Q4(c)(ii). Borazine (inorganic benzene): preparation and reactivity

1. Structure: a planar hexagon of alternating B and N atoms, all sp², with one H on each atom; each N gives its lone pair to the π system and each B has an empty 2p orbital, giving six π electrons. Isoelectronic and isostructural with C₆H₆.
2. Preparation (Stock, 1926): 3B₂H₆ + 6NH₃ →2B₃N₃H₆ + 12H₂
3. Better route: 3NH₄Cl + 3BCl₃ → B₃N₃H₃Cl₃ + 9HCl (B-trichloroborazine), then reduction with NaBH₄: 2B₃N₃H₃Cl₃ + 6NaBH₄ → 2B₃N₃H₆ + 3B₂H₆ + 6NaCl. Alkylammonium chlorides give N-alkyl derivatives.
4. Similar: both are colourless liquids with planar rings and equal ring bonds; both have six delocalised π electrons.
5. Different – polarity: N is much more electronegative than B, so the π density stays mainly on N; the ring is only partly aromatic and each B–N bond is polar (B δ+, N δ-).
6. Addition: borazine adds 3HCl across the B–N bonds, the electrophile H⁺ going to N and Cl⁻ to B, giving the saturated cyclohexane-like B₃N₃H₉Cl₃: B₃N₃H₆ + 3HCl → B₃N₃H₉Cl₃. It adds water and methanol in the same way and is hydrolysed on heating to boric acid, ammonia and hydrogen. Benzene adds none of these; it keeps its aromatic ring by electrophilic substitution.
7. Substitution: B–Cl bonds of B-trichloroborazine are much more labile than C–Cl bonds of chlorobenzene; Grignard reagents or hydrides replace Cl at boron easily.
8. Overall: borazine is more reactive and less stable than benzene; it is a cyclic analogue of an amine-borane rather than a true aromatic.

Application. heating borazine gives polymeric boron nitride, the graphite-like hexagonal BN used as a high-temperature lubricant and crucible material; B–N/C–C isosterism is also used to design BN analogues of aromatic drugs.