triethylborane

Triethylborane is prepared by the reaction of trimethyl borate with triethylaluminium:

:Et₃Al + (MeO)₃B → Et₃B + (MeO)₃Al

The molecule is monomeric, unlike H₃B and Et₃Al, which tend to dimerize. It has a planar BC₃ core.

Triethylborane was used to ignite the JP-7 fuel in the Pratt & Whitney J58 turbojet/ramjet engines powering the Lockheed SR-71 Blackbird{{cite web| url = http://www.marchfield.org/sr71a.htm| title = Lockheed SR-71 Blackbird| accessdate = 2009-05-05| publisher = March Field Air Museum| url-status = dead| archiveurl = https://web.archive.org/web/20000304181849/http://www.marchfield.org/sr71a.htm| archivedate = 2000-03-04}} 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.{{cite web| url = http://www.sr-71.org/blackbird/manual/1/1-22.php| title = Lockheed SR-71 Blackbird Flight Manual| accessdate = 2011-01-26| publisher = www.sr-71.org| archive-date = 2011-02-02| archive-url = https://web.archive.org/web/20110202220020/http://www.sr-71.org/blackbird/manual/1/1-22.php| url-status = live}}

Mixed with 10–15% triethylaluminium, it was used before lift-off to ignite the F-1 engines on the Saturn V rocket.{{cite book| page = 86| title = The Saturn V F-1 Engine: Powering Apollo Into History| author =A. Young| publisher = Springer| year = 2008| isbn = 978-0-387-09629-2}}

The Merlin engines that power the SpaceX Falcon 9 rocket use a triethylaluminium-triethylborane mixture (TEA-TEB) as a first- and second-stage ignitor.[http://www.spaceflightnow.com/falcon9/001/status.html Mission Status Center, June 2, 2010, 1905 GMT] {{Webarchive|url=https://web.archive.org/web/20100530232910/http://www.spaceflightnow.com/tracking/index.html|date=May 30, 2010}}, SpaceflightNow, accessed 2010-06-02, Quotation: "The flanges will link the rocket with ground storage tanks containing liquid oxygen, kerosene fuel, helium, gaseous nitrogen and the first stage ignitor source called triethylaluminum-triethylborane, better known as TEA-TEB."

The Firefly Aerospace Alpha launch vehicle's Reaver engines are also ignited by a triethylaluminium-triethylborane mixture.{{Cite web |title=Firefly_Space/status/1090319933534334977 |url=https://twitter.com/Firefly_Space/status/1090319933534334977 |access-date=2023-02-05 |website=Twitter |language=en}}

Industrially, triethylborane is used as an initiator in radical reactions, where it is effective even at low temperatures.{{cite encyclopedia|first1 = Robert J.|last1 = Brotherton|first2 = C. Joseph|last2 = Weber|first3 = Clarence R.|last3 = Guibert|first4 = John L.|last4 = Little|title = Boron Compounds|encyclopedia = Ullmann's Encyclopedia of Industrial Chemistry|date = 15 June 2000|publisher = Wiley-VCH|doi = 10.1002/14356007.a04_309|isbn = 3-527-30673-0}} 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.{{cite book|chapter-url = https://books.google.com/books?id=JEcSmEKtoT4C&pg=PT1847|series = Handbook of Reagents for Organic Synthesis|volume = 11|title = Reagents for Radical and Radical Ion Chemistry|editor-last = Crich|editor-first = David|year = 2008|publisher = John Wiley & Sons|isbn = 978-0-470-06536-5|chapter = Enoxytriethylborates and Enoxydiethylboranes|access-date = 2019-01-27|archive-date = 2022-02-19|archive-url = https://web.archive.org/web/20220219194543/https://books.google.com/books?id=JEcSmEKtoT4C&pg=PT1847|url-status = live}} 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, {{chem|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.{{cite journal|title = Highly regioselective generation of "thermodynamic" enolates and their direct characterization by NMR|first1 = Ei-ichi|last1 = Negishi|authorlink1 = Ei-ichi Negishi|first2 = Sugata|last2 = Chatterjee|journal = Tetrahedron Letters|volume = 24|issue = 13|year = 1983|pages = 1341–1344|doi = 10.1016/S0040-4039(00)81651-2}} The Et stands for ethyl group {{chem2|CH3CH2\s}}.

