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Borazine



Borazine
IUPAC name borazine
Other names borazol, inorganic benzene
Identifiers
CAS number 6569-51-3
SMILES B1NBNBN1
Properties
Molecular formula B3N3H6
Molar mass 80.50g mol−1
Appearance colourless liquid
Density 0.81 g cm−3
Melting point

−58 °C

Boiling point

55 °C

Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

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. [1] 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.

Additional recommended knowledge

Contents

Synthesis

Borazine is synthesized from diborane and ammonia in a 1:2 ratio at 250 - 300 °C with a conversion of 50%.

3 B2H6 + 6 NH3 → 2 B3H6N3 + 12 H2

An alternative more efficient route begins with lithium borohydride and ammonium chloride with improved chemical yield:

3 LiBH4 + 3 NH4Cl → B3H6N3 + 3 LiCl + 9 H2

In a two-step process to borazine, boron trichloride is first converted to trichloroborazine:

3 BCl3 + 3 NH4Cl → Cl3B3H3N3 + 9 HCl

The B-Cl bonds are subsequently converted to B-H bonds:

Cl3B3H3N3 + 3 NaBH4 → B3H6N3 + 3/2 B2H6 + 3 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 ΔHf 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 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 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.

Boron is the Lewis acid and nitrogen is the Lewis base.

Reactions

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.

 

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 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 para fashion by new boron - nitrogen bonds.

Applications

Borazine and borazine derivatives are potential precursors to boron nitride ceramics. 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 on rhodium.


References

  1. ^ (1926) "Boric acid solution, VIII Regarding knowledge of B2H6 and B5H11 author= Stock A., Pohland E journal=Berichte" (59): 2210-2215.
  • 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. Article
  • 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. Abstract
  • New perspectives in boron-nitrogen chemistry - I P. Paetzold Pure & Appl. Chern., Vol. 63, No. 3, pp. 345-350, 1991. Article
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Borazine". A list of authors is available in Wikipedia.
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