Borane

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Borane
Borane-2D-structure.svg
Borane-3D-balls.png
Borane-3D-vdW.png
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
PubChem CID
  • InChI=1S/BH3/h1H3
    Key: UORVGPXVDQYIDP-UHFFFAOYSA-N
  • B
Properties
BH3
Molar mass 13.83 g·mol−1
Appearancecolourless gas
Conjugate acid Boronium
Thermochemistry
Std molar
entropy (S⦵298)
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).

Borane, also known as 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. [2] 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. [3] It normally dimerizes to diborane in the absence of other chemicals. [4]

Contents

Structure and properties

BH3 is a trigonal planar molecule with D3h symmetry. The experimentally determined B–H bond length is 119  pm. [5]

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: [6]

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. [7] 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: [8]

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. [9] 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. [6] Aqueous solutions of BH3 are extremely unstable. [10] [11]

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: [6]

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

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.

Examples of amine borane complexes.png

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

Related Research Articles

<span class="mw-page-title-main">Boranes</span>

A borane is a compound with the formula BxHy or a related anion. Many such boranes are known. Most common are those with 1 to 12 boron atoms. Although they have few practical applications, the boranes exhibit structures and bonding that differs strongly from the patterns seen in hydrocarbons. Hybrids of boranes and hydrocarbons, the carboranes are also well developed.

Hydroboration–oxidation reaction is a two-step hydration reaction that converts an alkene into an alcohol. The process results in the syn addition of a hydrogen and a hydroxyl group where the double bond had been. Hydroboration–oxidation is an anti-Markovnikov reaction, with the hydroxyl group attaching to the less-substituted carbon. The reaction thus provides a more stereospecific and complementary regiochemical alternative to other hydration reactions such as acid-catalyzed addition and the oxymercuration–reduction process. The reaction was first reported by Herbert C. Brown in the late 1950s and it was recognized in his receiving the Nobel Prize in Chemistry in 1979.

<span class="mw-page-title-main">Diborane</span> Chemical compound

Diborane(6), commonly known as diborane, is the chemical compound with the formula B2H6. It is a highly toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Given its simple formula, borane is a fundamental boron compound. It has attracted wide attention for its electronic structure. Several of its derivatives are useful reagents.

<span class="mw-page-title-main">Herbert C. Brown</span> American chemist (1912–2004)

Herbert Charles Brown was an American chemist and recipient of the 1979 Nobel Prize in Chemistry for his work with organoboranes.

<span class="mw-page-title-main">Sodium borohydride</span> Chemical compound

Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, is an inorganic compound with the formula NaBH4. It is a white crystalline solid, usually encountered as an aqueous basic solution. Sodium borohydride is a reducing agent that finds application in papermaking and dye industries. It is also used as a reagent in organic synthesis.

<span class="mw-page-title-main">Organoboron chemistry</span> Study of compounds containing a boron-carbon bond

Organoboron chemistry or organoborane chemistry studies organoboron compounds, also called organoboranes. These chemical compounds combine boron and carbon; typically, they are organic derivatives of borane (BH3), as in the trialkyl boranes.

In organic chemistry, hydroboration refers to the addition of a hydrogen-boron bond to certain double and triple bonds involving carbon. This chemical reaction is useful in the organic synthesis of organic compounds.

<span class="mw-page-title-main">Corey–Itsuno reduction</span>

The Corey–Itsuno reduction, also known as the Corey–Bakshi–Shibata (CBS) reduction, is a chemical reaction in which a prochiral ketone is enantioselectively reduced to produce the corresponding chiral, non-racemic alcohol. The oxazaborolidine reagent which mediates the enantioselective reduction of ketones was previously developed by the laboratory of Itsuno and thus this transformation may more properly be called the Itsuno-Corey oxazaborolidine reduction.

<span class="mw-page-title-main">Ammonia borane</span> Chemical compound

Ammonia borane, also called borazane, is the chemical compound with the formula H3NBH3. The colourless or white solid is the simplest molecular boron-nitrogen-hydride compound. It has attracted attention as a source of hydrogen fuel, but is otherwise primarily of academic interest.

<span class="mw-page-title-main">Boron compounds</span>

Boron compounds are compounds containing the element boron. In the most familiar compounds, boron has the formal oxidation state +3. These include oxides, sulfides, nitrides, and halides.

<span class="mw-page-title-main">Catecholborane</span> Chemical compound

Catecholborane (abbreviated HBcat) is an organoboron compound that is useful in organic synthesis. This colourless liquid is a derivative of catechol and a borane, having the formula C6H4O2BH.

A frustrated Lewis pair (FLP) is a compound or mixture containing a Lewis acid and a Lewis base that, because of steric hindrance, cannot combine to form a classical adduct. Many kinds of FLPs have been devised, and many simple substrates exhibit activation.

