Borane

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Borane

Structural formula of borane





Ball-and-stick model of borane


Spacefill model of 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





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 D3h 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 reads 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
+ 3H2O → 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





  1. ^ Burg, Anton B.; Schlesinger, H. I. (May 1937). "Hydrides of boron. VII. Evidence of the transitory existence of borine ({{Chem|BH|3}}): Borine carbonyl and borine trimethylammine". Journal of the American Chemical Society. 59 (5): 780–787. doi:10.1021/ja01284a002..mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"""""""'""'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}


  2. ^ Tague, Thomas J.; Andrews, Lester (1994). "Reactions of Pulsed-Laser Evaporated Boron Atoms with Hydrogen. Infrared Spectra of Boron Hydride Intermediate Species in Solid Argon". Journal of the American Chemical Society. 116 (11): 4970–4976. doi:10.1021/ja00090a048. ISSN 0002-7863.


  3. ^ Kawaguchi, Kentarou (1992). "Fourier transform infrared spectroscopy of the BH3 ν3 band". The Journal of Chemical Physics. 96 (5): 3411. doi:10.1063/1.461942. ISSN 0021-9606.


  4. ^ abc Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9.


  5. ^ M. Page, G.F. Adams, J.S. Binkley, C.F. Melius "Dimerization energy of borane" J. Phys. Chem. 1987, vol. 91, pp 2675–2678. doi:10.1021/j100295a001


  6. ^ Hydrocarbon Chemistry, George A. Olah, Arpad Molner, 2d edition, 2003, Wiley-Blackwell
    ISBN 978-0471417828



  7. ^ Finn, Patricia.; Jolly, William L. (August 1972). "Asymmetric cleavage of diborane by water. The structure of diborane dihydrate". Inorganic Chemistry (PDF)|format= requires |url= (help). ACS Publications. 11 (8): 1941–1944. doi:10.1021/ic50114a043.


  8. ^ D'Ulivo, Alessandro (May 2010). "Mechanism of generation of volatile species by aqueous boranes". Spectrochimica Acta Part B: Atomic Spectroscopy. Elsevier B.V. 65 (5): 360–375. doi:10.1016/j.sab.2010.04.010.


  9. ^ Housecroft, C. E.; Sharpe, A. G. (2008). "Chapter 13: The Group 13 Elements". Inorganic Chemistry (3rd ed.). Pearson. p. 336. ISBN 978-0-13-175553-6.


  10. ^ ab Burkhardt, Elizabeth R.; Matos, Karl (July 2006). "Boron reagents in process chemistry: Excellent tools for selective reductions". Chemical Reviews. ACS Publications. 106 (7): 2617–2650. doi:10.1021/cr0406918.


  11. ^ Kollonitisch, J., "Reductive Ring Cleavage of Tetrahydrofurans by Diborane", J. Am. Chem. Soc. 1961, volume 83, 1515. doi: 10.1021/ja01467a056











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