# Borane

**Borane** is the name for the molecule BH₃, the simplest hydride of boron, and more broadly for the class of boron hydrides (polyboranes) of which it is the parent. Free BH₃ is a trigonal planar, electron-deficient gas that cannot be isolated at ordinary conditions because it rapidly dimerizes to diborane, B₂H₆; in practice borane is handled as Lewis base complexes such as BH₃·THF and BH₃·SMe₂, which are widely used reagents in organic synthesis.<sup>[1](https://webbook.nist.gov/cgi/cbook.cgi?ID=C13283313&Mask=28)</sup><sup> • </sup><sup>[2](https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Chemistry_of_the_Main_Group_Elements_(Barron)/06:_Group_13/6.04:_Boron_Hydrides)</sup>

| Key fact | Detail |
|---|---|
| Formula and mass | BH₃, molecular weight 13.835, CAS 13283-31-3<sup>[1](https://webbook.nist.gov/cgi/cbook.cgi?ID=C13283313&Mask=28)</sup> |
| Geometry | Trigonal planar, H–B–H angle 120°, with boron holding only six valence electrons<sup>[2](https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Chemistry_of_the_Main_Group_Elements_(Barron)/06:_Group_13/6.04:_Boron_Hydrides)</sup> |
| Dimerization | In the absence of a Lewis base, BH₃ dimerizes exothermically to diborane, B₂H₆ (boiling point −92.5 °C)<sup>[2](https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Chemistry_of_the_Main_Group_Elements_(Barron)/06:_Group_13/6.04:_Boron_Hydrides)</sup> |
| Bonding type | Bridging B–H–B units in diborane are three-center two-electron (3c2e) bonds, the hallmark of electron-deficient borane structures<sup>[2](https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Chemistry_of_the_Main_Group_Elements_(Barron)/06:_Group_13/6.04:_Boron_Hydrides)</sup> |
| Structural families | Higher boranes are classified as closo, nido, or arachno cages by Wade's rules<sup>[3](https://jovanaeducation.com/glossary/wades-rules)</sup> |
| Signature reaction | Hydroboration of alkenes, discovered in 1959 by H.C. Brown, gives syn, non-Markovnikov alcohols after oxidation<sup>[4](https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_(OpenStax)/08:_Alkenes_-_Reactions_and_Synthesis/8.05:_Hydration_of_Alkenes_-_Addition_of_HO_by_Hydroboration)</sup> |
| Recognition | William N. Lipscomb received the 1976 Nobel Prize in Chemistry for studies on borane structure; Herbert C. Brown shared the 1979 prize for boron reagents in organic synthesis<sup>[5](https://www.nobelprize.org/prizes/chemistry/1976/press-release/1000/)</sup><sup> • </sup><sup>[6](https://www.livescience.com/16384-nobel-prize-chemistry-list.html)</sup> |
| Hazard | Diborane has a NIOSH exposure limit of 0.1 ppm (8-hour TWA) and a revised IDLH of 15 ppm<sup>[7](https://www.cdc.gov/niosh/idlh/19287457.html)</sup> |

## The BH₃ molecule and its dimerization

Borane has three B–H bonds arranged around a boron atom with six valence electrons rather than the usual octet, which makes it a strong Lewis acid. In tetrahydrofuran solution it accepts an electron pair from a solvent molecule to form the stable BH₃–THF complex, and many other adducts are known, for example H₃B·PPh₃.<sup>[4](https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_(OpenStax)/08:_Alkenes_-_Reactions_and_Synthesis/8.05:_Hydration_of_Alkenes_-_Addition_of_HO_by_Hydroboration)</sup><sup> • </sup><sup>[2](https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Chemistry_of_the_Main_Group_Elements_(Barron)/06:_Group_13/6.04:_Boron_Hydrides)</sup>

Without a donor molecule, BH₃ molecules combine to form diborane. The dimer is a colorless gas, boiling at −92.5 °C, that is spontaneously and highly exothermically inflammable above 25 °C (ΔH = −2137.7 kJ/mol for the combustion reaction), so it is usually used in one of a range of solvated forms for safety.<sup>[2](https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Chemistry_of_the_Main_Group_Elements_(Barron)/06:_Group_13/6.04:_Boron_Hydrides)</sup> [Diborane](https://www.edgepedia.org/diborane) is generally synthesized by reacting boron trifluoride with a hydride source such as sodium borohydride or lithium aluminum hydride.<sup>[2](https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Chemistry_of_the_Main_Group_Elements_(Barron)/06:_Group_13/6.04:_Boron_Hydrides)</sup>

