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 (CH3CH2)3B or (C2H5)3B, abbreviated Et3B. It is soluble in organic solvents tetrahydrofuran and hexane.
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Names | |||
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Preferred IUPAC name
Triethylborane | |||
Other names
Triethylborine, triethylboron
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Identifiers | |||
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3D model (JSmol)
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ECHA InfoCard | 100.002.383 | ||
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CompTox Dashboard (EPA)
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Properties | |||
(CH3CH2)3B | |||
Molar mass | 98.00 g/mol | ||
Appearance | Colorless liquid | ||
Density | 0.677 g/cm3 | ||
Melting point | −93 °C (−135 °F; 180 K) | ||
Boiling point | 95 °C (203 °F; 368 K) | ||
Not applicable; highly reactive | |||
Hazards | |||
Occupational safety and health (OHS/OSH): | |||
Main hazards
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Spontaneously flammable in air; causes burns | ||
GHS labelling: | |||
Danger | |||
H225, H250, H301, H314, H330, H360 | |||
P201, P202, P210, P222, P233, P240, P241, P242, P243, P260, P264, P270, P271, P280, P281, P284, P301+P310, P301+P330+P331, P302+P334, P303+P361+P353, P304+P340, P305+P351+P338, P308+P313, P310, P320, P321, P330, P363, P370+P378, P403+P233, P403+P235, P405, P422, P501 | |||
NFPA 704 (fire diamond) | |||
Flash point | < −20 °C (−4 °F; 253 K) | ||
−20 °C (−4 °F; 253 K) | |||
Safety data sheet (SDS) | External SDS | ||
Related compounds | |||
Related compounds
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references
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Triethylborane is prepared by the reaction of trimethyl borate with triethylaluminium:[1]
The molecule is monomeric, unlike H3B and Et3Al, which tend to dimerize. It has a planar BC3 core.[1]
Triethylborane was used to ignite the JP-7 fuel in the Pratt & Whitney J58 turbojet/ramjet engines powering the Lockheed SR-71 Blackbird[2] and its predecessor, the A-12 OXCART. Triethylborane is suitable because it ignites readily upon exposure to oxygen. It was chosen as an ignition method for reliability reasons, and in the case of the Blackbird, because JP-7 fuel has very low volatility and is difficult to ignite. Conventional ignition plugs posed a high risk of malfunction. Triethylborane was used to start each engine and to ignite the afterburners.[3]
Mixed with 10–15% triethylaluminium, it was used before lift-off to ignite the F-1 engines on the Saturn V rocket.[4]
The Merlin engines that power the SpaceX Falcon 9 rocket use a triethylaluminium-triethylborane mixture (TEA-TEB) as a first- and second-stage ignitor.[5]
The Firefly Aerospace Alpha launch vehicle's Reaver engines are also ignited by a triethylaluminium-triethylborane mixture.[6]
Industrially, triethylborane is used as an initiator in radical reactions, where it is effective even at low temperatures.[1] As an initiator, it can replace some organotin compounds.
It reacts with metal enolates, yielding enoxytriethylborates that can be alkylated at the α-carbon atom of the ketone more selectively than in its absence. For example, the enolate from treating cyclohexanone with potassium hydride produces 2-allylcyclohexanone in 90% yield when triethylborane is present. Without it, the product mixture contains 43% of the mono-allylated product, 31% di-allylated cyclohexanones, and 28% unreacted starting material.[7] The choice of base and temperature influences whether the more or less stable enolate is produced, allowing control over the position of substituents. Starting from 2-methylcyclohexanone, reacting with potassium hydride and triethylborane in THF at room temperature leads to the more substituted (and more stable) enolate, whilst reaction at −78 °C with potassium hexamethyldisilazide, KN[Si(CH
3)
3]
2 and triethylborane generates the less substituted (and less stable) enolate. After reaction with methyl iodide the former mixture gives 2,2-dimethylcyclohexanone in 90% yield while the latter produces 2,6-dimethylcyclohexanone in 93% yield.[7][8] The Et stands for ethyl group CH3CH2−.
It is used in the Barton–McCombie deoxygenation reaction for deoxygenation of alcohols. In combination with lithium tri-tert-butoxyaluminum hydride it cleaves ethers. For example, THF is converted, after hydrolysis, to 1-butanol. It also promotes certain variants of the Reformatskii reaction.[9]
Triethylborane is the precursor to the reducing agents lithium triethylborohydride ("Superhydride") and sodium triethylborohydride.[10]
Triethylborane reacts with methanol to form diethyl(methoxy)borane, which is used as the chelating agent in the Narasaka–Prasad reduction for the stereoselective generation of syn-1,3-diols from β-hydroxyketones.[11][12]
Triethylborane is strongly pyrophoric, with an autoignition temperature of −20 °C (−4 °F),[13] burning with an apple-green flame characteristic for boron compounds. Thus, it is typically handled and stored using air-free techniques. Triethylborane is also acutely toxic if swallowed, with an LD50 of 235 mg/kg in rat test subjects.[14]
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