Pentaborane(9)

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Pentaborane(9)
Names
IUPAC name
Pentaborane(9)
Other names
Pentaborane, pentaboron nonahydride, stable pentaborane
Identifiers
3D model (
JSmol
)
ChEBI
ChemSpider
ECHA InfoCard
 100.039.253 Edit this at Wikidata
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 checkY
    Key: USBVLEBZPMQADS-UHFFFAOYSA-N checkY
  • 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
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
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)
NFPA 704 four-colored diamondHealth 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 4: Readily capable of detonation or explosive decomposition at normal temperatures and pressures. E.g. nitroglycerinSpecial hazard W: Reacts with water in an unusual or dangerous manner. E.g. sodium, sulfuric acid
4
4
4
W
Flash point 30 °C (86 °F; 303 K)
Explosive limits
 0.42%-?[1]
Lethal dose or concentration (LD, LC):
<50 mg/kg[3]
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
2.13 D
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

Pentaborane(9) is an

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.[6] 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.[7]

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.[8] 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.

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

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

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

nitrogen tetroxide.[10] In the Soviet Union, Valentin Glushko used it for the experimental RD-270M rocket engine, under development between 1962 and 1970.[11]

Other boranes were evaluated as fuels, including propylpentaborane (BEF-2) and ethyldecaborane (REF-3).[12] 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

feedstock. Instead, army engineers constructed a bespoke system, nicknamed the "Dragon Slayer".[13]

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

immediately dangerous to life and health, with a limit set at 1 ppm.[15]

See also

References

  1. ^ a b c d e f NIOSH Pocket Guide to Chemical Hazards. "#0481". National Institute for Occupational Safety and Health (NIOSH).
  2. WayBack Machine
    .
  3. ^ Pentaborane chemical and safety data
  4. ^ "Pentaborane". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  5. ^ "NFPA Hazard Rating Information for Common Chemicals". Archived from the original on 17 February 2015. Retrieved 13 March 2015.
  6. ISBN 0-8014-0412-6
    .
  7. ISBN 9780470132463
    .
  8. ISSN 0021-9606
    .
  9. ISBN 978-0-08-037941-8
    .
  10. ^ "N2O4/Pentaborane". Encyclopedia Astronautica. Archived from the original on 8 August 2007. Retrieved 11 August 2007.
  11. ^ "RD-270M". Encyclopedia Astronautica. Archived from the original on 26 August 2002. Retrieved 11 August 2007.
  12. ^ 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.
  13. U.S. Army Corps of Engineers
    . February 2001.
  14. ^ CDC – NIOSH Pocket Guide to Chemical Hazards
  15. ^ Documentation for Immediately Dangerous To Life or Health Concentrations (IDLHs)
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