Pentaborane(9)

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Pentaborane(9)
Pentaborane.png
Pentaborane(9)-from-xtal-view-1-Mercury-3D-bs.png
Names
IUPAC name
Pentaborane(9)
Other names
Pentaborane, pentaboron nonahydride, stable pentaborane
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.039.253 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 243-194-4
26757
RTECS number
  • RY8925000
UNII
UN number 1380
  • InChI=1S/B5H9/c6-1-5-2(6)8-4(5)9-3(5)7-1/h1-5H Yes check.svgY
    Key: USBVLEBZPMQADS-UHFFFAOYSA-N Yes check.svgY
  • InChI=1/B5H9/c6-2-1-3(2,6)5(1,8-3)4(1,2,7-2)9-5/h1-5H
    Key: XPIBKKWNZBDJNI-UHFFFAOYAS
  • [H]1[BH]2[H][BH]3[BH]24[BH]1[H][BH]4[H]3
Properties
B5H9
Molar mass 63.12 g/mol
AppearanceColorless 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 other hydrocarbons
Vapor pressure 171 mmHg (20°C) [1]
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
Extremely toxic, extremely flammable, can ignite spontaneously, corrosive
NFPA 704 (fire diamond)
[3]
NFPA 704.svgHealth 4: Very short exposure could cause death or major residual injury. E.g. VX gasFlammability 4: Will rapidly or completely vaporize at normal atmospheric pressure and temperature, or is readily dispersed in air and will burn readily. Flash point below 23 °C (73 °F). E.g. propaneInstability 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g. white phosphorusSpecial hazards (white): no code
4
4
2
Flash point 30 °C (86 °F; 303 K)
Explosive limits 0.42%-? [1]
Lethal dose or concentration (LD, LC):
<50 mg/kg [4]
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) [5]
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
2.13 D
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)

Pentaborane(9) is an inorganic compound with the formula B5H9. It is one of the most common boron hydride clusters, although it is a highly reactive compound. Because of its high reactivity with oxygen, it was once evaluated as rocket or jet fuel. Like many of the smaller boron hydrides, pentaborane is colourless, diamagnetic, and volatile. It is related to pentaborane(11) (B5H11).

Structure, synthesis, properties

Its structure is that of five atoms of boron 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 by Alfred Stock by pyrolysis of diborane at about 200 °C. [7] An improved synthesis starts from salts of octahydrotriborate (B3H−8), which is converted to the bromide B3H7Br− using HBr. Pyrolysis of this bromide gives pentaborane. [8]

5 B3H7Br− → 3 B5H9 + 5 Br− + 4 H2

In the U.S., pentaborane was produced on a commercial scale by Callery Chemical Company.

Above 150 °C, it decomposes, producing hydrogen. Unlike diborane, 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 2.13 D. [9] It is soluble in hydrocarbons like benzene, and cyclohexane, and in greases including those used in lab equipment.

Reactions

The chemistry of pentaborane is extensive. [10] Halogenation give the symmetrical derivatives B5H8X, which can be isomerised to place the halide on the base of the square pyramid. With strong bases such as alkyl lithium reagents, it can be deprotonated and the resulting lithium salts react with diverse electrophiles to give substituted derivatives. It is Lewis acidic, forming double adducts with two equivalents of trimethylphosphine. Pentaborane is used for the synthesis of other boron hydride clusters. It is also a precursor to metallaboranes. For example, it reacts with diiron nonacarbonyl to give B4H8Fe(CO)3.

History of its use as a fuel

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 of heat of combustion, pentaborane surpasses its equivalent carbon compounds because their self-linking element, carbon, weighs at least one atomic mass unit more than an atom of boron, 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 greatest specific impulse for a rocket motor is sometimes given as oxygen difluoride and pentaborane[ citation needed ]. During the early years of the space race and the missile gap, American rocket engineers thought they could more cheaply produce a rocket that would compete with the Soviets 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 by North American Aviation when the XB-70 Valkyrie was in the planning stages, but the aircraft ended up using hydrocarbon fuel instead. Pentaborane was also investigated to be used as a bipropellant with nitrogen tetroxide. [11] In the Soviet Union, Valentin Glushko used it for the experimental RD-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 and decaborane 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 procedure hydrolyzed the pentaborane with steam to yield hydrogen and a boric acid solution. The long delay occurred in part because there are no industrial plants consuming pentaborane as a feedstock. Instead, army engineers constructed a bespoke system, nicknamed the "Dragon Slayer". [14]

Safety

As one of the compounds that have a NFPA 704 (fire diamond) rating of 4 for every category, it is naturally extremely dangerous.

Above 30 °C it can form explosive concentration of vapors with air. Its vapors are heavier than air. It is pyrophoric—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 with halocarbons 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 some nerve agents.

