Dinitrogen tetroxide, commonly referred to as nitrogen tetroxide (NTO), and occasionally (usually among ex-USSR/Russian rocket engineers) as amyl, is the

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

N₂O₄. It is a useful

reagent In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

in chemical synthesis. It forms an equilibrium mixture with

nitrogen dioxide Nitrogen dioxide is a chemical compound with the formula . One of several nitrogen oxides, nitrogen dioxide is a reddish-brown gas. It is a paramagnetic, bent molecule with C2v point group symmetry. Industrially, is an intermediate in the s ...

. Its molar mass is 92.011 g/mol. Dinitrogen tetroxide is a powerful

oxidizer An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ''electron donor''). In ot ...

that is

hypergolic A hypergolic propellant is a rocket propellant combination used in a rocket engine, whose components spontaneously ignite when they come into contact with each other. The two propellant components usually consist of a fuel and an oxidizer. The ...

(spontaneously reacts) upon contact with various forms of

hydrazine Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly hazardous unless handled in solution as, for example, hydraz ...

, which has made the pair a common

bipropellant The highest specific impulse chemical rockets use liquid propellants (liquid-propellant rockets). They can consist of a single chemical (a monopropellant) or a mix of two chemicals, called bipropellants. Bipropellants can further be divided into ...

for rockets.

Structure and properties

Dinitrogen tetroxide could be regarded as two

nitro group In organic chemistry, nitro compounds are organic compounds that contain one or more nitro functional groups (). The nitro group is one of the most common explosophores (functional group that makes a compound explosive) used globally. The nit ...

s (-NO₂) bonded together. It forms an equilibrium mixture with

. The molecule is planar with an N-N bond distance of 1.78Å and N-O distances of 1.19Å. The N-N distance corresponds to a weak bond, since it is significantly longer than the average N-N single bond length of 1.45Å. This exceptionally weak σ bond (amounting to overlapping of the ''sp''² hybrid orbitals of the two NO₂ units) results from the simultaneous delocalization of the bonding electron pair across the whole N₂O₄ molecule, and the considerable electrostatic repulsion of the doubly occupied molecular orbitals of each NO₂ unit. Unlike NO₂, N₂O₄ is

diamagnetic Diamagnetism is the property of materials that are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagn ...

since it has no unpaired electrons.Holleman, A. F.; Wiberg, E. (2001) ''Inorganic Chemistry''. Academic Press: San Diego. . The liquid is also colorless but can appear as a brownish yellow liquid due to the presence of NO₂ according to the following equilibrium: : N₂O₄ ⇌ 2 NO₂() Higher temperatures push the equilibrium towards nitrogen dioxide. Inevitably, some dinitrogen tetroxide is a component of

smog Smog, or smoke fog, is a type of intense air pollution. The word "smog" was coined in the early 20th century, and is a portmanteau of the words ''smoke'' and ''fog'' to refer to smoky fog due to its opacity, and odour. The word was then inte ...

containing nitrogen dioxide. Solid is white, and melts at −11.2 °C.

Production

Nitrogen tetroxide is made by the

catalytic oxidation Catalytic oxidation are processes that rely on catalysts to introduce oxygen into organic and inorganic compounds. Many applications, including the focus of this article, involve oxidation by oxygen. Such processes are conducted on a large scale ...

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

(the

Ostwald process The Ostwald process is a chemical process used for making nitric acid (HNO3). The Ostwald process is a mainstay of the modern chemical industry, and it provides the main raw material for the most common type of fertilizer production. Historically a ...

): steam is used as a

diluent A diluent (also referred to as a filler, dilutant or thinner) is a diluting agent. Certain fluids are too viscous to be pumped easily or too dense to flow from one particular point to the other. This can be troublesome, because it might not be ...

to reduce the combustion temperature. In the first step, the ammonia is oxidized into

nitric oxide Nitric oxide (nitrogen oxide, nitrogen monooxide, or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes den ...

: : 4 NH₃ + 5 O₂ → 4 NO + 6 H₂O Most of the water is condensed out, and the gases are further cooled; the nitric oxide that was produced is oxidized to nitrogen dioxide, which is then dimerized into nitrogen tetroxide: : 2 NO + O₂ → 2 NO₂ : 2 NO₂ ⇌ N₂O₄ and the remainder of the water is removed as

nitric acid Nitric acid is an inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but samples tend to acquire a yellow cast over time due to decomposition into nitrogen oxide, oxides of nitrogen. Most com ...

