- Borazine
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IUPACName = borazine
OtherNames = borazol, inorganic benzene
Section1 = Chembox Identifiers
CASNo = 6569-51-3
PubChem =
SMILES = B1NBNBN1
Section2 = Chembox Properties
Formula = B3N3H6
MolarMass = 80.50 g/mol
Appearance = colourless liquid
Density = 0.81 g/cm3
MeltingPt = −58 °C
BoilingPt = 55 °C
Solubility =
Section3 = Chembox Hazards
MainHazards =
FlashPt =
Autoignition =Borazine is an
inorganic compound composed of the elementsboron ,nitrogen andhydrogen . In thiscyclic compound three hydroborane (BH) units and three amino units (NH) alternate.The compound was synthesised in 1926 by the chemistsAlfred Stock and Pohland by a reaction ofdiborane withammonia . [cite journal| title= Boric acid solution, VIII Regarding knowledge of B2H6 and B5H11author= Stock A., Pohland Ejournal=Berichte |issue=59|pages= 2210–2215 |year= 1926 ] The structure isisoelectronic and isostructural withbenzene . For this reason borazine is called inorganic benzene by a proposal of Nils Wiberg, and the compound is also called borazol from the German name for benzene, which is benzol.Synthesis
Borazine is synthesized from
diborane andammonia in a 1:2 ratio at 250 - 300°C with a conversion of 50%.:3 B2H6 + 6 NH3 → 2 B3H6N3 + 12 H2An alternative more efficient route begins withlithium borohydride andammonium chloride with improved chemical yield::3 LiBH4 + 3 NH4Cl → B3H6N3 + 3 LiCl + 9 H2In a two-step process to borazine,
boron trichloride is first converted to trichloroborazine::3 BCl3 + 3 NH4Cl → Cl3B3H3N3 + 9 HClThe B-Cl bonds are subsequently converted to B-H bonds::Cl3B3H3N3 + 3 NaBH4 → B3H6N3 + 3/2 B2H6 + 3 NaClProperties
Borazine is a colourless liquid with an aromatic smell. In water it decomposes to
boric acid , ammonia, and hydrogen. Borazine, with astandard enthalpy change of formation ΔHf of -531 kJ/mol, is thermally very stable.tructure
Borazine is
isostructural with benzene and bond lengths are identical just as in benzene. The distance between boron and nitrogen in the ring is 0.1436 nm. The carbon-carbon bond in benzene has a shorter length of 0.1397 nm. The boron-nitrogen bond length is between that of the boron nitrogen single bond with 0.151 nm and the boron nitrogendouble bond which is 0.131 nm. This suggests partial delocalisation of nitrogenlone pair electrons.Mesomers
The
electronegativity of boron (2.04 on thePauling scale ) compared to that of nitrogen (3.04) and also the electron deficiency on the boronatom and the lone pair on nitrogen favor alternativemesomer structures for borazine.Boron is the
Lewis acid and nitrogen is theLewis base .Reactions
Borazine is more reactive than benzene. It reacts with
hydrogen chloride in anaddition reaction . If borazine were truly aromatic like benzene this reaction would not occur without a Lewis acid catalyst.:B3N3H6 + 3HCl → B3N3H9Cl3:Addition reaction of borazine with hydrogen chloride
:B3N3H9Cl3 + NaBH4 → (BH4N)3:reduction with
sodium borohydride The addition reaction with
bromine takes place withoutcatalyst . Borazines interact with nucleophilic attack at boron and electrophilic attack at nitrogen. Heating borazine at 70°C expels hydrogen with formation of a borazinylpolymer or polyborazylene in which the monomer units are coupled in a para fashion by new boron-nitrogen bonds.Applications
Borazine and borazine derivatives are potential precursors to
boron nitride ceramic s. Boron nitride can be prepared by heating polyborazylene to 1000°C . Borazines are also starting materials for other potential ceramics such as boron carbonitrides:Borazine can also be used as a precursor to grow boron nitride thin films on surfaces, such as the
nanomesh structure which is formed onrhodium .External links
* [http://www.npi.gov.au/database/substance-info/profiles/15.html Australian National Pollutant Inventory: Boron and compounds]
References
* "Polymeric precursors to boron based ceramics" Larry G. Sneddon, Mario G. L. Mirabelli, Anne T. Lynch, Paul J. Fazen, Kai Su, and Jeffrey S. Beckdon Pure & Appl. Chem., Vol. 63, No. 3, pp. 407-410, 1991. [http://iupac.org/publications/pac/1991/pdf/6303x0407.pdf Article (dead)]
* "Synthesis of Novel Amorphous Boron Carbonitride Ceramics from the Borazine Derivative Copolymer via Hydroboration" Jong-Kyu Jeon, Yuko Uchimaru, and Dong-Pyo Kim Inorg. Chem., 43 (16), 4796 -4798, 2004. [http://pubs.acs.org/cgi-bin/abstract.cgi/inocaj/2004/43/i16/abs/ic035254a.html Abstract]
* "New perspectives in boron-nitrogen chemistry - I" P. Paetzold Pure & Appl. Chern., Vol. 63, No. 3, pp. 345-350, 1991. [http://www.iupac.org/publications/pac/1991/pdf/6303x0345.pdf Article]
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