Diborane

Diborane

Names
IUPAC name Diborane(6)
Other names Boroethane Boron hydride Diboron hexahydride
Properties
Chemical formula	B2H6
Molar mass	27.67 g/mol
Appearance	Colorless gas
Odor	Repulsive and sweet
Density	1.131 g/L
Melting point	−92.49 °C (−134.48 °F; 180.66 K)
Boiling point	−164.85 °C (−264.73 °F; 108.30 K)
Solubility in water	Reacts
Solubility	Reacts with alcohols Slightly soluble in diethyl ether, diglyme
Vapor pressure	39.5 atm (16.6 °C)
Thermochemistry
Std molar entropy (So298)	232.1 J·mol-1·K-1
Std enthalpy of formation (ΔfHo298)	36.4 kJ/mol
Hazards
Lethal dose or concentration (LD, LC):
LC50 (Median concentration)	40 ppm (rat, 4 h) 29 ppm (mouse, 4 h) 40–80 ppm (rat, 4 h) 159–181 ppm (rat, 15 min)
Related compounds
Related compounds	Borane
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Diborane is the inorganic chemical compound consisting of boron and hydrogen with the formula B₂H₆. It is a toxic, colorless and pyrophoric gas with a repulsively sweet odor, with a variety of uses in chemistry and industry.

Properties

Chemical

Diborane reacts exothermically with oxygen to form boron trioxide and water:

2 B₂H₆ + 6 O₂ → 2 B₂O₃ + 6 H₂O

Diborane reacts violently with water to form hydrogen and boric acid:

B₂H₆ + 6 H₂O → 2 B(OH)₃ + 6 H₂

It reacts similarly with alcohols, but yields borate esters instead of free acid. Reaction with methanol for example, gives hydrogen and trimethylborate:

B₂H₆ + 6 MeOH → 2 B(OMe)₃ + 6 H₂

Pyrolysis of diborane gives hydrogen and diverse boron hydride clusters. This method can be used to produce a variety of heavier boranes, like decaborane.

Treating diborane with sodium amalgam gives NaBH₄ and Na[B₃H₈].

When diborane is treated with lithium hydride in diethyl ether, lithium borohydride is formed:

B₂H₆ + 2 LiH → 2 LiBH₄

Diborane reacts with anhydrous hydrogen chloride or hydrogen bromide gas to give a boron halohydride:

B₂H₆ + HX → B₂H₅X + H₂ (X = Cl, Br)

Treating diborane with carbon monoxide at 470 K and 20 bar gives borane carbonyl (H₃BCO).

Diborane readily reduces carboxylic acids to the corresponding alcohols, whereas ketones react only sluggishly.

Physical

Diborane is a colorless gas with a terrible odor, which can be pyrophoric in air and will react with water and alcohols.

Availability

Diborane is not sold by lab suppliers, and usually has to be made in situ. Industrial companies may sell the compound in gas tanks, to other companies.

Preparation

A very accessible route relies on the direct reaction of borohydride salts with a non-oxidizing acid, such as phosphoric acid or dilute sulfuric acid.

2 BH₄⁻ + 2 H⁺ → 2 H₂ + B₂H₆

Similarly, oxidation of borohydride salts has been demonstrated and remains convenient for small-scale preparations. For example, using iodine as an oxidizer:

2 NaBH₄ + I₂ → 2 NaI + B₂H₆ + H₂

The industrial synthesis of diborane involves the reduction of boron trifluoride with strong reducing agents, such as sodium hydride, lithium hydride or lithium aluminium hydride:

8 BF₃ + 6 LiH → B₂H₆ + 6 LiBF₄

Two laboratory methods start from boron trichloride with lithium aluminium hydride or from boron trifluoride ether solution with sodium borohydride. Both methods result in as much as 30% yield:

4 BCl₃ + 3 LiAlH₄ → 2 B₂H₆ + 3 LiAlCl₄

4 BF₃ + 3 NaBH₄ → 2 B₂H₆ + 3 NaBF₄

Projects

• Demonstration of pyrophoricity
• Make green fire
• Reagents for hydroboration
• Make borane–tetrahydrofuran adduct
• Synthesis of decaborane

Handling

Safety

Diborane is toxic, pyrophoric and reacts with water.

Storage

Should not be stored for long. Can be kept dissolved in an inert solvent, under argon gas.

Disposal

Should be burned outside.

References

Relevant Sciencemadness threads

• how to prepare diborane in situ?
• B2H6 can be prepared by NaBH4 and dimethyl sulfate?