Triethylborane

Names Preferred IUPAC name Other names Identifiers ChemSpider ECHA InfoCard 100.002.383 Edit this at Wikidata EC Number UNII 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): Spontaneously flammable in air; causes burns GHS labelling: GHS02: Flammable GHS05: Corrosive GHS06: Toxic GHS08: Health hazard 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

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 Et ₃B. It is soluble in organic solvents tetrahydrofuran and hexane.

Triethylborane is prepared by the reaction of trimethyl borate with triethylaluminium:^[1]

Et₃Al + (MeO)₃B → Et₃B + (MeO)₃Al

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]

MH + Et₃B → MBHEt₃ (M = Li, Na)

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]

• Organoboranes
• Pentaborane
• Zip fuel