. The gas is essentially pure nitrogen dioxide, which is condensed into dinitrogen tetroxide in a brine-cooled liquefier. Dinitrogen tetroxide can also be made through the reaction of concentrated nitric acid and metallic copper. This synthesis is practical in a laboratory setting. Dinitrogen tetroxide can also be produced by heating metal nitrates. The oxidation of copper by nitric acid is a complex reaction forming various nitrogen oxides of varying stability which depends on the concentration of the nitric acid, presence of oxygen, and other factors. The unstable species further react to form nitrogen dioxide which is then purified and condensed to form dinitrogen tetroxide.

Use as a rocket propellant

Nitrogen tetroxide is used as an oxidizing agent in one of the most important rocket propellant systems because it can be stored as a liquid at room temperature.

Pedro Paulet Pedro Eleodoro Paulet Mostajo (2 July 1874 or 4 July 1875 – 30 January 1945) was a Peruvian diplomat and engineer. Some early rocket experts described him as a pioneer in aeronautics, saying that he was the first person to build a liquid-propel ...

, a

Peruvian Peruvians (''/peruanas'') are the citizens of Peru. What is now Peru has been inhabited for several millennia by cultures such as the Caral before the Spanish conquest in the 16th century. Peruvian population decreased from an estimated 5–9 ...

polymath A polymath or polyhistor is an individual whose knowledge spans many different subjects, known to draw on complex bodies of knowledge to solve specific problems. Polymaths often prefer a specific context in which to explain their knowledge, ...

, reported in 1927 that he had experimented in the 1890s with a rocket engine that used spring-loaded nozzles that periodically introduced vaporized nitrogen tetroxide and a

petroleum benzine Petroleum benzine is a hydrocarbon-based solvent mixture that is classified by its physical properties (e.g. boiling point, vapor pressure) rather than a specific chemical composition. The chemical composition of a petroleum distillate can be m ...

to a

spark plug A spark plug (sometimes, in British English, a sparking plug, and, colloquially, a plug) is a device for delivering electric current from an ignition system to the combustion chamber of a spark-ignition engine to ignite the compressed fuel/air ...

for ignition, with the engine putting out 300 pulsating explosions per minute. Paulet would go on to visit the German rocket association Verein für Raumschiffahrt (VfR) and on March 15, 1928, Valier applauded Paulet's liquid-propelled rocket design in the VfR publication ''Die Rakete'', saying the engine had "amazing power". Paulet would soon be approached by

Nazi Germany Nazi Germany, officially known as the German Reich and later the Greater German Reich, was the German Reich, German state between 1933 and 1945, when Adolf Hitler and the Nazi Party controlled the country, transforming it into a Totalit ...

to help develop rocket technology, though he refused to assist and never shared the formula for his propellant. In early 1944, research on the usability of dinitrogen tetroxide as an oxidizing agent for rocket fuel was conducted by German scientists, although the Germans only used it to a very limited extent as an additive for S-Stoff (fuming nitric acid). It became the storable oxidizer of choice for many rockets in both the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

and

USSR The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...

by the late 1950s. It is a

hypergolic propellant A hypergolic propellant is a rocket propellant combination used in a rocket engine, whose components spontaneously ignite when they come into contact with each other. The two propellant components usually consist of a fuel and an oxidizer. The ...

in combination with a

-based

rocket fuel Rocket propellant is used as reaction mass ejected from a rocket engine to produce thrust. The energy required can either come from the propellants themselves, as with a chemical rocket, or from an external source, as with ion engines. Overvi ...

. One of the earliest uses of this combination was on the Titan family of rockets used originally as ICBMs and then as

launch vehicle A launch vehicle is typically a rocket-powered vehicle designed to carry a payload (a crewed spacecraft or satellites) from Earth's surface or lower atmosphere to outer space. The most common form is the ballistic missile-shaped multistage ...

s for many spacecraft. Used on the U.S.

Gemini Gemini most often refers to: * Gemini (constellation), one of the constellations of the zodiac * Gemini (astrology), an astrological sign Gemini may also refer to: Science and technology Space * Gemini in Chinese astronomy, the Gemini constellat ...

and

Apollo Apollo is one of the Twelve Olympians, Olympian deities in Ancient Greek religion, ancient Greek and Ancient Roman religion, Roman religion and Greek mythology, Greek and Roman mythology. Apollo has been recognized as a god of archery, mu ...

spacecraft and also on the

Space Shuttle The Space Shuttle is a retired, partially reusable launch system, reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. ...

