What molecule am I?
Diborane (B2H6) is a boron hydride with useful and hazardous properties. The simplest boron hydride is borane1 (BH3), but it spontaneously dimerizes to diborane or forms complexes with electron-rich molecules such as ammonia.
Diborane has been known in the literature since the 1920s, but early chemists did not understand its structure. As an example, in 1927 Max Ulmann2 at the University of Tübingen (Germany) wrote that the molecule, for which he used the formula BH2•BH4, contained “2 negative H-systems, 1 positive and 1 negative B-system, and 4 positive H-nuclei; i.e., a stable ‘noble gas’ arrangement in every partial system.”
Another German chemist, Alfred Stock at the University of Karlsruhe, demonstrated in the early 1930s that diborane has an ethane-like structure. This finding was confirmed in 1937 by S. H. Bauer at Caltech (Pasadena, CA), who used electron diffraction spectrometry on diborane vapor to visualize the structure.
Diborane has many uses in organic and inorganic chemistry. An important one is the direct hydroboration of olefins to produce organoboranes. In an early example from 1957, Herbert C. Brown*3 and B. C. Subba Rao at Purdue University (West Lafayette, IN), reported the diglyme4-promoted addition of diborane to 1-hexene5 to produce tri-n-hexylborane6. The ambient-temperature reaction was rapid and almost quantitative. In addition to borane, hydroboration reactions can be carried out using mono- and dialkyl boranes and borane adducts with Lewis bases such as ammonia–borane7 and tetrahydrofuran–borane8.
Diborane can also be used as a reducing agent, for example to reduce carboxylic acids to alcohols and to generate hydrogen from its reactions with water or alcohols.
1. CAS Reg. No. 13283-31-3.
2. Not to be confused with Fritz Ullmann, the German chemist best known for developing Ullmann's Encyclopedia of Industrial Chemistry, a major reference work published from 1914 to the present.
3. Brown was the leading developer of hydroboration reactions, for which he won the 1979 Nobel Prize in Chemistry.
4. CAS Reg. No. 111-96-6.
5. CAS Reg. No. 592-41-6.
6. CAS Reg. No. 1188-92-7.
7. CAS Reg. No. 13774-81-7.
8. CAS Reg. No. 14044-65-6.
Diborane hazard information
| Hazard class* | GHS code and hazard statement | |
|---|---|---|
| Flammable gases, category 1 | H220—Extremely flammable gas |  |
| Pyrophoric gases | [None]—Catches fire spontaneously if exposed to air | |
| Gases under pressure, compressed gas | H281—Contains gas under pressure; may explode if heated |  |
| Acute toxicity, inhalation, category 1 | H330—Fatal if inhaled |  |
| EUH018**—In use may form flammable/explosive vapor–air mixtures | ||
*Globally Harmonized System (GHS) of Classification and Labeling of Chemicals. Explanation of pictograms.
**Not a GHS code.
MOTW update
Ozone1 (O3) was the Molecule of the Week for June 3, 2008. It is a highly reactive compound that is useful in chemical synthesis. It is present in Earth’s troposphere, where it absorbs almost all UV-C radiation from the Sun.
Last month, Daochuan Li and 15 co-workers at Sun Yat-Sen University (Guangzhou, China) described the effect of ozone on the oral microbiome. They collected oral microbiome samples from 29 individuals who had been exposed to filtered air containing 282 ppb ozone, far more than normal air at 5.1 ppb. The results showed significantly impaired lung function and reduced microbiome diversity. Males exhibited greater sensitivity to ozone exposure than females.
1. CAS Reg. No. 10028-15-6.
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Diborane fast facts
| CAS Reg. No. | 19287-45-7 |
| SciFinder name | Diborane(6) |
| Empirical formula | B2H6 |
| Molar mass | 27.67 g/mol |
| Appearance | Colorless gas |
| Boiling point | –92.5 °C |
| Water solubility | Reacts |
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