Triethylaluminium
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Triethylaluminium is one of the simplest examples of an organoaluminium compound. Despite its name it has the
formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwee ...
Al2( C2H5)6 (abbreviated as Al2Et6 or TEA), as it exists as a
dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ...
. This colorless liquid is
pyrophoric A substance is pyrophoric (from grc-gre, πυροφόρος, , 'fire-bearing') if it ignites spontaneously in air at or below (for gases) or within 5 minutes after coming into contact with air (for liquids and solids). Examples are organolith ...
. It is an industrially important compound, closely related to
trimethylaluminium Trimethylaluminium is one of the simplest examples of an organoaluminium compound. Despite its name it has the formula Al2( CH3)6 (abbreviated as Al2Me6 or TMA), as it exists as a dimer. This colorless liquid is pyrophoric. It is an industriall ...
.


Structure and bonding

The structure and bonding in Al2R6 and
diborane Diborane(6), generally known as diborane, is the chemical compound with the formula B2H6. It is a toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Diborane is a key boron compound with a variety of applications. It has attracte ...
are analogous (R = alkyl). Referring to Al2Me6, the Al-C(terminal) and Al-C(bridging) distances are 1.97 and 2.14 Å, respectively. The Al center is tetrahedral. The carbon atoms of the bridging ethyl groups are each surrounded by five neighbors: carbon, two hydrogen atoms and two aluminium atoms. The ethyl groups interchange readily intramolecularly. At higher temperatures, the dimer cracks into monomeric AlEt3.


Synthesis and reactions

Triethylaluminium can be formed via several routes. The discovery of an efficient route was a significant technological achievement. The multistep process uses aluminium metal,
hydrogen gas Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, a ...
, and
ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds). Ethylene i ...
, summarized as follows: :2 Al + 3 H2 + 6 C2H4 → Al2Et6 Because of this efficient synthesis, triethylaluminium is one of the most available organoaluminium compounds. Triethylaluminium can also be generated from
ethylaluminium sesquichloride Ethylaluminium sesquichloride, also called EASC, is an industrially important organoaluminium compound used primarily as a precursor to triethylaluminium and as a catalyst component in Ziegler–Natta type systems for olefin and diene polymeriza ...
(Al2Cl3Et3), which arises by treating aluminium powder with chloroethane. Reduction of ethylaluminium sesquichloride with an
alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
such as sodium gives triethylaluminium: :6 Al2Cl3Et3 + 18 Na → 3 Al2Et6 + 6 Al + 18 NaCl


Reactivity

The Al–C bonds of triethylaluminium are polarized to such an extent that the carbon is easily
protonated In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brønsted–Lowry acid, ...
, releasing ethane: :Al2Et6 + 6 HX → 2 AlX3 + 6 EtH For this reaction, even weak acids can be employed such as terminal
acetylene Acetylene (systematic name: ethyne) is the chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in its pure ...
s and alcohols. The linkage between the pair of aluminium centres is relatively weak and can be cleaved by Lewis bases (L) to give
adduct An adduct (from the Latin ''adductus'', "drawn toward" alternatively, a contraction of "addition product") is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all co ...
s with the formula AlEt3L: :Al2Et6 + 2 L → 2 LAlEt3


Applications


Precursors to fatty alcohols

Triethylaluminium is used industrially as an intermediate in the production of fatty alcohols, which are converted to
detergent A detergent is a surfactant or a mixture of surfactants with cleansing properties when in dilute solutions. There are a large variety of detergents, a common family being the alkylbenzene sulfonates, which are soap-like compounds that are more ...
s. The first step involves the
oligomerization In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomers.Quote: ''Oligomer molecule: A molecule of intermediate relativ ...
of ethylene by the ''Aufbau'' reaction, which gives a mixture of trialkylaluminium compounds (simplified here as
octyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloalk ...
groups): :Al2(C2H5)6 + 18 C2H4 → Al2(C8H17)6 Subsequently, these trialkyl compounds are oxidized to aluminium
alkoxide In chemistry, an alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They are written as , where R is the organic substituent. Alkoxides are strong bases and, whe ...
s, which are then hydrolysed: :Al2(C8H17)6 + 3 O2 → Al2(OC8H17)6 :Al2(OC8H17)6 + 6 H2O → 6 C8H17OH + 2 "Al(OH)3"


Co-catalysts in olefin polymerization

A large amount of TEAL and related aluminium alkyls are used in Ziegler-Natta catalysis. They serve to activate the transition metal catalyst both as a reducing agent and an
alkylating agent Alkylation is the transfer of an alkyl group from one molecule to another. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene (or their equivalents). Alkylating agents are reagents for effectin ...
. TEAL also functions to scavenge water and oxygen.


