Borane
A
borane is an inorganic
chemical compound of
boron and
hydrogen. The lighter boranes are notably unstable â€"
diborane ignites in air to burn with a green flame â€" but higher ones are much less so.
Decaborane is stable and crystalline, reacting with neither air nor water.
They are named by analogy with the
alkanes, which are
carbon-hydrogen compounds. The
salts of boranes are called
borohydrides. The bonding in boranes is not explicable by a standard
covalent bonding scheme, and is best described by
3-center-2-electron bonds.
Organoboranes are compounds containing carbon as well as boron and hydrogen.
German chemist Alfred Stock was the first scientist to characterize the series of boron-hydrogen compounds by analogy with
hydrocarbons. The boranes remained a laboratory curiosity until World War Two, where there was some interest in using
uranium borohydride as a volatile uranium compound for isotope separation. Dr
Herbert C. Brown,
Nobel prize winner in 1979, started working on boranes at the University of Chicago in 1942 under the auspices of this project, and never really stopped.
Borane-based reagants are now widely used in organic synthesis;
sodium borohydride is the standard reactant for converting
aldehydes and
ketones to alcohols.
The US and USSR both spent very substantial amounts in the fifties and early sixties researching boron-based high-energy fuels (ethylboranes, for example) for very fast aircraft such as the
XB-70 Valkyrie. The development of advanced surface-to-air missiles made the fast aircraft redundant, and the fuel programs were shut down, although
triethylborane (TEB) was later used to light the engines of the
SR-71 Blackbird high-speed plane.
[http://incolor.inebraska.com/hwolfe/history/sr71.pdf]The chemistry of boranes is dominated by boron possessing only three valence electrons, but four valence orbitals. Imagining a covalently-bonded system for BH
3, boron, with its three valence electrons, will bond to three hydrogen atoms, which in turn each share one electron with the boron to give it a valence electron count of six. However, the
octet rule states that a top row atom such as boron must fill its valence orbitals for maximum stability, and therefore possess eight valence electrons, yet the boron in discrete BH
3 has only six valence electrons, and hence a vacant p-orbital. The
unsaturation of borane results in a highly-reactive species that only exists in the gas phase. It readily dimerises to form
diborane and, with larger numbers of boron atoms,
clusters.
Cluster formation overcomes the electron deficiency of boranes by utilising a molecular orbital bonding scheme that gives rise to
3-center-2-electron bonds. Using empirical rules developed by K. Wade and later improved by M. Mingos, known as
Polyhedral skeletal electron pair theory or
Wade's/Mingos' rules, the structure of a boron cluster can often be unambiguously determined from the chemical formula.
Image:Diborane-3D-sticks.png|DiboraneImage:Pentaborane.png|PentaboraneImage:Decaborane-3D-balls.png|DecaboraneDiborane is manufactured in kilotons annually; it is used as a
dopant in semiconductors as well as in organic synthesis. Diborane can be prepared by reacting a hydride agent such as
sodium borohydride to
boron trifluoride or by adding
sodium borohydride to
sulfuric acid. It can be produced industrially by
reducing borax with
aluminium and
hydrogen at high pressure with a
aluminium chloride catalyst.
[Modern Inorganic Chemistry W.L. Jolly, ISBN 0-07-032-760-2]Boranes are generally at least somewhat toxic; the exposure limit for diborane is 100 parts per billion.
*
National Pollutant Inventory - Boron and compounds*
BoronWeb