, it continues to be used as station-keeping propellant on most geo-stationary satellites, and many deep-space probes. It is also the primary oxidizer for Russia's

Proton rocket Proton (, formal designation: UR-500) is an expendable launch system used for both commercial and Russian government space launches. The first Proton rocket was launched in 1965. Modern versions of the launch system are still in use , making it ...

. When used as a propellant, dinitrogen tetroxide is usually referred to simply as ''nitrogen tetroxide'' and the abbreviation ''NTO'' is extensively used. Additionally, NTO is often used with the addition of a small percentage of

that reacts to form

dinitrogen trioxide Dinitrogen trioxide (also known as nitrous anhydride) is the inorganic compound with the formula . It is a nitrogen oxide. It forms upon mixing equal parts of nitric oxide and nitrogen dioxide and cooling the mixture below −21°C (−6°F): : + ...

, which inhibits

stress-corrosion cracking Stress corrosion cracking (SCC) is the growth of crack formation in a corrosive environment. It can lead to unexpected and sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. SCC ...

of titanium alloys, and in this form, propellant-grade NTO is referred to as ''

mixed oxides of nitrogen Mixed oxides of nitrogen (MON) are solutions of dinitrogen trioxide (N2O3) in dinitrogen tetroxide/nitrogen dioxide (N2O4 and NO2). It may be used as an oxidizing agent in rocket propulsion systems. Mixed oxides of nitrogen are produced by disso ...

'' (''MON'') and can be distinguished by its green-blue color. Larger additions of nitric oxide, up to 25-30%, also lower the freezing point of NTO, improving storability in space conditions. Most spacecraft now use MON instead of NTO; for example, the Space Shuttle reaction control system used MON3 (NTO containing 3% NO by weight).

The Apollo-Soyuz mishap

On 24 July 1975, NTO poisoning affected three U.S.

astronaut An astronaut (from the Ancient Greek (), meaning 'star', and (), meaning 'sailor') is a person trained, equipped, and deployed by a List of human spaceflight programs, human spaceflight program to serve as a commander or crew member of a spa ...

s on the final descent to Earth after the Apollo-Soyuz Test Project flight. This was due to a switch accidentally left in the wrong position, which allowed the

attitude control Spacecraft attitude control is the process of controlling the orientation of a spacecraft (vehicle or satellite) with respect to an inertial frame of reference or another entity such as the celestial sphere, certain fields, and nearby objects, ...

thrusters to fire after the cabin fresh air intake was opened, allowing NTO fumes to enter the cabin. One crew member lost consciousness during descent. Upon landing, the crew was hospitalized for five days for chemical-induced

pneumonia Pneumonia is an Inflammation, inflammatory condition of the lung primarily affecting the small air sacs known as Pulmonary alveolus, alveoli. Symptoms typically include some combination of Cough#Classification, productive or dry cough, ches ...

and

pulmonary edema Pulmonary edema (British English: oedema), also known as pulmonary congestion, is excessive fluid accumulation in the tissue or air spaces (usually alveoli) of the lungs. This leads to impaired gas exchange, most often leading to shortness ...

Power generation using N₂O₄

The tendency of N₂O₄ to reversibly break into NO₂ has led to research into its use in advanced power generation systems as a so-called dissociating gas. "Cool" dinitrogen tetroxide is compressed and heated, causing it to dissociate into

at half the molecular weight. This hot nitrogen dioxide is expanded through a turbine, cooling it and lowering the pressure, and then cooled further in a heat sink, causing it to recombine into nitrogen tetroxide at the original molecular weight. It is then much easier to compress to start the entire cycle again. Such dissociative gas

Brayton cycle The Brayton cycle, also known as the Joule cycle, is a thermodynamic cycle that describes the operation of certain heat engines that have air or some other gas as their working fluid. It is characterized by isentropic process, isentropic compre ...

s have the potential to considerably increase efficiencies of power conversion equipment. The high molecular weight and smaller volumetric expansion ratio of nitrogen dioxide compared to steam allows the turbines to be more compact. N₂O₄ was the main component of the "nitrin" working fluid in the decommissioned Pamir-630D portable nuclear reactor which operated from 1985 to 1987.

