Borazine

Borazine is an inorganic aromatic compound formed as an azo-derivative of boron. Borazine is also named ‘borazole’ as it contains azole groups in its heterocyclic ring structure. This cyclic structure of the borazine exemplifies the inorganic benzene in chemistry. It is not the same as benzene, though it highly resembles it in its structure and some chemical properties. The derivatives of borazine have commercial uses. Due to its resemblance with organic benzene, the compound borazine itself is widely used for research and studies of its use as a commercial product.

What is Borazine?

Borazine is a chemical compound highly similar to benzene. Thus, they are termed ‘inorganic benzene’ due to the presence of inorganic compounds in their structure in the form of a benzene ring.

Structure of borazine

The structure of borazine is as follows.

• It is an aromatic heterocyclic ring containing six members as part of the ring, which forms a hexagonal ring structure
• The chemical molecular formula of borazine is B3H6N3
• The ring has three BH and NH groups forming 12σ and 3π bonds
• Though the structure is similar to benzene, borazine does not form a perfect hexagon like benzene. The bond angles of the borazine heterocyclic ring differ as per the members of the ring
• In a borazine ring, the bond angle at the borons is 117.1°, and the bond angle at the nitrogen atoms is 122°
• Though the bond angle differs, the bond length is the same in the borazine ring, just like the benzene ring. The bond length in the borazine ring is 1.429Å
• The bond length does not differ because the BH and NH atoms are alternatively present as consecutive members. The bond length between B (boron) of the BH and N (nitrogen) of the NH remains the same

Physical properties of borazine compounds

• They are colourless, volatile liquids having an aromatic odour
• They have a boiling point of about 64.5 °C and a melting point of about -58 °C
• Their molecular weight ranges about 78g

Chemical properties of borazine

The chemical properties of the borazine ring are similar to benzene but not the same. Their chemical properties are given below.

Isoelectronic and isomorphous with benzene 

• Borazine has similar structural and same electron configurations as benzene. Thus, they are isoelectronic with benzene
• Borazine has the same structure as a hexagonal ring as benzene. Thus, they are isostructural with each other

Electronegativity and polarity of borazine

• The atomic number of boron is 5, whereas the atomic number of nitrogen is 7. 
• Nitrogen tends to have a higher number of electrons in its valence shell (5) than boron (3)
• Thus, the nuclear attraction in nitrogen is higher, making the atom compact and tough to remove the electron from the valence shell
• Nitrogen has space for accepting three electrons in its valence shell and boron tends to give away three electrons from its valence shell
• The nuclear attraction in boron is lesser than the nitrogen atom, yet it is reasonably electronegative
• On the Pauling scale, the electronegativity of boron is 2.04, and nitrogen is 3.04
• The high electronegativity difference between boron and nitrogen makes borazine a highly polar molecule

Mesomerism

• Borazine exhibits mesomerism due to the electron availability in the nitrogen atom and deficiency in the boron atom
• They tend to shift their electrons to form double bonds, resulting in an ionic molecule of borazine

Lewis acid and base

The borazine molecule is highly electronegative and polar due to the presence of 

• The boron is a Lewis acid, accepting the free electrons and
• Nitrogen as Lewis base, donating the free electrons to obtain molecular stability. 

Chemical reactions of borazine

The chemical properties of borazine contribute to the chemical reactions undergone by the molecule.

Hydrolysis 

• Borazine, being a polar molecule, readily undergoes hydrolysis
• On dissolving with water, borazine yields boric acid, ammonia and hydrogen

B3H6N3 + H2O → H3BO3 + NH3 + H2 

(an approximate depiction of the chemical reaction; stoichiometric numbers not included)

Polymerisation 

• Borazine is polymerised into polypropylene by heating it at 70 °C
• The polymer is formed when borazine loses some hydrogen atoms from its sides

n B3N3H6 → 1/n[B3N3H4]n

Synthesis of borazine 

A reaction between boron trichloride and ammonium chloride efficiently yields borazine as a 2-step process.

Boron trichloride with ammonium chloride yields trichloroborazine.

3 BCl3 + 3 NH4Cl → Cl3B3H3N3 + 9 HCl

On further reduction with sodium borohydride, it yields borazine.

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

• An alternatively efficient way to synthesise borazine is with sodium borohydride and ammonium sulphate, which is a single-step process

6 NaBH4 + 3 (NH4)2SO4 → 2 B3N3H6 + 3 Na2SO4 + 18 H2

Conclusion

Borazine is an inorganic aromatic compound that is quite similar to benzene in its structure and few electrochemical properties. Borazine is a 6-membered heterocyclic ringed compound that is colourless and volatile, having an aromatic fragrance. The borazine molecule weighs about 78g. The general molecular formula of borazine is B3H6N3. It has 3 NH and 3 BH alternating in its ring structure. It is a highly polar compound due to the difference in the electronegativity of the members of the cyclic ring. The electronegativity of boron is 2.04, and nitrogen is 3.04, as per the Pauling scale. This article covered examples of borazine.

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