|Jmol-3D images||Image 1|
|Molar mass||13.83 g mol−1|
|Solubility in water||hydrolyses|
|Solubility in Ammonia||3.2 mol L−1|
|Std enthalpy of
|106.69 kJ mol−1|
|187.88 kJ mol−1 K−1|
|Molecular shape||trigonal planar|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)|
Borane (also systematically named trihydridoboron), also called borine, is an inorganic compound with the chemical formula BH
3 (also written [BH
3]). It is a colourless gas that only persists at elevated temperatures or in dilution. Borane is the simplest of member of the boranes.
In 1937, the discovery of carbonyltrihydridoboron, the adduct of borane with carbon monoxide, among other borane adducts, played in important role in exploring the chemistry of "normal" boranes at a time when three-centre two-electron bonding was not yet known. This discovery also implied the existence of borane, however, it was not until some years later that direct evidence was observed.
The dominant behaviour of borane is its dimerisation to form diborane, as shown by the enthalpy of the reaction, which is predicted to be near -40 kcal/mol. For such an exothermic process, the concentration of BH3 is negligible in solution.
- 2 BH3 → B2H6
Although borane in principle acts as a Lewis acid, the many 1:1 adducts are invariably prepared from diborane or via ligand exchange of an existing adduct.
- B2H6 + 2 L → 2 LBH3
The stability sequence of these estimated spectroscopically and thermochemically is:-
- PF3 < CO < Et2O < Me2O < C4H8O < C4H8S < Et2S < Me2S < Py < Me3N < H–
Solute properties of diborane
Diborane dissolves in diethyl ether and diglyme and is present as the dimer. In Tetrahydrofuran, THF, it is present as a loose 1:1 adduct, THF.BH3. A diborane solution in THF is commercially available as is a solution of the DMS complex. Gaseous borane, diborane(6) dissolves in polar compounds such as amines and tetrahydrofuran. This dissolution property of diborane makes it a widely used laboratory chemical, for example, as a reagent in the production of ethylborane. Borane is commercially available as a Lewis acid–base adduct with various ligands, including solutions of borane dimethylsulfide, ammonia borane (and other amines), and borane tetrahydrofuran. Borane has a solubility in liquid ammonia of 3.2 mol L−1, above which it precipitates out as the ammoniate. Any attempt to deammoniate the crystallised product thermally, only results in its decomposition.
Borane as a reactive intermediate
- 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.
Another example is hydroboration reaction, in which "diborane" adds to an alkene. In this reaction, either the diborane dissociates forming BH3 as an intermediate or one B–H–B bridge opens to produce an electron deficient boron atom. The addition to alkenes is rapid, quantitative and reversible. The addition is anti-markovnikov, that is to say that boron adds to the less substituted C atom, the attack taking place on the less hindered side[clarification needed] of the molecule.
|This section does not cite any references or sources. (November 2013)|
3L + OH−
1−n + H
2 + n L
Because of this capture of the hydroxide (OH−
), borane has Arrhenius-acidic character. Borane can capture three hydroxides. Its hydroxylation products are borinate (H
), boronate (HBO2−
2), and borate (BO3−
3). Borane does not form stable aqueous solutions due to hydrolysis.
3L + 3 H
2O → B(OH)
1-n + 3 H
2 + n L
There are two main methods for producing borane. One common method is the cleaving reaction of diborane with dimethyl sulfide. The other method is the partial oxidation of a boranuide salt under a coordinating borane solvent such as trimethylamine.
- (THF)BH3 + 3 CH2=CHR → B(CH2CH2R)3 + THF
This reaction is regioselective, and 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 the THF adduct of borane adduct.
- 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.
- 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.
- 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
- Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419.
- Brown, H. C.; Chandrasekharan, J.; Wang, K. K. (1983). "Hydroboration-kinetics and mechanism". Pure and Applied Chemistry 55 (9): 1387–1414. doi:10.1351/pac198355091387. ISSN 0033-4545.
- Hydrocarbon Chemistry, George A. Olah, Arpad Molner, 2d edition, 2003, Wiley-Blackwell ISBN 978-0471417828
- 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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