Borazine

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Borazine
Borazine-dimensions-2D.png
Borazine-elpot-3D-vdW.png Borazine-3D-vdW.png
Identifiers
CAS number 6569-51-3 YesY
PubChem 138768
ChemSpider 122374 YesY
ChEBI CHEBI:33119 YesY
Jmol-3D images Image 1
Properties
Molecular formula B3H6N3
Molar mass 80.50 g/mol
Appearance colourless liquid
Density 0.81 g/cm3
Melting point −58 °C
Boiling point 161 °C; 55 °C at 105 Pa
Hazards
NFPA 704
Flammability code 2: Must be moderately heated or exposed to relatively high ambient temperature before ignition can occur. Flash point between 38 and 93 °C (100 and 200 °F). E.g., diesel fuel Health code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g., chloroform Reactivity code 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g., calcium Special hazards (white): no codeNFPA 704 four-colored diamond
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 YesY (verify) (what is: YesY/N?)
Infobox references

Borazine is an inorganic compound with the chemical formula (BH)3(NH)3. In this cyclic compound, the three BH units and three NH units alternate. The compound is isoelectronic and isostructural with benzene. Like benzene, borazine is a colourless liquid.[1] For this reason borazine is sometimes referred to as "inorganic benzene".

Synthesis[edit]

The compound was reported in 1926 by the chemists Alfred Stock and Erich Pohland by a reaction of diborane with ammonia.[2]

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:

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:

2 Cl3B3H3N3 + 6 NaBH4 → 2 B3H6N3 + 3 B2H6 + 6 NaCl

Properties[edit]

Borazine is a colourless liquid with an aromatic smell. In water it hydrolyzes to boric acid, ammonia, and hydrogen. Borazine, with a standard enthalpy change of formation ΔHf of −531 kJ/mol, is thermally very stable.

Structure and bonding[edit]

Borazine is isostructural with benzene. The six B-N bonds have length of 1.436 Å. The carbon–carbon bond in benzene is shorter length at 1.397 Å. The boron–nitrogen bond length 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.

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.

Borazin Mesomers.png

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

Reactions[edit]

Although often compared with benzene, borazine is far more reactive. With hydrogen chloride it forms an adduct, whereas benzene is unreactive toward HCl.

Polyborazylene
B3N3H6 + 3 HCl → B3N3H9Cl3
Addition reaction of borazine with hydrogen chloride
B3N3H9Cl3 + NaBH4 → (BH4N)3
reduction with sodium borohydride

The addition reaction with bromine does not require a catalyst. Borazines undergo nucleophilic attack at boron and electrophilic attack at nitrogen. Heating borazine at 70 °C expels 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. 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.

synthetic route to boron carbonitrides, first step a hydroboration reaction to an oligomeric precursor followed by step two: pyrolysis

Polyborazylene has been proposed as a recycled hydrogen storage medium for hydrogen fuel cell vehicle applications, using a "single pot" process for digestion and reduction to recreate ammonia borane.[3]

Among other B-N type compounds mixed amino-nitro substituted borazines have been predicted to outperform carbon based explosives such as CL-20.[4]

See also[edit]

References[edit]

  1. ^ Duward Shriver; Peter Atkins (2010). Inorganic Chemistry (Fifth ed.). New York: W. H. Freeman and Company. p. 328. ISBN 978-1429218207. 
  2. ^ Stock, Alfred; Pohland, Erich (October 1926). "Borwasserstoffe, VIII. Zur Kenntnis des B2H6 und des B5H11" [Boric acid solution, VIII Regarding knowledge of B2H6 and B5H11]. Berichte (in German) 59 (9): 2210–2215. doi:10.1002/cber.19260590906. 
  3. ^ Davis, B. L.; Dixon, D. A.; Garner, E. B.; Gordon, J. C.; Matus, M. H.; Scott, B.; Stephens, F. H. (2009). "Efficient Regeneration of Partially Spent Ammonia Borane Fuel". Angewandte Chemie International Edition 48 (37): 6812–6816. doi:10.1002/anie.200900680. PMID 19514023.  edit
  4. ^ Koch, E.-C; Klapötke, T. M. (2012). Boron-Based High Explosives. Propellants, Explosives, Pyrotechnics 37. pp. 335–344. doi:10.1002/prep.201100157.