Preparation, Properties and Structure of Borazine or Borazole
Borazine, also known as borazole, is a polar inorganic compound with the chemical formula B3N3H6. The compound is isoelectronic and isostructural with benzene. For this reason borazine is sometimes referred to as “inorganic benzene.
Preparation of Borazine or Borazole:
Borazine can be prepared by the following methods-
1. Stock and Pohland's Method
Borazine was originally prepared by Stock and Pohland, in 1926, by the action of ammonia on diborane. The adduct, B2H6.2NH3, is first formed Which then gets decomposed by heating in a closed tube at 200°C.
3B2H6 + 6NH3 → 3[B2H6.2NH3] ---200°C → 2B3N3H6 + 12H2O
This method gives low yield of borazine because of simultaneous formation of solid polymeric by-products.
2. Laboratory Method
B3N3H6 can be prepared in the laboratory by heating a mixture of LiBH4 and NH4Cl in vacuum at 230°C. This method gives 30% borazine.
3NH4Cl + 3LiBH4 ---230°C → B3N3H6 + 3LiCl + 9H2.
3. Synthesis Using Sodium Borohydride:
A more efficient synthesis involves sodium borohydride and ammonium sulfate:
6NaBH4 + 3(NH4)2SO4 → 2B3N3H6 + 3Na2SO4 + 18H2
Properties of Borazine or Borazole:
1. It is colorless liquid with an aromatic odor.
2. It shows unsaturated reaction.
B3N3H6 + 3HCl → B3N3H9Cl3
B3N3H6 + 3Br2 → B3N3H6Br6
B3N3H6 + 3H2O → B3N3H12O3
3. It forms two series of methyl derivatives-
a. N- methyl derivatives
b. B- methyl derivatives
4. It is isoelectronic with benzene.
5. When borazine is pyrolysed above 340°C, B3N6H10 and B5N5H8 are produced. These products are boron- nitrogen analogues of diphenyl and naphthalene respectively.
6. Borazine gets slowly hydrolysed by water to produce boric acid, ammonia and Hydrogen.Hydrolysis is favoured by the increase in temperature.
B3N3H6 + H2O → H3BO3 + NH3 + H2
Structure of Borazine or Borazole:
The molecule has a D3h symmetry point group, with bond angles of approximately 117.1° at boron atoms and 122.9° at nitrogen atoms. In Borazine both Boron and Nitrogen are sp2 hybridised. Each N-atom has one lone pair of electrons, while each B-atom has an empty p-orbital. (B-N) bond in borazine is a Dative bond, which arises from the sidewise overlapping between the filled p-orbitals of N-atom and empty p-orbitals of B-atom. In this cyclic compound, the three BH units and three NH units are alternate to each-other. It is isoelectronic with benzene (C6H6) and exhibits similar connectivity.