Pentaborane
Encyclopedia
Pentaborane, also called pentaborane(9) to distinguish it from B5H11, 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 toward oxygen, it was once evaluated as rocket or jet fuel
. Like many of the smaller boron hydrides, pentaborane is colourless, diamagnetic and volatile.
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. An improved synthesis starts from salts of B3H8-, which is converted to the bromide B3H7Br- using is HBr. Pyrolysis
of this bromide gives pentaborane. In the US, pentaborane was produced by Callery Chemical Company.
Pentaborane is a colorless mobile liquid with a pungent odor (resembling garlic, acetylene, or sour milk). Above 150 °C, it decomposes, producing hydrogen; when it occurs in a closed container, the consequent rise of pressure can be dangerous. It is much more stable in presence of water than diborane
. It is soluble in hydrocarbons, benzene
, and cyclohexane
, and in grease
s including those used in lab equipment. In storage, it decomposes negligibly, yielding small amount of hydrogen and solid residue.
. Pentaborane is used for synthesis of other boron hydride clusters.
, pentaborane surpasses its equivalent carbon compounds because their self-linking element, carbon
, weighs two unified atomic mass units 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. 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. In the Soviet Union, Valentin Glushko
used it for the experimental RD-270M
rocket engine, under development between 1962-1970.
Other boranes were evaluated as fuels, including propylpentaborane (BEF-2) and ethyldecaborane
(REF-3). 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 would also be toxic. Long after the pentaborane was considered unworkable, the total United States
stock of the chemical, 1900 pounds, was destroyed in the year 2000, when a safe and inexpensive means for doing so was finally engineered. The process used hydrolysis
with steam
, yielding hydrogen and a boric acid
solution. The system was nicknamed "Dragon Slayer".
s 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 agent
s.
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
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 toward oxygen, it was once evaluated as rocket or jet fuel
Jet fuel
Jet fuel is a type of aviation fuel designed for use in aircraft powered by gas-turbine engines. It is clear to straw-colored in appearance. The most commonly used fuels for commercial aviation are Jet A and Jet A-1 which are produced to a standardized international specification...
. Like many of the smaller boron hydrides, pentaborane is colourless, diamagnetic and volatile.
Structure, synthesis, properties
Its structure is that of five atoms of boronBoron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
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
Alfred Stock
Alfred Stock was a German inorganic chemist. He did pioneering research on the hydrides of boron and silicon, coordination chemistry, mercury, and mercury poisoning...
by pyrolysis
Pyrolysis
Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...
of diborane
Diborane
Diborane is the chemical compound consisting of boron and hydrogen with the formula B2H6. It is a colorless gas at room temperature with a repulsively sweet odor. Diborane mixes well with air, easily forming explosive mixtures. Diborane will ignite spontaneously in moist air at room temperature...
at about 200 °C. An improved synthesis starts from salts of B3H8-, which is converted to the bromide B3H7Br- using is HBr. Pyrolysis
Pyrolysis
Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...
of this bromide gives pentaborane. In the US, pentaborane was produced by Callery Chemical Company.
Pentaborane is a colorless mobile liquid with a pungent odor (resembling garlic, acetylene, or sour milk). Above 150 °C, it decomposes, producing hydrogen; when it occurs in a closed container, the consequent rise of pressure can be dangerous. It is much more stable in presence of water than diborane
Diborane
Diborane is the chemical compound consisting of boron and hydrogen with the formula B2H6. It is a colorless gas at room temperature with a repulsively sweet odor. Diborane mixes well with air, easily forming explosive mixtures. Diborane will ignite spontaneously in moist air at room temperature...
. It is soluble in hydrocarbons, benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
, and cyclohexane
Cyclohexane
Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is used as a nonpolar solvent for the chemical industry, and also as a raw material for the industrial production of adipic acid and caprolactam, both of which being intermediates used in the production of nylon...
, and in grease
Grease (lubricant)
The term grease is used to describe semisolid lubricants. Although the word grease is also used to describe rendered fat of animals, in the context of lubrication, grease typically applies to a material consisting of a soap emulsified with mineral or vegetable oil...
s including those used in lab equipment. In storage, it decomposes negligibly, yielding small amount of hydrogen and solid residue.
