Borazine
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215314514
2008-05-27T17:34:42Z
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{{Chembox new
| ImageFileL1 = Borazine-dimensions-2D.png
| ImageSizeL1 = 150 px
| ImageFileR1 = Borazine-3D-vdW.png
| ImageSizeR1 = 150 px
| IUPACName = borazine
| OtherNames = borazol, inorganic benzene
| Section1 = {{Chembox Identifiers
| CASNo = 6569-51-3
| PubChem =
| SMILES = B1NBNBN1
}}
| Section2 = {{Chembox Properties
| Formula = B<sub>3</sub>N<sub>3</sub>H<sub>6</sub>
| MolarMass = 80.50 g/mol
| Appearance = colourless liquid
| Density = 0.81 g/cm<sup>3</sup>
| MeltingPt = −58 °C
| BoilingPt = 55 °C
| Solubility =
}}
| Section3 = {{Chembox Hazards
| MainHazards =
| FlashPt =
| Autoignition =
}}
}}
'''Borazine''' is an [[inorganic compound]] composed of the elements [[boron]], [[nitrogen]] and [[hydrogen]]. In this [[cyclic compound]] three hydroborane (BH) units and three amino units (NH) alternate.
The compound was synthesised in 1926 by the chemists [[Alfred Stock]] and Pohland by a reaction of [[diborane]] with [[ammonia]]. <ref>{{cite journal| title= Boric acid solution, VIII Regarding knowledge of B2H6 and B5H11
author= Stock A., Pohland E
journal=[[Berichte]] |issue=59|pages= 2210–2215 |year= 1926 }}</ref> The structure is [[isoelectronic]] and [[isostructural]] with [[benzene]] and for this reason borazine is called '''inorganic benzene''' by a proposal of Nils Wiberg and the compound also goes by the name of '''borazol''' from the German name for benzene which is benzol.
== Synthesis ==
Borazine is synthesized from [[diborane]] and [[ammonia]] in a 1:2 ratio at 250 - 300 [[°C]] with a [[conversion (chemistry)|conversion]] of 50%.
:3 B<sub>2</sub>H<sub>6</sub> + 6 NH<sub>3</sub> → 2 B<sub>3</sub>H<sub>6</sub>N<sub>3</sub> + 12 H<sub>2</sub>
An alternative more efficient route begins with [[lithium borohydride]] and [[ammonium chloride]] with improved chemical yield:
:3 LiBH<sub>4</sub> + 3 NH<sub>4</sub>Cl → B<sub>3</sub>H<sub>6</sub>N<sub>3</sub> + 3 LiCl + 9 H<sub>2</sub>
In a two-step process to borazine, [[boron trichloride]] is first converted to trichloroborazine:
:3 BCl<sub>3</sub> + 3 NH<sub>4</sub>Cl → Cl<sub>3</sub>B<sub>3</sub>H<sub>3</sub>N<sub>3</sub> + 9 [[hydrogen chloride|HCl]]
The B-Cl bonds are subsequently converted to B-H bonds:
:Cl<sub>3</sub>B<sub>3</sub>H<sub>3</sub>N<sub>3</sub> + 3 NaBH<sub>4</sub> → B<sub>3</sub>H<sub>6</sub>N<sub>3</sub> + 3/2 B<sub>2</sub>H<sub>6</sub> + 3 [[Sodium chloride|NaCl]]
==Properties==
Borazine is a colourless liquid with an aromatic smell. In water it decomposes to [[boric acid]], ammonia, and hydrogen. Borazine, with a [[standard enthalpy change of formation]] ΔH<sub>f</sub> of -531 kJ/mol, is thermally very stable.
===Structure===
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 [[nanometre|nm]], the carbon carbon bond in benzene has a length of 0.1397 nm. The boron nitrogen bond is between that of the boron nitrogen [[covalent bond|single bond]] with 0.151 nm and the boron nitrogen [[double bond]] which is 0.131 nm. This suggests partial delocalisation of nitrogen [[lone pair]] electrons.
===Mesomers ===
The [[electronegativity]] of boron (2.04 on the [[Pauling scale]]) compared to that of nitrogen (3.04) and also the electron deficiency on the boron [[atom]] and the lone pair on nitrogen favor alternative [[mesomer]] structures for borazine.
[[Image:Borazin Mesomers.png|center|600px]]
Boron is the [[Lewis acid]] and nitrogen is the [[Lewis base]].
==Reactions==
{{Cleanup-section|date=March 2008}}
Borazine is more reactive than benzene. It reacts with [[hydrogen chloride]] in an [[addition reaction]]. If borazine were truly aromatic like benzene this reaction would not occur without a Lewis acid catalyst. <!-- now what exactly is benzene supposed to do with HCl and a LA catalyst?-->
[[Image:Polyborazylene Polymer.png|thumb|right|Polyborazylene]]
:B<sub>3</sub>N<sub>3</sub>H<sub>6</sub> + 3HCl → B<sub>3</sub>N<sub>3</sub>H<sub>9</sub>Cl<sub>3</sub>
:<small>Addition reaction of borazine with hydrogen chloride</small>
:B<sub>3</sub>N<sub>3</sub>H<sub>9</sub>Cl<sub>3</sub> + NaBH<sub>4</sub> → (BH<sub>4</sub>N)<sub>3</sub>
:<small>reduction with [[sodium borohydride]]</small>
The addition reaction with [[bromine]] takes place without [[catalyst]]. Borazines interact with nucleophilic attack at boron and electrophilic attack at nitrogen. Heating borazine at 70 [[°C]] expulses hydrogen with formation of a '''borazinyl''' [[polymer]] or '''polyborazylene''' in which the monomer units are coupled in a [[arene substitution patterns|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''':
[[Image:Boroncaronitrides.gif|center|synthetic route to boron carbonitrides, first step a hydroboration reaction to an oligomeric precursor followed by step two: pyrolysis]]
Borazine can also be used as a precursor to grow boron nitride thin films on surfaces, such as the [[nanomesh]] structure which is formed on [[rhodium]].
{{commonscat|borazine}}
== External links ==
* [http://www.npi.gov.au/database/substance-info/profiles/15.html Australian National Pollutant Inventory: Boron and compounds]
==References==
{{reflist}}
* ''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]
[[Category:Boron compounds]]
[[Category:Heterocyclic compounds]]
[[Category:Inorganic polymers]]
[[ar:بورازين]]
[[de:Borazin]]
[[it:Borazina]]
[[lb:Borazin]]
[[ja:ボラジン]]
[[pl:Borazol]]
[[ru:Боразол]]
[[zh:环硼氮烷]]