Diborane

Diborane, B₂H₆, is the simplest of the boron hydrides (boranes).

Preparation. Industrially, diborane is made exactly as described in Section 2.2.3: treating boron trifluoride with sodium hydride around 450 K, trapping the product immediately to prevent further pyrolysis. On a small scale it can also be made by reacting iodine with sodium borohydride in diglyme: 2NaBH₄ + I₂ → B₂H₆ + 2NaI + H₂. Heating magnesium boride with HCl gives a mixture of volatile boranes, from which diborane can be isolated: 2Mg₃B₂ + 12HCl → 6MgCl₂ + B₄H₁₀ + H₂, and B₄H₁₀ + H₂ → 2B₂H₆.

Properties. Boranes are colourless, diamagnetic compounds of low thermal stability. Diborane is a sweet-smelling, extremely toxic, highly reactive gas at room temperature. On heating it converts into progressively higher boranes, liberating hydrogen at each step -- for example 5B₂H₆ →(388 K, sealed tube)→ 2B₅H₁₁ + 4H₂, and further reactions at other temperatures give B₄H₁₀, B₁₀H₁₄, B₅H₉ (two different routes) and, ultimately, decomposition to the elements: B₂H₆ →(red hot)→ 2B + 3H₂.

Diborane reacts with water and alkali to give boric acid and metaborate respectively: B₂H₆ + 6H₂O → 2H₃BO₃ + 6H₂ and B₂H₆ + 2NaOH + 2H₂O → 2NaBO₂ + 6H₂. At room temperature, pure diborane does not react with air or oxygen, but the impure gas ignites, giving boron trioxide and a large release of heat: B₂H₆ + 3O₂ → B₂O₃ + 3H₂O, ΔH = -2165 kJ mol⁻¹. With methyl alcohol, it gives trimethyl borate: B₂H₆ + 6CH₃OH → 2B(OCH₃)₃ + 6H₂.

Hydroboration. Diborane adds across alkenes and alkynes in an ether solvent at room temperature -- a reaction called hydroboration, widely used in synthetic organic chemistry, particularly to achieve anti-Markovnikov addition: B₂H₆ + 6RCH=CHR → 2(RCH₂-CHR)₃B.

Reaction with ionic hydrides. Treated with metal hydrides, diborane gives metal borohydrides: B₂H₆ + 2LiH →(ether)→ 2LiBH₄ and B₂H₆ + 2NaH →(diglyme)→ 2NaBH₄.

Reaction with ammonia. With excess ammonia at low temperature, diborane forms a diammoniate adduct; on further heating to a high temperature in a closed vessel, this converts into borazole (borazine), often called 'inorganic benzene' for its six-membered B₃N₃ ring: 3B₂H₆ + 6NH₃ →(-153 K)→ 3(B₂H₆.2NH₃), then 3(B₂H₆.2NH₃) →(high temp, closed vessel)→ 2B₃N₃H₆ + 12H₂. …