Reactions
The chemistry of pentaborane is extensive. Halogenation give the symmetrical derivatives B5H8X which can be isomerised to place the halide on the base. 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 trimethylphosphineTrimethylphosphine
Trimethylphosphine is the organophosphorus compound with the formula P3, commonly abbreviated PMe3. This colorless liquid has a strongly unpleasant odour, which is characteristic of alkylphosphines. It is a pyramidal molecule with C3v symmetry, similar to ammonia and phosphine . As a ligand, its...
. Pentaborane is used for synthesis of other boron hydride clusters.
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 combustionHeat of combustion
The heat of combustion is the energy released as heat when a compound undergoes complete combustion with oxygen under standard conditions. The chemical reaction is typically a hydrocarbon reacting with oxygen to form carbon dioxide, water and heat...
, pentaborane surpasses its equivalent carbon compounds because their self-linking element, carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, weighs two unified atomic mass units more than an atom of boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
, 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
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...
for a rocket motor is sometimes given as oxygen difluoride
Oxygen difluoride
Oxygen difluoride is the chemical compound with the formula F2O. As predicted by VSEPR theory, the molecule adopts a "V" shaped structure like H2O, but it has very different properties, being a strong oxidizer.-Preparation:...
and pentaborane. During the early years of the space race
Space Race
The Space Race was a mid-to-late 20th century competition between the Soviet Union and the United States for supremacy in space exploration. Between 1957 and 1975, Cold War rivalry between the two nations focused on attaining firsts in space exploration, which were seen as necessary for national...
and the missile gap
Missile gap
The missile gap was the term used in the United States for the perceived disparity between the number and power of the weapons in the U.S.S.R. and U.S. ballistic missile arsenals during the Cold War. The gap only existed in exaggerated estimates made by the Gaither Committee in 1957 and United...
, 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
North American Aviation
North American Aviation was a major US aerospace manufacturer, responsible for a number of historic aircraft, including the T-6 Texan trainer, the P-51 Mustang fighter, the B-25 Mitchell bomber, the F-86 Sabre jet fighter, the X-15 rocket plane, and the XB-70, as well as Apollo Command and Service...
when the XB-70 Valkyrie
XB-70 Valkyrie
The North American Aviation XB-70 Valkyrie was the prototype version of the proposed B-70 nuclear-armed deep-penetration strategic bomber for the United States Air Force's Strategic Air Command...
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. In the Soviet Union, Valentin Glushko
Valentin Glushko
Valentin Petrovich Glushko or Valentyn Petrovych Hlushko was a Soviet engineer, and the principal Soviet designer of rocket engines during the Soviet/American Space Race.-Biography:...
used it for the experimental RD-270M
RD-270
RD-270 - is a single chamber liquid bipropellant rocket engine designed by Energomash in 1960-1970. It was to be used on the first stages of proposed heavy-lift UR-700 and UR-900 rocket families. It has the highest thrust among single chambered engines of USSR and Russia, 640 metric tons at the...
rocket engine, under development between 1962-1970.
Other boranes were evaluated as fuels, including propylpentaborane (BEF-2) and ethyldecaborane
Decaborane
Decaborane, also called decaborane, is the borane with the chemical formula B10H14. 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....
(REF-3). Diborane
Diborane
Diborane is the chemical compound consisting of boron and hydrogen with the formula B2H6. It is a colorless gas at room temperature with a repulsively sweet odor. Diborane mixes well with air, easily forming explosive mixtures. Diborane will ignite spontaneously in moist air at room temperature...
and decaborane
Decaborane
Decaborane, also called decaborane, is the borane with the chemical formula B10H14. 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....
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 would also be toxic. Long after the pentaborane was considered unworkable, the total United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
stock of the chemical, 1900 pounds, was destroyed in the year 2000, when a safe and inexpensive means for doing so was finally engineered. The process used hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
with steam
Steam
Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...
, yielding hydrogen and a boric acid
Boric acid
Boric acid, also called hydrogen borate or boracic acid or orthoboric acid or acidum boricum, is a weak acid of boron often used as an antiseptic, insecticide, flame retardant, as a neutron absorber, and as a precursor of other chemical compounds. It exists in the form of colorless crystals or a...
solution. The system was nicknamed "Dragon Slayer".
Safety
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 halocarbonHalocarbon
Halocarbon compounds are chemicals in which one or more carbon atoms are linked by covalent bonds with one or more halogen atoms resulting in the formation of organofluorine compounds, organochlorine compounds, organobromine compounds, and organoiodine compounds...
s 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 agent
Nerve agent
Nerve agents are a class of phosphorus-containing organic chemicals that disrupt the mechanism by which nerves transfer messages to organs...
s.