## Bonding in diborane

Diborane is the archetypal electron-deficient compound. Its structure, confirmed by IR spectroscopy and electron diffraction, has four terminal B–H bonds that are ordinary covalent bonds, plus two bridging B–H–B units. Each bridge is a three-center two-electron bond, ordinarily described as the combination of two boron sp³ orbitals and one hydrogen s orbital; because a single electron pair links three nuclei, the molecule holds together with fewer electrons than a classical structure would require.<sup>[2](https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Chemistry_of_the_Main_Group_Elements_(Barron)/06:_Group_13/6.04:_Boron_Hydrides)</sup> A refinement notes that the 120° H–B–H angle of the terminal hydrides suggests the BH₂ fragments are better treated as sp² hybridized, with the bridge built from one sp² and one p orbital on each boron.<sup>[2](https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Chemistry_of_the_Main_Group_Elements_(Barron)/06:_Group_13/6.04:_Boron_Hydrides)</sup>

This kind of multicenter bonding is characteristic of boranes generally: two types of 3c2e bonds, B–B–B and B–H–B, occur across the family.<sup>[8](https://pubs.rsc.org/en/content/articlehtml/2018/cp/c7cp07422k)</sup>

## History

**Alfred Stock's boranes.** The German chemist [Alfred Stock](https://www.edgepedia.org/alfred-stock) (1876–1946) began work on the boron hydrides around 1910 and achieved the first successful synthesis in 1912, by reacting magnesium diboride with dilute hydrochloric acid. Because the compounds are extremely air- and moisture-sensitive and spontaneously flammable, he developed specialized high-vacuum laboratory equipment that allowed research under vacuum, at low temperature, and with very small quantities of substance. In a series of studies he reliably characterized B₂H₆, B₄H₁₀, B₅H₉, B₅H₁₁, B₆H₁₀, and B₁₀H₁₄, and provided preliminary evidence for dodecaborane and hexaborane.<sup>[9](https://www.sciencedirect.com/science/article/pii/S0010854525007921)</sup> The German Chemical Society established the Alfred Stock Memorial Prize in 1950 for outstanding independent experimental research in inorganic chemistry.<sup>[9](https://www.sciencedirect.com/science/article/pii/S0010854525007921)</sup>

**Structure and the 1976 Nobel Prize.** For decades the bonding in these molecules resisted explanation; a 1937 Nature review noted the unsatisfactory devices, from singlet linkages to polyvalent hydrogen, that had been invoked to explain B₂H₆.<sup>[10](https://www.nature.com/articles/139381b0.pdf)</sup> William N. Lipscomb of Harvard University solved the problem from the early 1950s onward, using topological methods to identify feasible bonding combinations, [X-ray diffraction](https://www.edgepedia.org/x-ray-diffraction) to determine geometric structures, and quantum mechanical calculations to predict stability and reactivity. The [Royal Swedish Academy of Sciences](https://www.edgepedia.org/royal-swedish-academy-of-sciences) awarded him the 1976 [Nobel Prize in Chemistry](https://www.edgepedia.org/nobel-prize-in-chemistry) "for his studies on the structure of boranes illuminating problems of chemical bonding."<sup>[5](https://www.nobelprize.org/prizes/chemistry/1976/press-release/1000/)</sup>

**Hydroboration and the 1979 Nobel Prize.** Herbert C. Brown discovered the hydroboration reaction at [Purdue University](https://www.edgepedia.org/purdue-university) in 1959: addition of a B–H bond of borane to an alkene gives an organoborane, RBH₂, which oxidation with basic hydrogen peroxide converts to an alcohol.<sup>[4](https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_(OpenStax)/08:_Alkenes_-_Reactions_and_Synthesis/8.05:_Hydration_of_Alkenes_-_Addition_of_HO_by_Hydroboration)</sup> Brown shared the 1979 Nobel Prize in Chemistry with [Georg Wittig](https://www.edgepedia.org/georg-wittig) "for their development of the use of boron- and phosphorus-containing compounds, respectively, into important reagents in organic synthesis."<sup>[6](https://www.livescience.com/16384-nobel-prize-chemistry-list.html)</sup>

