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 B_nH_n+4)
• arachnoboranes (general formula B_nH_n+6)

For example:

• B₂H₆ : diborane
• B₄H₁₀ : tetraborane

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.