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| Names | |
|---|---|
| IUPAC name Pentaborane(9) | |
| Other names Pentaborane, pentaboron nonahydride, stable pentaborane | |
| Identifiers | |
3D model (JSmol) | |
| ChEBI | |
| ChemSpider |
|
| ECHA InfoCard | 100.039.253 |
| EC Number |
|
| 26757 | |
| RTECS number |
|
| UNII | |
| UN number | 1380 |
| |
| |
| Properties | |
| B5H9 | |
| Molar mass | 63.12 g/mol |
| Appearance | Colorless liquid |
| Odor | pungent, like sour milk[1] |
| Density | 0.618 g/mL |
| Melting point | −46.8 °C (−52.2 °F; 226.3 K) |
| Boiling point | 58.4 °C (137.1 °F; 331.5 K)[2] |
| Reacts | |
| Solubility | Benzene,Cyclohexane, and in otherhydrocarbons |
| Vapor pressure | 171 mmHg (20°C)[1] |
| Hazards | |
| Occupational safety and health (OHS/OSH): | |
Main hazards | Extremely toxic, extremely flammable, can ignite spontaneously, corrosive |
| GHS labelling: | |
     | |
| H226,H250,H315,H318,H330,H335,H336,H370,H372 | |
| NFPA 704 (fire diamond) | |
| Flash point | 30 °C (86 °F; 303 K) |
| Explosive limits | 0.42%-?[1] |
| Lethal dose or concentration (LD, LC): | |
LD50 (median dose) | <50 mg/kg[3] |
LC50 (median concentration) | 3 ppm (mouse, 4 hr) 6 ppm (rat, 4 hr) 3.4 ppm (mouse, 4 hr) 35 ppm (dog, 15 min) 244 ppm (monkey, 2 min) 67 ppm (rat, 5 min) 40 ppm (mouse, 5 min) 31 ppm (rat, 15 min) 19 ppm (mouse, 15 min) 15 ppm (rat, 30 min) 11 ppm (mouse, 30 min) 10 ppm (rat, 1 hr) 6 ppm (mouse, 1 hr)[4] |
| NIOSH (US health exposure limits): | |
PEL (Permissible) | TWA 0.005 ppm (0.01 mg/m3)[1] |
REL (Recommended) | TWA 0.005 ppm (0.01 mg/m3) ST 0.015 ppm (0.03 mg/m3)[1] |
IDLH (Immediate danger) | 1 ppm[1] |
| Structure | |
| C4v | |
| 2.13D | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Pentaborane(9) is aninorganic compound with the formulaB5H9. It is one of the most commonboron hydride clusters, although it is a highly reactive compound. Because of its high reactivity with oxygen, it was once evaluated asrocket orjet fuel. Like many of the smaller boron hydrides, pentaborane is colourless,diamagnetic, and volatile. It is related topentaborane(11) (B5H11).
Its structure is that of five atoms ofboron arranged in a square pyramid. Each boron has a terminal hydride ligand and four hydrides span the edges of the base of the pyramid. It is classified as a nido cage.
It was first prepared byAlfred Stock bypyrolysis ofdiborane at about 200 °C.[7] An improved synthesis starts from salts ofoctahydrotriborate (B3H−8), which is converted to the bromideB3H7Br− using HBr.Pyrolysis of this bromide gives pentaborane.[8]
In the U.S., pentaborane was produced on a commercial scale by Callery Chemical Company.
Above 150 °C, it decomposes, producing hydrogen. Unlikediborane, It is quite stable at room temperature if stored properly. It is much more stable in presence of water than diborane.
Pentaborane is a highly polar compound, with a dipole moment of 2.13D.[9] It is soluble in hydrocarbons likebenzene, andcyclohexane, and ingreases including those used in lab equipment.
The chemistry of pentaborane is extensive.[10] Halogenation give the symmetrical derivativesB5H8X, which can be isomerised to place the halide on the base of the square pyramid. With strong bases such asalkyl lithium reagents, it can be deprotonated and the resulting lithium salts react with diverse electrophiles to give substituted derivatives. It isLewis acidic, forming double adducts with two equivalents oftrimethylphosphine. Pentaborane is used for the synthesis of other boron hydride clusters. It is also a precursor tometallaboranes. For example, it reacts withdiiron nonacarbonyl to giveB4H8Fe(CO)3.
Pentaborane was evaluated by both the U.S. and Russian armed services as a so-called "exotic fuel". Because simple boron compounds burn with a characteristic green flame, the nickname for this fuel in the U.S. industry was "Green Dragon". In terms ofheat of combustion, pentaborane surpasses its equivalent carbon compounds because their self-linking element,carbon, weighs at least onedalton more than an atom ofboron does, and some boranes contain more hydrogen than the carbon equivalent. The ease of breaking the chemical bonds of the compound is also taken into consideration.
Interest in this substance began as a possible fuel for high-speed jets. The propellant mix that would produce the greatestspecific impulse for a rocket motor is sometimes given asoxygen difluoride and pentaborane[citation needed]. During the early years of thespace race and themissile gap, American rocket engineers thought they could more cheaply produce a rocket that would compete with theSoviets by using an existing first stage and putting an upper stage with an engine that produces thrust at a very high specific impulse atop it, so projects were begun to investigate this fuel.
This pentaborane was considered for use as a fuel byNorth American Aviation when theXB-70 Valkyrie was in the planning stages, but the aircraft ended up using hydrocarbon fuel instead. Pentaborane was also investigated to be used as abipropellant withnitrogen tetroxide.[11] In the Soviet Union,Valentin Glushko used it for the experimentalRD-270M rocket engine, under development between 1962 and 1970.[12]
Other boranes were evaluated as fuels, including propylpentaborane (BEF-2) and ethyldecaborane (REF-3).[13]Diborane anddecaborane and their derivates were also investigated.
Problems with this fuel include its toxicity and its characteristic of bursting into flame on contact with the air. Furthermore, its exhaust (when used in a jet engine) would also be toxic.
The US destroyed its last stockpiles of "Green Dragon" in 2000, long after the pentaborane had been discarded as unworkable. The destruction procedurehydrolyzed the pentaborane withsteam to yield hydrogen and aboric acid solution. The long delay occurred in part because there are no industrial plants consuming pentaborane as afeedstock. Instead, army engineers constructed a bespoke system, nicknamed the "Dragon Slayer".[14]
Above 30 °C it can form explosive concentration of vapors with air. Its vapors are heavier than air. It ispyrophoric—can ignite spontaneously in contact with air, when even slightly impure. It can also readily form shock sensitive explosive compounds, and reacts violently with some fire suppressants, notably withhalocarbons and water. It is highly toxic and symptoms of lower-level exposure may occur with up to 48 hours delay. Its acute toxicity is comparable to somenerve agents.
Occupational exposure limits for pentaborane set by theOccupational Safety and Health Administration andNational Institute for Occupational Safety and Health stand at 0.005 ppm (0.01 mg/m3) over an eight-hour time-weighted average, with ashort-term exposure limit of 0.015 ppm (0.03 mg/m3).[15] The acute toxicity of pentaborane has caused it to be consideredimmediately dangerous to life and health, with a limit set at 1 ppm.[16]
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