## Higher boranes and Wade's rules

Larger boranes form cages in which the boron atoms occupy vertices of closed or open deltahedra (triangulated polyhedra). IUPAC nomenclature places a structural descriptor before the name: a closed deltahedron is closo (Greek for cage), one missing a vertex is nido (Latin for nest), and one missing two vertices is arachno (Greek for spider's web).<sup>[11](https://www.degruyterbrill.com/document/doi/10.1515/pac-2018-0205/html)</sup><sup> • </sup><sup>[3](https://jovanaeducation.com/glossary/wades-rules)</sup> Kenneth Wade's electron-counting rules connect formula to shape: a cluster with n skeletal atoms and n+1 skeletal electron pairs is closo, with n+2 pairs nido, and with n+3 pairs arachno.<sup>[3](https://jovanaeducation.com/glossary/wades-rules)</sup> Neutral boron hydrides are named with the number of boron atoms followed in parentheses by the number of hydrogen atoms, as in diborane(6), pentaborane(9), and decaborane(14).<sup>[11](https://www.degruyterbrill.com/document/doi/10.1515/pac-2018-0205/html)</sup>

## Uses in synthesis and industry

**Hydroboration.** When an alkene reacts with BH₃ in THF solution, addition to the double bond occurs three times to give a trialkylborane, R₃B; oxidation then replaces each C–B bond with C–OH. The addition is syn, with boron attaching to the less substituted carbon, so the sequence delivers an overall syn non-Markovnikov hydration of the alkene, complementary to oxymercuration–demercuration.<sup>[4](https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_(OpenStax)/08:_Alkenes_-_Reactions_and_Synthesis/8.05:_Hydration_of_Alkenes_-_Addition_of_HO_by_Hydroboration)</sup> Bulky dialkylboranes made from borane are more selective reagents; 9-BBN, prepared from 1,5-cyclooctadiene and borane, adds only to unhindered alkenes and is one of the few boranes that can be manipulated in air.<sup>[12](https://chem.libretexts.org/Courses/Westminster_College/CHE_261_-_Organic_Chemistry_I/09%3A_Electrophilic_Addition_Reactions_of_Alkenes_and_Alkynes/9.09%3A_Addition_of_Boron_Hydrides_to_Alkenes._Organoboranes)</sup>

**Reductions.** Borane is considered the best reagent for reducing carboxylic acids to primary alcohols, and a stoichiometric amount can selectively reduce a carboxylic acid in the presence of an amide.<sup>[13](https://orca.cardiff.ac.uk/id/eprint/41305/18/Asim%27s-PhD-Thesis%20%281%29%20dec%20page%20removed.pdf)</sup> The most common complexes for such work are borane–dimethyl sulfide (BH₃·SMe₂) and borane–tetrahydrofuran (BH₃·THF).<sup>[13](https://orca.cardiff.ac.uk/id/eprint/41305/18/Asim%27s-PhD-Thesis%20%281%29%20dec%20page%20removed.pdf)</sup>

Beyond its role in rocket propellant mixtures, diborane serves as a reducing agent, vulcanizes rubber, catalyzes the polymerization of hydrocarbons, speeds up flame propagation, and acts as a semiconductor dopant.<sup>[14](https://pubchem.ncbi.nlm.nih.gov/compound/Diborane)</sup> During World War II and after, boron hydrides were pursued as high-energy aircraft and rocket fuels, and solid propellants based on them showed very high burning rates attractive for rapid-response rockets.<sup>[15](https://ntrs.nasa.gov/api/citations/19700078681/downloads/19700078681.pdf)</sup><sup> • </sup><sup>[16](https://www.chem.ucla.edu/dept/Faculty/hawthorne/lecture/lecture2.htm)</sup>