Chemical reactions

Intermediate in the manufacture of nitric acid

Nitric acid is manufactured on a large scale via N₂O₄. This species reacts with water to give both

nitrous acid Nitrous acid (molecular formula ) is a weak and monoprotic acid known only in solution, in the gas phase, and in the form of nitrite () salts. It was discovered by Carl Wilhelm Scheele, who called it " phlogisticated acid of niter". Nitrous ac ...

and

: : N₂O₄ + H₂O → HNO₂ + HNO₃ The coproduct HNO₂ upon heating

disproportionates In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation state. The reverse of disproportionatio ...

to NO and more nitric acid. When exposed to oxygen, NO is converted back into nitrogen dioxide: : 2 NO + O₂ → 2 NO₂ The resulting NO₂ and N₂O₄ can be returned to the cycle to give the mixture of nitrous and nitric acids again.

Synthesis of metal nitrates

N₂O₄ undergoes

molecular autoionization In chemistry, molecular autoionization (or self-ionization) is a chemical reaction between molecules of the same substance to produce ions. If a pure liquid partially dissociates into ions, it is said to be self-ionizing. In most cases the ox ...

to give O⁺ O₃⁻ with the former

nitrosonium The nitrosonium ion is , in which the nitrogen atom is bonded to an oxygen atom with a bond order of 3, and the overall diatomic species bears a positive charge. It can be viewed as nitric oxide with one electron removed. This ion is usually o ...

ion being a strong oxidant. Various anhydrous

transition metal nitrate complex A transition metal nitrate complex is a coordination compound containing one or more nitrate ligands. Such complexes are common starting reagents for the preparation of other compounds. Ligand properties Nitrate is isostructural with but less b ...

es can be prepared from N₂O₄ and base metal. : 2 N₂O₄ + M → 2 NO + M(NO₃)₂ where M = Cu, Zn, or Sn. If metal nitrates are prepared from N₂O₄ in completely anhydrous conditions, a range of covalent metal nitrates can be formed with many transition metals. This is because there is a thermodynamic preference for the nitrate ion to bond covalently with such metals rather than form an ionic structure. Such compounds must be prepared in anhydrous conditions, since the nitrate ion is a much weaker ligand than water, and if water is present the simple nitrate of the hydrated metal ion will form. The anhydrous nitrates concerned are themselves covalent, and many, e.g. anhydrous

copper nitrate Copper(II) nitrate describes any member of the family of inorganic compounds with the formula Cu( NO3)2(H2O)x. The hydrates are hygroscopic blue solids. Anhydrous copper nitrate forms blue-green crystals and sublimes in a vacuum at 150-200 °C ...

, are volatile at room temperature. Anhydrous titanium nitrate sublimes in vacuum at only 40 °C. Many of the anhydrous transition metal nitrates have striking colours. This branch of chemistry was developed by Cliff Addison and Norman Logan at the

University of Nottingham The University of Nottingham is a public research university in Nottingham, England. It was founded as University College Nottingham in 1881, and was granted a royal charter in 1948. Nottingham's main campus (University Park Campus, Nottingh ...

in the UK during the 1960s and 1970s when highly efficient desiccants and dry boxes started to become available.

With organic compounds

In even slightly basic solvents, N₂O₄ adds to

alkene In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins. The Internationa ...

s radically, giving mixtures of

nitro compound In organic chemistry, nitro compounds are organic compounds that contain one or more nitro functional groups (). The nitro group is one of the most common explosophores (functional group that makes a compound explosive) used globally. The nit ...

s and

nitrite ester In organic chemistry, alkyl nitrites are a group of organic compounds based upon the molecular structure , where R represents an alkyl group. Formally they are alkyl esters of nitrous acid. They are distinct from nitro compounds (). The first ...

s. Pure or in entirely nonbasic solvents, the compounds autoionizes as above, to give

nitroso compound In organic chemistry, nitroso refers to a functional group in which the nitric oxide () group is attached to an organic moiety. As such, various nitroso groups can be categorized as ''C''-nitroso compounds (e.g., nitrosoalkanes; ), ''S''-nitros ...

s and

nitrate ester In organic chemistry, a nitrate ester is an organic functional group with the formula , where R stands for any organyl group. They are the esters of nitric acid and alcohols. A well-known example is nitroglycerin, which is not a ''nitro'' comp ...

References

External links

International Chemical Safety Card 0930

* ttps://www.cdc.gov/niosh/npg/npgd0454.html NIOSH Pocket Guide to Chemical Hazards Nitrogen tetroxide
Air Liquide Gas Encyclopedia: NO₂ / N₂O₄
* {{DEFAULTSORT:Dinitrogen Tetroxide Nitrogen oxides Rocket oxidizers Oxidizing agents Gases with color