Reagent in organic and organometallic chemistry

Triethylaluminium has niche uses as a precursor to other organoaluminium compounds, such as
diethylaluminium cyanide Diethylaluminum cyanide ("Nagata's reagent") is the organoaluminum compound with formula ((C2H5)2AlCN)n. This colorless compound is usually handled as a solution in toluene. It is a reagent for the hydrocyanation of α,β-unsaturated ketones. S ...
: :\ce\ \tfrac 1 n \ce_n + \ce


Pyrophoric agent

Triethylaluminium ignites on contact with air and will ignite and/or decompose on contact with water, and with any other oxidizer—it is one of the few substances sufficiently pyrophoric to ignite on contact with cryogenic
liquid oxygen Liquid oxygen—abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries—is the liquid form of molecular oxygen. It was used as the oxidizer in the first liquid-fueled rocket invented in 1926 by Robert H. Goddard, an applica ...
. The
enthalpy of combustion The heating value (or energy value or calorific value) of a substance, usually a fuel or food (see food energy), is the amount of heat released during the combustion of a specified amount of it. The ''calorific value'' is the total energy relea ...
, ΔcH°, is (–22.36 kJ/ g). Its easy ignition makes it particularly desirable as a
rocket engine A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, in accordanc ...
ignitor Ignitor is the Italian name for a planned tokamak device, developed by ENEA. , the device has not been constructed. Started in 1977 by Prof. Bruno Coppi at MIT, Ignitor based on the 1970s Alcator machine at MIT which pioneered the high magnetic ...
. The
SpaceX Space Exploration Technologies Corp. (SpaceX) is an American spacecraft manufacturer, launcher, and a satellite communications corporation headquartered in Hawthorne, California. It was founded in 2002 by Elon Musk with the stated goal of ...
Falcon 9 Falcon 9 is a partially reusable medium lift launch vehicle that can carry cargo and crew into Earth orbit, produced by American aerospace company SpaceX. The rocket has two stages. The first (booster) stage carries the second stage and payl ...
rocket A rocket (from it, rocchetto, , bobbin/spool) is a vehicle that uses jet propulsion to accelerate without using the surrounding air. A rocket engine produces thrust by reaction to exhaust expelled at high speed. Rocket engines work entirely fr ...
uses a triethylaluminium-
triethylborane Triethylborane (TEB), also called triethylboron, is an organoborane (a compound with a B–C bond). It is a colorless pyrophoric liquid. Its chemical formula is or , abbreviated . It is soluble in organic solvents tetrahydrofuran and hexane. Pr ...
mixture as a first-stage ignitor. Triethylaluminium thickened with
polyisobutylene Polyisobutene (polyisobutylene) is a class of organic polymers prepared by polymerization of isobutene. The polymers often have the formula Me3C H2CMe2sub>nX (Me = CH3, X = H, F). They are typically colorless gummy solids. Polymerization is typ ...
is used as an
incendiary weapon Incendiary weapons, incendiary devices, incendiary munitions, or incendiary bombs are weapons designed to start fires or destroy sensitive equipment using fire (and sometimes used as anti-personnel weaponry), that use materials such as napalm, th ...
, as a pyrophoric alternative to
napalm Napalm is an incendiary mixture of a gelling agent and a volatile petrochemical (usually gasoline (petrol) or diesel fuel). The name is a portmanteau of two of the constituents of the original thickening and gelling agents: coprecipitated al ...
; e.g., in the M74 clip holding four rockets for the M202A1 launchers. In this application it is known as TPA, for ''thickened pyrotechnic agent'' or ''thickened pyrophoric agent''. The usual amount of the thickener is 6%. The amount of thickener can be decreased to 1% if other diluents are added. For example,
n-hexane Hexane () is an organic compound, a straight-chain alkane with six carbon atoms and has the molecular formula C6H14. It is a colorless liquid, odorless when pure, and with boiling points approximately . It is widely used as a cheap, relatively ...
, can be used with increased safety by rendering the compound non-pyrophoric until the diluent evaporates, at which point a combined fireball results from both the triethylaluminium and the hexane vapors.''Encyclopedia of Explosives and Related Items'', Vol.8, US Army The M202 was withdrawn from service in the mid-1980s owing to safety, transport, and storage issues. Some saw limited use in the Afghanistan War against caves and fortified compounds.


See also

*
Triethylborane Triethylborane (TEB), also called triethylboron, is an organoborane (a compound with a B–C bond). It is a colorless pyrophoric liquid. Its chemical formula is or , abbreviated . It is soluble in organic solvents tetrahydrofuran and hexane. Pr ...
, used as an ignitor in the
Pratt & Whitney J58 The Pratt & Whitney J58 (company designation JT11D-20) is an American jet engine that powered the Lockheed A-12, and subsequently the YF-12 and the SR-71 aircraft. It was an afterburning turbojet engine with a unique compressor bleed to the a ...
turbojet The turbojet is an airbreathing jet engine which is typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet which includes inlet guide vanes, a compressor, a combustion chamber, and ...
/
ramjet A ramjet, or athodyd (aero thermodynamic duct), is a form of airbreathing jet engine that uses the forward motion of the engine to produce thrust. Since it produces no thrust when stationary (no ram air) ramjet-powered vehicles require an ass ...
engines. *
Trimethylaluminium Trimethylaluminium is one of the simplest examples of an organoaluminium compound. Despite its name it has the formula Al2( CH3)6 (abbreviated as Al2Me6 or TMA), as it exists as a dimer. This colorless liquid is pyrophoric. It is an industriall ...


References

{{Aluminium compounds Organoaluminium compounds Rocket propellants Rocket fuels Incendiary weapons