One of a class of binary compounds of boron and hydrogen, often referred
to as boron hydrides. The term borane is sometimes used to denote substances
which may be considered to be derivatives of the boron-hydrogen compounds, such
as boron trichloride (BCl3), and diiododecaborane (B10H12I2).
The simplest borane is diborane (B2H6); other boranes
of increasingly higher molecular weight are known, one of the least volatile
of which is an apparently polymeric solid of composition (BH)x.
Certain boranes, such as BH3, and B3H7, are
not known as such, but can be prepared in the form of adducts with
electron-donor molecules.
The most spectacular projected large-scale use of the boranes and their
derivatives is in the field of high-energy fuels for jet planes and rockets.
The thermal decomposition of diborane (6) [B2H6] has been
used to produce coatings of pure elementary boron for neutron-detecting devices
and for applications requiring hard, corrosion-resistant surfaces. Boranes can
also be used as vulcanizing agents for natural and synthetic rubbers, and are
especially effective in the preparation of silicone rubbers.
The molecular structures possessed by the boranes are exhibited by no
other class of substances. Because of the lack of sufficient electrons for the
formation of the requisite number of covalent bonds, normal covalently bonded
structures of the hydrocarbon type are not possible. The boranes are sometimes
referred to as electron-deficient substances. In no case are the simple chain
and ring configurations of carbon chemistry encountered in the more complex
boranes. Instead, the boron atoms are situated at the comers of polyhedrons. An
example of such a structure is that of pentaborane (9) [B5H9].
Boron nomenclature uses a prefix to designate the number of boron atoms in the
molecule and a numeral suffix in parentheses to indicate the number of hydrogen
atoms.
As a class, the boranes are quite reactive substances and are generally
decomposed, at times explosively, on contact with air. Their reactivities with
air and water decrease with increasing molecular weight. Because boranes react
readily with air, laboratory investigations are almost invariably carried out
in all-glass vacuum apparatus or in inert-atmosphere dry boxes. With the
possible exception of decaborane (14) [B10H14], the known
boranes are not indefinitely stable at room temperature. They decompose more or
less rapidly to yield elementary hydrogen and boranes richer in boron. The
known derivatives of the boranes ( other than BH3) are relatively
few in number. Several halo, alkyl, and amino boranes have been reported but,
in general, these have not been extensively characterized.