Trihydridoboron, also known as borane or borine, is an unstable and highly reactive molecule with the chemical formula BH
3. The preparation of borane carbonyl, BH3(CO), played an important role in exploring the chemistry of boranes, as it indicated the likely existence of the borane molecule. However, the molecular species BH3 is a very strong Lewis acid. Consequently it is highly reactive and can only be observed directly as a continuously produced, transitory, product in a flow system or from the reaction of laser ablated atomic boron with hydrogen.
|Systematic IUPAC name
3D model (JSmol)
|Molar mass||13.83 g·mol−1|
|187.88 kJ mol−1 K−1|
Std enthalpy of
|106.69 kJ mol−1|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Structure and propertiesEdit
- BX3 +BH4− → HBX3− + (BH3) (X=F, Cl, Br, I)
- 2 BH3 → B2H6
The standard enthalpy of dimerization of BH3 is estimated to be −170 kJ mol−1. The boron atom in BH3 has 6 valence electrons. Consequently it is a strong Lewis acid and reacts with any Lewis base, L to form an adduct.
- BH3 + L → L—BH3
in which the base donates its lone pair, forming a dative covalent bond. Such compounds are thermodynamically stable, but may be easily oxidised in air. Solutions containing borane dimethylsulfide and borane–tetrahydrofuran are commercially available; in tetrahydrofuran a stabilising agent is added to prevent the THF from oxidising the borane. A stability sequence for several common adducts of borane, estimated from spectroscopic and thermochemical data, is as follows:
- PF3 < CO < Et2O < Me2O < C4H8O < C4H8S < Et2S < Me2S < Py < Me3N < H−
3 + 3H
2O → B(OH)
3 + 3 H
- B2H6 ⇌ 2BH3
- BH3 +B2H6 → B3H7 +H2 (rate determining step)
- BH3 + B3H7 ⇌ B4H10
- B2H6 + B3H7 → BH3 + B4H10
- ⇌ B5H11 + H2
Further steps give rise to successively higher boranes, with B10H14 as the most stable end product contaminated with polymeric materials, and a little B20H26.
- (THF)BH3 + 3 CH2=CHR → B(CH2CH2R)3 + THF
This reaction is regioselective, Other borane derivatives can be used to give even higher regioselectivity. The product trialkylboranes can be converted to useful organic derivatives. With bulky alkenes one can prepare species such as [HBR2]2, which are also useful reagents in more specialised applications. Borane dimethylsulfide which is more stable than borane–tetrahydrofuran may also be used.
Hydroboration can be coupled with oxidation to give the hydroboration-oxidation reaction. In this reaction, the boryl group in the generated organoborane is substituted with a hydroxyl group.
Reductive amination is an extension of the hydroboration-oxidation reaction, wherein a carbon–nitrogen double bond is undergoing hydroboration. The carbon–nitrogen double bond is created by the reductive elimination of water from a hemiaminal, formed by the addition of an amine to a carbonyl molecule, hence the adjective 'reductive'.
Borane(5) is the dihydrogen complex of borane. Its molecular formula is BH5 or possibly BH3(η2-H2). It is only stable at very low temperatures and its existence is confirmed in very low temperature. Borane(5) and methanium (CH5+) are isoelectronic. Its conjugate base is the borohydride anion.
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