Diborane

• Borethan
• Diborhexahydrid

Colorless gas with a sweet odor repellent

Gaseous

1.17 kg · m -3 ( 15 ° C, 1 bar)

-164.85 ° C

-92.5 ° C

2.8 MPa ( 0 ° C)

Risk

36.4 kJ / mol

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Diborane is a chemical compound consisting of the elements boron and hydrogen. It has the molecular formula B2H6 and is the simplest compound from the class of boron -hydrogen compounds ( boranes ).

• 4.1 Structure
• 4.2 Chemical Properties

History

Diborane was synthesized for the first time in the 19th century by the hydrolysis of metal borides, but not thoroughly investigated. From 1912 to 1936, the great pioneer of the chemistry of boron hydrides, Alfred Stock explored these compounds, which led to methods for the synthesis and handling of highly reactive, volatile and often toxic boron hydrides. He first proposed a similar structure for the ethane diborane. The diffraction of electron beams according to experiments of SH Bauer supported his proposed structure.

In the communication of HI Schlesinger L. Pauling ( who also took the ethane structure), the three-center bond but was not explicitly discussed in his classic review in the early 1940s. However, the review discussed the C2v structure profoundly: " It can be seen that this assumption easily many of the chemical properties of diboranes explains ... ".

In 1943, the undergraduate student published at Balliol College, Oxford Christopher Longuet- Higgins along with RP Bell today the accepted structure of diborane. However, this was already described in 1921 in the former Soviet Union. In the years after the proposal of Longuet-Higgins/Bell there was an extensive discussion about the correct structure. The debate ended in 1951 with the electron diffraction measurements of K. Hedberg and V. Schomaker, which confirmed the up to now accepted structure of diborane.

William Nunn Lipscomb Jr. confirmed the molecular structure of boranes by X-ray crystallography in the 1950s and developed theories to explain this binding. Later he used the same methods for similar problems, including the structure of carboranes. Lipscomb was awarded the Nobel Prize in Chemistry for his efforts in 1976.

Occurrence

Diborane does not occur naturally.

Production and representation

For producing diborane, there are several possibilities:

Reaction of lithium hydride with boron trifluoride:

Reaction of lithium tetrahydroaluminate with boron trichloride in diethyl ether:

Reaction of sodium borohydride with boron trifluoride in ethylene glycol dimethyl ether:

Reaction of sodium borohydride with iodine in ethylene glycol dimethyl ether or THF:

Technical production

Borane is technically hydrogenation of diboron trioxide (B2O3 ) by using aluminum metal and aluminum chloride as catalyst at temperatures above 150 ° C and at a hydrogen pressure of 750 bar produced.

Properties

Diborane is a colorless, flammable, highly toxic gas with a pungent, sweetish, disagreeable odor. It is a metastable compound at standard conditions. Above 50 ° C begins its decomposition into hydrogen and higher boranes ( Tetra, pentaborane, hexaborane, decaborane and others). The ignition temperature of pure diborane is 145 ° C. If it contains traces of higher boranes, it may be in the air spontaneously spontaneous combustion and burns it under high heat.

In practice, therefore, it ignites already at about 45 ° C. With an air volume rate 0.8 up to 88%, it forms explosive mixtures. Since the gas is therefore to be handled with caution, it is allowed to react mostly to aminoborane complexes with amines ().

This (liquid) substances serve as a precursor for diborane and can be stored and transported without danger. The recovery is performed by addition of strong acids (eg hydrochloric acid).

When dissolved in water diborane reacts with it. On the laboratory scale can be represented with iodine in diglyme diborane by oxidation of sodium borohydride.

Structure

Diborane is the simplest compound from the class of boranes as monomeric borane ( BH3 ) is not stable and the metastable adduct of diborane forms. The boron nuclei here are tetrahedrally surrounded by four hydrogen nuclei. The two bridging hydrogen nuclei form a two -electron three - center bond, to compensate for the electron deficiency of the boron nuclei. Two electrons are located here in an orbital three atoms distributed.

Chemical Properties

The boron in diborane has due to the two-electron three-center bonds to the two bridge hydrogen atoms is electron deficient. Characterized diborane is a monovalent Lewis acid reacts with Lewis bases, in an acid -base reaction with cleavage in monomeric boranes.

The salts of diborane contain the Monoboranat / Tetrahydroboranat / Tetrahydridoboranat ion BH4 and Monoboranate called / Tetrahydroboranate. An important representative is the sodium borohydride produced by the reaction of diborane with sodium hydride.

Use

Diborane is the most important reagent for hydroboration, (any R ) where alkenes are connected via the B -H bonds to trialkylborane.

This reaction is regioselective and the resulting trialkyl can be easily converted into other useful organic derivatives.

Diborane is used as a reducing agent such as a complement to the reactivity of lithium aluminum hydride. The compound slightly reduced carboxylic acids to the corresponding alcohols. It was also taken into consideration as rocket fuel, but it proved to be totally unsuitable as it forms during the combustion of boron oxide, which clogs the engines. Diborane is still used for rubber vulcanization. Boron compounds are generally used as catalysts in the polymerization of hydrocarbons, or in the production of anti-Markovnikov products.

Safety

The inhalation of diborane causes a cough, sore throat, dizziness, difficulty breathing, nausea and fatigue. It is highly toxic and extremely flammable.