Boron Nitride (Inorganic Graphite)

Much of B—N chemistry relates that a BN unit isoelectronic with the CC unit. The electronegativity and atomic radius of carbon are intermediate between boron and nitrogen. Therefore there are many azoborane analogues of hydrocarbon in which a pair of carbon atoms replace B and N unit. Boron Nitride an important ceramic material that is isoelectronic with graphite. Boron Nitride has a similar sheet-like structure except that rings are aligned in an eclipsed fashion rather than staggered in graphite. Hexagonal Boron Nitride is colourless, electrically nonconducting, and is more resistant to chemical attack than graphite.

Preparation of Boron Nitride 

Boron reacts with N or NH₃ on heating to form a binary nitride BN which is a white refractory solid. This compound is also obtained by fusing borax with NH₄Cl.

Na₂B₄O₅(OH)₄ + 2NH₄Cl -----> 2NaCl + 2BN + 2B₂O₃ + 6H₂O

Pure Boron Nitride is best obtained by heating boron amide. This decomposes first to boron imide and finally into Boron Nitride.

2B(NH₂)₃ -----> B₂(NH)₃ + 3NH₃

                 B₂(NH)₃ ------> 2BN + NH₃

Boron Nitride is also prepared by heating borazine--

B₃N₃H₆ ------> 3BN + 3H₂

Property of Boron Nitride

Boron Nitride is a white powder of density 2.34. It melts under pressure at 3000 ⁰C. It is a high melting refractory solid, very stable, and unreactive. It is unchanged by aqueous acids or alkalis, Cl₂, H₂, O_2, etc.

It is decomposed by steam --

2BN + 3H₂O = B₂O₃ + 2NH₃

It reacts with F₂ and HF at a lower temperature --

2BN + 3F₂ = 2BF₃ + N₂

BN + 4HF = NH₄BF₄

It is decomposed by alkali on fusion --

BN + KOH + H₂O = KBO₂ + NH₃

Structure of Boron Nitride

One B atom and one N atom together have the same no of valence electron as two carbon atoms. Thus Boron Nitride has a graphite-like layer structure. The layers are made up of hexagonal rings of alternate B and N atoms joined together. The B—N distance (1.45 A) in Boron Nitride is comparable to the C—C distance (1.42 A) in graphite. The layer spacing in Boron Nitride and graphite is also comparable. Both have the same density. This form of Boron Nitride is often called Inorganic Graphite. Each B and N is sp² hybridized as each six C-atom in graphite. The three sp² hybrid orbitals of each B overlaps with hybrid orbitals of three neighboring N atoms. The unhybridized p-orbital then accepts a lone pair forming a π-bond. Through resonance, all the B—N bonds become equivalent in Boron Nitride.

Structure of Boron Nitride and Graphite

Uses of Boron Nitride 

Boron Nitride possesses the same hardness as diamond and can withstand a temperature of more than 3000 ⁰C. Due to this property Boron Nitride is used for coating crucible linings, it is also used as a lubricant.