A borane is an inorganic chemical compound of
boron and hydrogen. The lighter boranes are notably unstable
- diborane ignites in air to burn with a green flame - 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
standard ball-and-stick models; electrons are shared between the boron
atoms.
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 XB70 . The development
of advanced surface-to-air missiles made the fast aircraft redundant,
and the fuel programs were shut down: boranes were used to light the engines
of the SR71 high-speed plane.
There are 2 major classes of boranes:
- nidoboranes (general formula BnHn+4)
- arachnoboranes (general formula BnHn+6)
For example:
Diborane is manufactured in kilotons annually; it is used as a
dopant in semiconductors as well as in organic synthesis. It is
produced by reacting methyl borate with sodium hydride at elevated
temperatures to get sodium trimethoxyborohydride, which then reacts
with boron trifluoride to produce diborane.
Tetraborane is formed in the reaction of magnesium boride with acid.
Boranes are generally at least somewhat toxic; the exposure limit for
diborane is 100 parts per billion.