<span class="mw-page-title-main">Borane dimethylsulfide</span> Chemical compound

Borane dimethylsulfide (BMS) is a chemical compound with the chemical formula BH3·S(CH3)2. It is an adduct between borane molecule and dimethyl sulfide molecule. It is a complexed borane reagent that is used for hydroborations and reductions. The advantages of BMS over other borane reagents, such as borane-tetrahydrofuran, are its increased stability and higher solubility. BMS is commercially available at much higher concentrations than its tetrahydrofuran counterpart and does not require sodium borohydride as a stabilizer, which could result in undesired side reactions. In contrast, BH3·THF requires sodium borohydride to inhibit reduction of THF to tributyl borate. BMS is soluble in most aprotic solvents.

<span class="mw-page-title-main">Borane–tetrahydrofuran</span> Chemical compound

Borane–tetrahydrofuran is an adduct derived from borane and tetrahydrofuran (THF). These solutions, which are colorless, are used for reductions and hydroboration, reactions that are useful in synthesis of organic compounds. The use of borane–tetrahydrofuran has been displaced by borane–dimethylsulfide, which has a longer shelf life and effects similar transformations.

<span class="mw-page-title-main">1,2-Dimethyldiborane</span> Chemical compound

1,2-Dimethyldiborane is an organoboron compound with the formula [(CH3)BH2]2. Structurally, it is related to diborane, but with methyl groups replacing terminal hydrides on each boron. It is the dimer of methylborane, CH3BH2, the simplest alkylborane. 1,2-Dimethyldiborane can exist in a cis- and a trans arrangement. 1,2-Dimethyldiborane is an easily condensed, colorless gas that ignites spontaneously in air.

<span class="mw-page-title-main">Tetramethyldiborane</span> Chemical compound

Dimethylborane, (CH3)2BH is the simplest dialkylborane, consisting of a methyl group substituted for a hydrogen in borane. As for other boranes it normally exists in the form of a dimer called tetramethyldiborane or tetramethylbisborane or TMDB ((CH3)2BH)2. Other combinations of methylation occur on diborane, including monomethyldiborane, trimethyldiborane, 1,2-dimethylborane, 1,1-dimethylborane and trimethylborane. At room temperature the substance is at equilibrium between these forms. The methylboranes were first prepared by H. I. Schlesinger and A. O. Walker in the 1930s.

<span class="mw-page-title-main">Trimethyldiborane</span> Chemical compound

Trimethyldiborane, (CH3)3B2H3 is a molecule containing boron carbon and hydrogen. It is an alkylborane, consisting of three methyl group substituted for a hydrogen in diborane. It can be considered a mixed dimer: (CH3)2BH2BH(CH3) or dimethylborane and methylborane. called 1,2-dimethyldiborane. Other combinations of methylation occur on diborane, including monomethyldiborane, 1,2-dimethyldiborane, tetramethyldiborane, 1,1-dimethylborane and trimethylborane. At room temperature the substance is at equilibrium between these forms, so it is difficult to keep it pure. The methylboranes were first prepared by H. I. Schlesinger and A. O. Walker in the 1930s.

<span class="mw-page-title-main">Methyldiborane</span> Chemical compound

Methyldiborane, CH3B2H5, or monomethyldiborane is the simplest of alkyldiboranes, consisting of a methyl group substituted for a hydrogen in diborane. As with other boranes it exists in the form of a dimer with a twin hydrogen bridge that uses three-center two-electron bonding between the two boron atoms, and can be imagined as methyl borane (CH3BH2) bound to borane (BH3). Other combinations of methylation occur on diborane, including 1,1-dimethylborane, 1,2-dimethyldiborane, trimethyldiborane, tetramethyldiborane, and trimethylborane (which is not a dimer). At room temperature the substance is at equilibrium between these molecules.

<span class="mw-page-title-main">Borane carbonyl</span> Chemical compound

Borane carbonyl is the inorganic compound with the formula H3BCO. This colorless gas is the adduct of borane and carbon monoxide. It is usually prepared by combining borane-ether complexes and CO. The compound is mainly of theoretical and pedagogical interest.

<span class="mw-page-title-main">1,1-Dimethyldiborane</span> Chemical compound

1,1-Dimethyldiborane is the organoboron compound with the formula (CH3)2B(μ-H)2BH2. A pair of related 1,2-dimethyldiboranes are also known. It is a colorless gas that ignites in air.

References

  1. ↑ "Borane".
  2. ↑ 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.
  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. ↑ 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.
  5. ↑ 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.
  6. 1 2 3 Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN   978-0-08-037941-8.
  7. ↑ 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.
  8. ↑ 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.
  9. ↑ Hydrocarbon Chemistry, George A. Olah, Arpad Molner, 2d edition, 2003, Wiley-Blackwell ISBN   978-0471417828
  10. ↑ 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.
  11. ↑ 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.
  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. 1 2 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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