Wikipedia

Borane

Not to be confused with Boran or bornane.
This article is about a specific compound. For the class of compounds, see boranes. For the edition of the Hebrew Bible known as BH5, see Biblia Hebraica Quinta.

Borane is an inorganic compound with the chemical formula BH
3. Because it tends to dimerize or form adducts, borane is very rarely observed. It normally dimerizes to diborane in the absence of other chemicals.[2] It can be observed directly as a continuously produced, transitory, product in a flow system or from the reaction of laser ablated atomic boron with hydrogen.[3]

Borane
Structural formula of borane
Ball-and-stick model of borane
Spacefill model of borane
Names
IUPAC names
Borane[1]
Systematic IUPAC name
Borane (substitutive)
Trihydridoboron (additive)
Other names
  • borine
  • boron trihydride
  • hydrogen boride
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
44
  • InChI=1S/BH3/h1H3
    Key: UORVGPXVDQYIDP-UHFFFAOYSA-N
  • B
Properties
BH3
Molar mass 13.83 g·mol−1
Appearance colourless gas
Conjugate acid Boronium
Thermochemistry
187.88 kJ mol−1 K−1
106.69 kJ mol−1
Structure
D3h
trigonal planar
0 D
Related compounds
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Structure and properties

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BH3 is a trigonal planar molecule with D3h symmetry. The experimentally determined B–H bond length is 119 pm.[4]

In the absence of other bases, it dimerizes to form diborane. Thus, it is an intermediate in the preparation of diborane according to the reaction:[5]

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.[6] The boron atom in BH3 has 6 valence electrons. Consequently, it is a strong Lewis acid and reacts with any Lewis base ('L' in equation below) to form an adduct:[7]

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.[8] 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.[5] Aqueous solutions of BH3 are extremely unstable.[9][10]

BH
3 + 3H2O → B(OH)
3 + 3 H
2

Reactions

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Molecular species BH3 is a very strong Lewis acid. It can be isolated in the form of various adducts, such as borane carbonyl, BH3(CO).[11]

Molecular BH3 is believed to be a reaction intermediate in the pyrolysis of diborane to produce higher boranes:[5]

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.[12]

Borane adducts are widely used in organic synthesis for hydroboration, where BH3 adds across the C=C bond in alkenes to give trialkylboranes:[13]

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

This reaction is regioselective.[14] Other borane derivatives can be used to give even higher regioselectivity.[15] 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.[16][15]

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.[17]

As a Lewis acid

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Phosphine-boranes, with the formula R3−nHnPBH3, are adducts of organophosphines and borane. Borane adducts with amines are more widely used.[18] Borane makes a strong adduct with triethylamine; using this adduct requires harsher conditions in hydroboration. This can be advantageous for cases such as hydroborating trienes to avoid polymerization. More sterically hindered tertiary and silyl amines can deliver borane to alkenes at room temperature.

 

Borane(5) is the dihydrogen complex of borane. Its molecular formula is BH5 or possibly BH3(η2-H2).[19] It is only stable at very low temperatures and its existence is confirmed in very low temperature.[20][21] Borane(5) and methanium (CH5+) are isoelectronic.[22] Its conjugate base is the borohydride anion.

See also

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References

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  1. ^ "Borane".
  2. ^ Carey, Francis A.; Sundberg, Richard J. (2007). Advanced Organic Chemistry: Part B: Reactions and Synthesis (5th ed.). New York: Springer. p. 337. ISBN 978-0387683546.
  3. ^ 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.
  4. ^ Kawaguchi, Kentarou (1992). "Fourier transform infrared spectroscopy of the BH3 ν3 band". The Journal of Chemical Physics. 96 (5): 3411–3415. Bibcode:1992JChPh..96.3411K. doi:10.1063/1.461942. ISSN 0021-9606.
  5. ^ a b c Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  6. ^ Page, M.; Adams, G.F.; Binkley, J.S.; Melius, C.F. (1987). "Dimerization energy of borane". J. Phys. Chem. 91 (11): 2675–2678. doi:10.1021/j100295a001.
  7. ^ Carey, Francis A.; Sundberg, Richard J. (2007). Advanced Organic Chemistry: Part B: Reactions and Synthesis (5th ed.). New York: Springer. p. 337. ISBN 978-0387683546.
  8. ^ Hydrocarbon Chemistry, George A. Olah, Arpad Molner, 2d edition, 2003, Wiley-Blackwell ISBN 978-0471417828
  9. ^ Finn, Patricia; Jolly, William L. (August 1972). "Asymmetric cleavage of diborane by water. The structure of diborane dihydrate". Inorganic Chemistry. 11 (8): 1941–1944. doi:10.1021/ic50114a043.
  10. ^ D'Ulivo, Alessandro (May 2010). "Mechanism of generation of volatile species by aqueous boranes". Spectrochimica Acta Part B: Atomic Spectroscopy. 65 (5): 360–375. doi:10.1016/j.sab.2010.04.010.
  11. ^ Burg, Anton B.; Schlesinger, H. I. (May 1937). "Hydrides of boron. VII. Evidence of the transitory existence of borine (BH
    3    ): Borine carbonyl and borine trimethylammine". Journal of the American Chemical Society. 59 (5): 780–787. doi:10.1021/ja01284a002.
  12. ^ 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.
  13. ^ Carey, Francis A.; Sundberg, Richard J. (2007). Advanced Organic Chemistry: Part B: Reactions and Synthesis (5th ed.). New York: Springer. p. 337. ISBN 978-0387683546.
  14. ^ Carey, Francis A.; Sundberg, Richard J. (2007). Advanced Organic Chemistry: Part B: Reactions and Synthesis (5th ed.). New York: Springer. p. 338. ISBN 978-0387683546.
  15. ^ a b Burkhardt, Elizabeth R.; Matos, Karl (July 2006). "Boron reagents in process chemistry: Excellent tools for selective reductions". Chemical Reviews. 106 (7): 2617–2650. doi:10.1021/cr0406918. PMID 16836295.
  16. ^ Kollonitisch, J. (1961). "Reductive Ring Cleavage of Tetrahydrofurans by Diborane". J. Am. Chem. Soc. 83 (6): 1515. doi:10.1021/ja01467a056.
  17. ^ Carey, Francis A.; Sundberg, Richard J. (2007). Advanced Organic Chemistry: Part B: Reactions and Synthesis (5th ed.). New York: Springer. p. 344. ISBN 978-0387683546.
  18. ^ Carboni, B.; Mounier, L. (1999). "Recent developments in the chemistry of amine- and phosphine-boranes". Tetrahedron. 55 (5): 1197. doi:10.1016/S0040-4020(98)01103-X.
  19. ^ Szieberth, Dénes; Szpisjak, Tamás; Turczel, Gábor; Könczöl, László (19 August 2014). "The stability of η2-H2 borane complexes – a theoretical investigation". Dalton Transactions. 43 (36): 13571–13577. doi:10.1039/C4DT00019F. PMID 25092548.
  20. ^ Tague, Thomas J.; Andrews, Lester (1 June 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.
  21. ^ Schreiner, Peter R.; Schaefer III, Henry F.; Schleyer, Paul von Ragué (1 June 1994). "The structure and stability of BH5. Does correlation make it a stable molecule? Qualitative changes at high levels of theory". The Journal of Chemical Physics. 101 (9): 7625. Bibcode:1994JChPh.101.7625S. doi:10.1063/1.468496.
  22. ^ A Life of Magic Chemistry: Autobiographical Reflections Including Post-Nobel Prize Years and the Methanol Economy, 159p
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