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Borane

Names
Systematic IUPAC name borane (substitutive) trihydridoboron (additive)
Other names borine boron trihydride
Identifiers
CAS Number	13283-31-3
3D model (JSmol)	Interactive image
ChEBI	CHEBI:30149
ChemSpider	6091
Gmelin Reference	44
PubChem CID	6331
InChI InChI=1S/BH3/h1H3 Key: UORVGPXVDQYIDP-UHFFFAOYSA-N
SMILES B
Properties
Chemical formula	BH3
Molar mass	13.83 g·mol−1
Appearance	colourless gas
Conjugate acid	Boronium
Thermochemistry
Std molar entropy (So298)	187.88 kJ mol−1 K−1
Std enthalpy of formation (ΔfHo298)	106.69 kJ mol−1
Structure
Point group	D3h
Molecular shape	trigonal planar
Dipole moment	0 D
Related compounds
Related compounds	diborane
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

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. ^[1] 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.^[2]

Structure and properties

BH₃ is trigonal planar molecule with D_3h symmetry The experimentally determined B–H bond length is 119 pm.^[3]

In the absence of other chemical species, it reacts with itself to form diborane. Thus, it is an intermediate in the preparation of diborane according to the reaction:^[4]

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.^[5]
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.^[6] 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⁻

BH₃ has some soft acid characteristics as sulfur donors form more stable complexes than do oxygen donors.^[4] Aqueous solutions of BH₃ are extremely unstable. ^[7][8]

BH
₃ + 3 H₂O → B(OH)
₃ + 3 H
₂

Reactions

Molecular BH₃ is believed to be a reaction intermediate in the pyrolysis of diborane to produce higher boranes:^[4]

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.

Borane ammoniate, which is produced by a displacement reaction of other borane adducts, eliminates elemental hydrogen on heating to give borazine (HBNH)₃.^[9]

Borane adducts are widely used in organic synthesis for hydroboration, where BH₃ adds across the C=C bond in alkenes to give trialkylboranes:

(THF)BH₃ + 3 CH₂=CHR → B(CH₂CH₂R)₃ + THF

This reaction is regioselective, Other borane derivatives can be used to give even higher regioselectivity.^[10] 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.^[11][10]

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'.

References