the laboratory and on a technical scale. Large amounts are...
- Sodium hydrideSodium hydrideSodium hydride is the chemical compound with the empirical formula NaH. It is primarily used as a strong base in organic synthesis. NaH is representative of the saline hydrides, meaning it is a salt-like hydride, composed of Na+ and H− ions, in contrast to the more molecular hydrides such as...
Further reading
on-line version- Chapter 5 in Full text version
External links
- Usage of LiAlH4 in Organic Syntheses
- Condensed phase thermochemistry data from Nist webbook
- Materials Safety Data Sheet from Cornell University
- Sandia National Laboratory - Hydride information center
- Synthesis of LAH
- Reduction reactions, University of Birmingham, Teaching Resources - 4th Year
- PubChem LiAlH4 summary