UPSC Civil Services (Mains), Chemistry optional · 2020 · Paper I

Q7(b)(i). Borazine, the “inorganic benzene”

Question

What is inorganic benzene? Write down its structural formula.

Borazine, B₃N₃H₆: a planar six-membered ring of alternating B and N atoms, each carrying one H; it is isoelectronic and isostructural with benzene.

a flat hexagon of three BH and three NH units alternating; all B–N bonds are equal (between single and double), and each ring atom is sp². The N lone pairs are delocalised into the empty B 2p orbitals: a 6π-electron ring, as…
N is more electronegative than B, so the π cloud is polarised towards N (B^(δ+), N^(δ-)); borazine is much more reactive and adds HCl or H₂O across the B=N bonds (B gets Cl or OH, N gets H).
3B₂H₆ + 6NH₃ → 2B₃N₃H₆ + 12H₂ on heating; or NH₄Cl with BCl₃ to give B₃N₃H₃Cl₃, reduced by NaBH₄.

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

Structure: a flat hexagon of three BH and three NH units alternating; all B–N bonds are equal (between single and double), and each ring atom is sp². The N lone pairs are delocalised into the empty B 2p orbitals: a 6π-electron ring, as in benzene.
Difference from benzene: N is more electronegative than B, so the π cloud is polarised towards N (B^(δ+), N^(δ-)); borazine is much more reactive and adds HCl or H₂O across the B=N bonds (B gets Cl or OH, N gets H).
Preparation: 3B₂H₆ + 6NH₃ → 2B₃N₃H₆ + 12H₂ on heating; or NH₄Cl with BCl₃ to give B₃N₃H₃Cl₃, reduced by NaBH₄.

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