## Hazards

Diborane is a colorless gas with a repulsive, sweet odor, with an odor threshold of 2.5 ppm and a lower explosive limit of 0.8%.<sup>[14](https://pubchem.ncbi.nlm.nih.gov/compound/Diborane)</sup><sup> • </sup><sup>[7](https://www.cdc.gov/niosh/idlh/19287457.html)</sup> The NIOSH recommended exposure limit is 0.1 ppm (0.1 mg/m³) as an 8-hour time-weighted average, matching the OSHA permissible exposure limit; the revised immediately-dangerous-to-life-or-health concentration is 15 ppm, based on acute inhalation toxicity data in animals.<sup>[7](https://www.cdc.gov/niosh/idlh/19287457.html)</sup> Acute exposure symptoms include chest tightness, shortness of breath, nonproductive cough, headache, dizziness, and fever, and animal studies show liver and kidney damage and pulmonary edema.<sup>[17](https://portugues.cdc.gov/niosh/npg/npgd0183.html)</sup> The higher boranes share the family's traits: decaborane has a musty odor and, like the other boron hydrides, is toxic to the central nervous system.<sup>[18](https://www.acs.org/molecule-of-the-week/archive/d/decaborane.html)</sup>

## References

1. [Borane, NIST Chemistry WebBook, SRD 69](https://webbook.nist.gov/cgi/cbook.cgi?ID=C13283313&Mask=28)
2. [6.4: Boron Hydrides, Chemistry LibreTexts](https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Chemistry_of_the_Main_Group_Elements_(Barron)/06:_Group_13/6.04:_Boron_Hydrides)
3. [Wade's rules, JOVANA Education glossary](https://jovanaeducation.com/glossary/wades-rules)
4. [8.5: Hydration of Alkenes – Addition of H₂O by Hydroboration, Chemistry LibreTexts](https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_(OpenStax)/08:_Alkenes_-_Reactions_and_Synthesis/8.05:_Hydration_of_Alkenes_-_Addition_of_HO_by_Hydroboration)
5. [Press release: The 1976 Nobel Prize in Chemistry, NobelPrize.org](https://www.nobelprize.org/prizes/chemistry/1976/press-release/1000/)
6. [Nobel Prize in Chemistry: 1901–Present, Live Science](https://www.livescience.com/16384-nobel-prize-chemistry-list.html)
7. [Diborane – IDLH, NIOSH, CDC](https://www.cdc.gov/niosh/idlh/19287457.html)
8. [A systematic examination of classical and multi-center bonding in heteroborane clusters, PCCP](https://pubs.rsc.org/en/content/articlehtml/2018/cp/c7cp07422k)
9. [The chemistry of lower boranes: from synthesis and structure to reactivity and applications, ScienceDirect](https://www.sciencedirect.com/science/article/pii/S0010854525007921)
10. [Hydrides of Boron, Nature (1937)](https://www.nature.com/articles/139381b0.pdf)
11. [Nomenclature for boranes and related species (IUPAC Recommendations 2019)](https://www.degruyterbrill.com/document/doi/10.1515/pac-2018-0205/html)
12. [9.9: Addition of Boron Hydrides to Alkenes. Organoboranes, Chemistry LibreTexts](https://chem.libretexts.org/Courses/Westminster_College/CHE_261_-_Organic_Chemistry_I/09%3A_Electrophilic_Addition_Reactions_of_Alkenes_and_Alkynes/9.09%3A_Addition_of_Boron_Hydrides_to_Alkenes._Organoboranes)
13. [Chapter 1: Introduction to Borane and Its Reagents, Cardiff University PhD thesis](https://orca.cardiff.ac.uk/id/eprint/41305/18/Asim%27s-PhD-Thesis%20%281%29%20dec%20page%20removed.pdf)
14. [Diborane, PubChem](https://pubchem.ncbi.nlm.nih.gov/compound/Diborane)
15. [Historical Development of Diborane, NASA Technical Reports](https://ntrs.nasa.gov/api/citations/19700078681/downloads/19700078681.pdf)
16. [Hawthorne Faculty Research Lecture, UCLA](https://www.chem.ucla.edu/dept/Faculty/hawthorne/lecture/lecture2.htm)
17. [NIOSH Pocket Guide to Chemical Hazards – Diborane, CDC](https://portugues.cdc.gov/niosh/npg/npgd0183.html)
18. [Decaborane, Molecule of the Week, American Chemical Society](https://www.acs.org/molecule-of-the-week/archive/d/decaborane.html)

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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Organic substances › Alcohols, ethers, and organooxygen groups › Organosulfur, selenium, and heavier main-group organo derivatives › Heavier main-group organometaloids (B, Si, P, and neighbors) › Organoboron compounds › Boranes and organoboranes › Boranes (binary boron hydrides)*

*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026; Oct 25, 2026 · Last review: Oct 15, 2026*

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