Occupational exposure limits for pentaborane set by the Occupational Safety and Health Administration and National Institute for Occupational Safety and Health stand at 0.005 ppm (0.01 mg/m3) over an eight-hour time-weighted average, with a short-term exposure limit of 0.015 ppm (0.03 mg/m3). [15] The acute toxicity of pentaborane has caused it to be considered immediately dangerous to life and health, with a limit set at 1 ppm. [16]

See also

Related Research Articles

<span class="mw-page-title-main">Boron hydride clusters</span>

Boron hydride clusters are compounds with the formula BxHy or related anions, where x ≥ 3. Many such cluster compounds are known. Common examples are those with 5, 10, and 12 boron atoms. Although they have few practical applications, the borane hydride clusters exhibit structures and bonding that differs strongly from the patterns seen in hydrocarbons. Hybrids of boranes and hydrocarbons, the carboranes are also well developed.

<span class="mw-page-title-main">Diborane</span> Chemical compound

Diborane(6), commonly known as diborane, is the chemical compound with the formula B2H6. It is a highly toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Given its simple formula, borane is a fundamental boron compound. It has attracted wide attention for its electronic structure. Several of its derivatives are useful reagents.

Monomethylhydrazine (MMH) is a highly toxic, volatile hydrazine derivative with the chemical formula CH6N2. It is used as a rocket propellant in bipropellant rocket engines because it is hypergolic with various oxidizers such as nitrogen tetroxide and nitric acid. As a propellant, it is described in specification MIL-PRF-27404.

<span class="mw-page-title-main">Decaborane</span> Chemical compound

Decaborane, also called decaborane(14), is the inorganic compound with the chemical formula B10H14. It is classified as a borane and more specifically a boron hydride cluster. This white crystalline compound is one of the principal boron hydride clusters, both as a reference structure and as a precursor to other boron hydrides. It is toxic and volatile, giving off a foul odor, like that of burnt rubber or chocolate.

<span class="mw-page-title-main">Hexaborane(10)</span> Chemical compound

Hexaborane, also called hexaborane(10) to distinguish it from hexaborane(12) (B6H12), is a boron hydride cluster with the formula B6H10. It is a colorless liquid that is unstable in air.

<span class="mw-page-title-main">Stibine</span> Chemical compound

Stibine (IUPAC name: stibane) is a chemical compound with the formula SbH3. A pnictogen hydride, this colourless, highly toxic gas is the principal covalent hydride of antimony, and a heavy analogue of ammonia. The molecule is pyramidal with H–Sb–H angles of 91.7° and Sb–H distances of 170.7 pm (1.707 Å). The smell of this compound from usual sources (like from reduction of antimony compounds) is reminiscent of arsine, i.e. garlic-like.

<span class="mw-page-title-main">Heptachlor</span> Chemical compound

Heptachlor is an organochlorine compound that was used as an insecticide. Usually sold as a white or tan powder, heptachlor is one of the cyclodiene insecticides. In 1962, Rachel Carson's Silent Spring questioned the safety of heptachlor and other chlorinated insecticides. Due to its highly stable structure, heptachlor can persist in the environment for decades. In the United States, the Environmental Protection Agency has limited the sale of heptachlor products to the specific application of fire ant control in underground transformers. The amount that can be present in different foods is regulated.

<span class="mw-page-title-main">Hydrogen selenide</span> Chemical compound

Hydrogen selenide is an inorganic compound with the formula H2Se. This hydrogen chalcogenide is the simplest and most commonly encountered hydride of selenium. H2Se is a colorless, flammable gas under standard conditions. It is the most toxic selenium compound with an exposure limit of 0.05 ppm over an 8-hour period. Even at extremely low concentrations, this compound has a very irritating smell resembling that of decayed horseradish or "leaking gas", but smells of rotten eggs at higher concentrations.

<span class="mw-page-title-main">Nitrogen trifluoride</span> Chemical compound

Nitrogen trifluoride is the inorganic compound with the formula. It is a colorless, non-flammable, toxic gas with a slightly musty odor. In contrast with ammonia, it is nonbasic. It finds increasing use within the manufacturing of flat-panel displays, photovoltaics, LEDs and other microelectronics. NF
3 is a greenhouse gas, with a global warming potential (GWP) 17,200 times greater than that of CO
2 when compared over a 100-year period.

<span class="mw-page-title-main">Propyl acetate</span> Chemical compound

Propyl acetate, also known as propyl ethanoate, is an organic compound. Nearly 20,000 tons are produced annually for use as a solvent. This colorless liquid is known by its characteristic odor of pears. Due to this fact, it is commonly used in fragrances and as a flavor additive. It is formed by the esterification of acetic acid and propan-1-ol, often via Fischer–Speier esterification, with sulfuric acid as a catalyst and water produced as a byproduct.

<span class="mw-page-title-main">Boron compounds</span>

Boron compounds are compounds containing the element boron. In the most familiar compounds, boron has the formal oxidation state +3. These include oxides, sulfides, nitrides, and halides.

