Borane

borane, any of a homologous series of inorganic compounds of boron and hydrogen or their derivatives.

The boron hydrides were first systematically synthesized and characterized during the period 1912 to roughly 1937 by the German chemist Alfred Stock. He called them boranes in analogy to the alkanes (saturated hydrocarbons), the hydrides of carbon (C), which is the neighbour of boron in the periodic table. Because the lighter boranes were volatile, sensitive to air and moisture, and toxic, Stock developed high-vacuum methods and apparatus for studying them. American work on boranes began in 1931, carried out by Hermann I. Schlesinger and Anton B. Burg. Boranes remained primarily of academic interest until World War II, when the U.S. government supported research to find volatile uranium compounds (borohydrides) for isotope separation, and the 1950s, when it supported programs to develop high-energy fuels for rockets and jet aircraft. (Boranes and their derivatives have much higher heats of combustion than hydrocarbon fuels.) William Nunn Lipscomb, Jr., received the 1976 Nobel Prize for Chemistry “for his studies on the structure of boranes illuminating problems of chemical bonding,” while one of Schlesinger’s students, Herbert Charles Brown, shared the 1979 prize for his hydroboration reaction (1956), the remarkably easy addition of BH₃ (in the form of BH₃ · S) to unsaturated organic compounds (i.e., alkenes and alkynes) in ether solvents (S) at room temperature to yield organoboranes quantitatively (that is, in a reaction that proceeds wholly, or almost wholly, to completion). The hydroboration reaction in turn opened up new avenues of research in the area of stereospecific organic synthesis.

The boranes that were prepared by Stock had the general composition B_nH_{n + 4} and B_nH_{n + 6}, but more complex species, both neutral and negative (anionic), are known. The hydrides of boron are more numerous than those of any other element except carbon. The simplest isolable borane is B₂H₆, diborane(6). (The Arabic numeral in parentheses indicates the number of hydrogen atoms.) It is one of the most extensively studied and most synthetically useful chemical intermediates. It is commercially available, and for years many boranes and their derivatives were prepared from it, either directly or indirectly. Free BH₃ (and B₃H₇) are very unstable, but they can be isolated as stable adducts (addition products) with Lewis bases (electron-donor molecules)—e.g., BH₃ · N(CH₃)₃. Boranes may be solids, liquids, or gases; in general, their melting and boiling points increase with increasing complexity and molecular weight.