File:2-Methylcyclohexanone to 2,2- and 2,6-dimethylcyclohexanone.png

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.{{cite encyclopedia|first1 = Yoshinori|last1 = Yamamoto|first2 = Takehiko|last2 = Yoshimitsu|first3 = John L.|last3 = Wood|authorlink3 = John L. Wood|first4 = Laura Nicole|last4 = Schacherer|title = Triethylborane|encyclopedia = Encyclopedia of Reagents for Organic Synthesis|doi = 10.1002/047084289X.rt219.pub3|date = 15 March 2007|publisher = Wiley|isbn = 978-0-471-93623-7}}

Triethylborane is the precursor to the reducing agents lithium triethylborohydride ("Superhydride") and sodium triethylborohydride.{{cite book|last1 = Binger|first1 = P.|last2 = Köster|first2 = R.| chapter=Sodium Triethylhydroborate, Sodium Tetraethylborate, and Sodium Triethyl-1-Propynylborate |title = Inorganic Syntheses|series = Inorganic Syntheses|year = 1974|volume = 15|pages = 136–141|doi = 10.1002/9780470132463.ch31|isbn = 978-0-470-13246-3}}

: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.{{cite journal|title = A Novel Method for the In situ Generation of Alkoxydialkylboranes and Their Use in the Selective Preparation of 1,3-syn Diols|last1 = Chen|first1 = Kau-Ming|last2 = Gunderson|first2 = Karl G.|last3 = Hardtmann|first3 = Goetz E.|last4 = Prasad|first4 = Kapa|last5 = Repic|first5 = Oljan|last6 = Shapiro|first6 = Michael J.|journal = Chemistry Letters|year = 1987|volume = 16|issue = 10|pages = 1923–1926|doi-access = free|doi = 10.1246/cl.1987.1923}}{{Cite book|first = Jaemoon|last = Yang|chapter-url = https://books.google.com/books?id=A_vUdr6ABGIC&pg=PA151|chapter = Diastereoselective Syn-Reduction of β-Hydroxy Ketones|title = Six-Membered Transition States in Organic Synthesis|year = 2008|publisher = John Wiley & Sons|pages = 151–155|isbn = 978-0-470-19904-6|access-date = 2019-01-27|archive-date = 2022-02-19|archive-url = https://web.archive.org/web/20220219194529/https://books.google.com/books?id=A_vUdr6ABGIC&pg=PA151|url-status = live}}

Triethylborane is strongly pyrophoric, with an autoignition temperature of {{convert|−20|C}},{{Cite web |url=http://www.engineeringtoolbox.com/fuels-ignition-temperatures-d_171.html |title=Fuels and Chemicals - Autoignition Temperatures |access-date=2017-08-26 |archive-date=2015-05-04 |archive-url=https://web.archive.org/web/20150504022056/http://www.engineeringtoolbox.com/fuels-ignition-temperatures-d_171.html |url-status=live }} 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.{{Cite web |url=https://www.sigmaaldrich.com/MSDS/MSDS/DisplayMSDSPage.do?country=US&language=en&productNumber=257192&brand=ALDRICH&PageToGoToURL=https%3A%2F%2Fwww.sigmaaldrich.com%2Fcatalog%2Fproduct%2Faldrich%2F257192%3Flang%3Den |title=Archived copy |access-date=2020-09-26 |archive-date=2022-02-19 |archive-url=https://web.archive.org/web/20220219194531/https://www.sigmaaldrich.com/US/en/sds/ALDRICH/257192 |url-status=live }}

• Organoboranes
• Pentaborane
• Zip fuel

{{Reflist}}{{Boron compounds}}

Category:Alkylboranes

Category:Rocket fuels

Category:Pyrophoric materials