Perchloryl fluoride is a reactive gas with the chemical formula ClO
3F. It has a characteristic sweet odor that resembles gasoline and kerosene. It is toxic and is a powerful oxidizing and fluorinating agent. It is the acid fluoride of perchloric acid.

<span class="mw-page-title-main">Nitroethane</span> Chemical compound

Nitroethane is an organic compound having the chemical formula C2H5NO2. Similar in many regards to nitromethane, nitroethane is an oily liquid at standard temperature and pressure. Pure nitroethane is colorless and has a fruity odor.

<span class="mw-page-title-main">Bromochloromethane</span> Chemical compound

Bromochloromethane or methylene bromochloride and Halon 1011 is a mixed halomethane. It is a heavy low-viscosity liquid with refractive index 1.4808.

<span class="mw-page-title-main">Dichlorofluoromethane</span> Chemical compound

Dichlorofluoromethane or Freon 21 or R 21 is a halomethane or hydrochlorofluorocarbon with the formula CHCl2F. It is a colorless and odorless gas. It is produced by fluorination of chloroform using a catalyst such as antimony trifluoride:

Selenium hexafluoride is the inorganic compound with the formula SeF6. It is a very toxic colourless gas described as having a "repulsive" odor. It is not widely encountered and has no commercial applications.

<span class="mw-page-title-main">Pentaborane(11)</span> Chemical compound

Pentaborane(11) is inorganic compound with the chemical formula B5H11. It is an obscure boron hydride cluster, especially relative to the heavily studied pentaborane(9) (B5H9). With two more hydrogen atoms than nido-pentaborane(9), pentaborane(11) is classified as an arachno- cluster.

<span class="mw-page-title-main">Octahydrotriborate</span> Chemical compound

Octahydrotriborate is the boron hydride anion B3H8−. It forms a variety of salts that are colorless and air-stable. The tetrabutylammonium salt is soluble in organic solvents such as acetonitrile and methylene chloride. The anion is an intermediate is the synthesis of various higher boron hydrides, such as pentaborane(9). B3H8− can be viewed as the conjugate base of triborane B3H9.

<span class="mw-page-title-main">Boranes</span> Class of chemical compounds

A borane is a compound with the formula BRxHy although examples include multi-boron derivatives. A large family of boron hydride clusters is also known. In addition to some applications in organic chemistry, the boranes have attracted much attention as they exhibit structures and bonding that differs strongly from the patterns seen in hydrocarbons. Hybrids of boranes and hydrocarbons, the carboranes, are also a well developed class of compounds.

References

  1. 1 2 3 4 5 6 NIOSH Pocket Guide to Chemical Hazards. "#0481". National Institute for Occupational Safety and Health (NIOSH).
  2. ↑ DeQuasie, Andrew. The Green Flame. Chapter 4. Excerpted and archived at the WayBack Machine.
  3. ↑ OSHA Occupational Chemical Database PENTABORANE
  4. ↑ Pentaborane chemical and safety data
  5. ↑ "Pentaborane". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  6. "NFPA Hazard Rating Information for Common Chemicals". Archived from the original on 17 February 2015. Retrieved 13 March 2015.
  7. ↑ Stock, A. (1933). The Hydrides of Boron and Silicon. New York: Cornell University Press. ISBN   0-8014-0412-6.
  8. ↑ Miller, V. R.; Ryschkewitsch, G. E. (1974). "Pentaborane(9) (B 5 H 9 )". Inorganic Syntheses. Vol. 15. pp. 118–122. doi:10.1002/9780470132463.ch26. ISBN   9780470132463.
  9. ↑ Hrostowski, Henry J.; Myers, Rollie J. (29 December 2004). "The Microwave Spectra, Structure, and Dipole Moment of Stable Pentaborane". The Journal of Chemical Physics. 22 (2): 262. doi:10.1063/1.1740048. ISSN   0021-9606.
  10. ↑ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN   978-0-08-037941-8.
  11. ↑ "N2O4/Pentaborane". Encyclopedia Astronautica. Archived from the original on 8 August 2007. Retrieved 11 August 2007.
  12. ↑ "RD-270M". Encyclopedia Astronautica. Archived from the original on 26 August 2002. Retrieved 11 August 2007.
  13. ↑ McDonald, G. (13 November 1957). "Thermal Stability of a Commercial Propyl Pentaborane (HEF-2) in the range 147 to 190 °C" (PDF). National Advisory Committee for Aeronautics.
  14. ↑ ""Dragon Slayer" neutralizes super fuel" (PDF). Engineer Update. 25 (2). U.S. Army Corps of Engineers. February 2001.
  15. ↑ CDC – NIOSH Pocket Guide to Chemical Hazards
  16. ↑ Documentation for Immediately Dangerous To Life or Health Concentrations (IDLHs)
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