Borazine

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. 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 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: