Борейн

Substance identification

Borain (borane, borine, boron trihydride) is the simplest hydride compound of boron. Chemical formula BH3, CAS number 13283-31-3. Synonyms: borane, borine, boron trihydride, boron hydride (BH3), boron hydride.

Basic Attributes

• Molecular Weight: 13.834
• Exact weight: 14.033
• PSA Features: 0

Physico-chemical characteristics

• Density: 0.898 g/cm3 at -57 °C
• Melting point: -58.0 °C
• Boiling point: 53 °C at 760 Torr pressure

Substance class and purpose

Borain (BH3) is the simplest boron hydride, an electron-deficient molecule with an unfilled valence shell of the boron atom, which is why it exists in a free form extremely unstable and in real conditions dimerizes to diborane B2H6 or is stabilized in the form of complexes with electron pair donors (tetrahydrofuran, dimethyl sulfide, amines, phosphines). Borane complexes are widely used in organic synthesis for the reactions of hydroboration of alkenes and alkynes—the addition of boron and hydrogen at a multiple bond—and also as reducing agents that directly reduce carboxylic acids to alcohols and amides to amines. One significant application is as a source of borane for the asymmetric reduction of ketones using the Corey-Bakshi-Shibata method with an oxazaborolidine catalyst.

Electronic structure and reactivity

The boron atom in the BH3 molecule has sp2 hybridization and a vacant p orbital, making the molecule a strong Lewis acid. It is this vacant orbital that is responsible for both spontaneous dimerization into diborane through the formation of three-center two-electron B-H-B bonds, and for the easy formation of donor-acceptor complexes with any molecules possessing a lone electron pair.

Implications for industrial and laboratory synthesis

Hydroboration using borane complexes is one of the key methods for the regioselective and stereoselective introduction of hydroxyl groups into organic molecules through the intermediate formation of alkylboranes followed by oxidation with hydrogen peroxide. This approach is widely used in the synthesis of pharmaceutical substances and fine organic products.

Storage and transportation

BH3 is not stored in its free form due to instability - in practice, the substance is used exclusively in the form of stable complexes (borane-THF, borane-dimethyl sulfide), which are stored in sealed containers at low temperatures, in an inert gas atmosphere, away from sources of moisture and ignition. Before use, the complexes are dosed with a syringe through the septum membrane, avoiding contact of the contents with atmospheric air and moisture, which can destroy the complex with the rapid release of hydrogen and loss of the reducing activity of the reagent during the subsequent reaction, therefore the reagent is dosed immediately before use, and the remainder is not subject to long-term storage after opening the container.