Dinitrogen tetroxide , commonly called nitrogen tetroxide (NTO) , and sometimes (commonly among ex-USSR/Russian rocket engineers) as amyl , is the chemical compound N2O4 . It is a useful reagent in chemical synthesis . It forms an equilibrium mixture with nitrogen dioxide . Its molar mass is 92.011 g/mol.
Dinitrogen tetroxide is a powerful oxidizer that is hypergolic (spontaneously reacting) upon contact with various forms of hydrazine, which has made the pair a common bipropellant for rockets.
Structure and Properties
Dinitrogen tetroxide can be thought of as having two nitro groups (-NO 2 ) bonded together. It forms an equilibrium mixture with nitrogen dioxide.  The molecule is planar with an N-N bond distance of 1.78 and a NO distance of 1.19. The NN distance corresponds to a weak bond, as it is much longer than the average NN single bond length of 1.45.  This exceptionally weak bond ( the amount of overlapping of the sp 2 hybrid orbitals of the two NO 2 units  ) results in the simultaneous delocalization of the bonding electron pair throughout the entire N 2 O 4 molecule, and significantly each NO 2Electrostatic repulsion of molecular orbitals doubly occupied by the unit. 
Unlike any 2 , N 2 O 4 is diamagnetic because it has no unpaired electrons.  The liquid is also colorless, but can appear as a brownish yellow liquid due to the presence of not 2 according to the following equilibrium:
n 2 o 4 2 no 2
Higher temperatures push the equilibrium towards nitrogen dioxide. Essentially, some dinitrogen tetroxide is a component of nitrogen dioxide-containing smog.
Nitrogen tetroxide is made by the catalytic oxidation of ammonia: steam is used as a diluent to lower combustion temperatures. In the first step, ammonia is oxidized to nitric oxide:
4 NH 3 + 5 O 2 → 4 NO + 6 H 2 O
Most of the water condenses, and the gases are further cooled; The produced nitric oxide is oxidized to nitrogen dioxide, which is then dimerized to nitrogen tetroxide:
2 NO + O 2 → 2 NO 22 no 2 n 2 o 4
And the remaining water is removed as nitric acid. The gas is essentially pure nitrogen dioxide, which is condensed into dinitrogen tetroxide in a brine-cooled liquefier.
Dinitrogen tetroxide can also be prepared by the reaction of concentrated nitric acid and metallic copper. This synthesis is more practical in a laboratory setting and is commonly used as a demonstration or experiment in undergraduate chemistry laboratories. The oxidation of copper by nitric acid is a complex reaction that forms various nitrogen oxides of varying stability depending on the concentration of nitric acid, the 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 rocket propellant
Nitrogen tetroxide is used as an oxidizing agent in one of the most important rocket propellants because it can be stored as a liquid at room temperature. As early as 1944, research on the usefulness of dinitrogen tetroxide as an oxidizing agent for rocket fuel was carried out by German scientists, However the Germans only used it to a very limited extent as an additive to s-stauf (fuming nitric acid). It became the preferred oxidizer of choice for many rockets in both the United States and the USSR by the late 1950s. It is a hypergolic propellant in combination with a hydrazine based rocket fuel. One of the earliest uses of this combination was on the Titan family of rockets, originally as ICBMs and then as launch vehicles for several spacecraft. Also used on the US Gemini and Apollo spacecraft and the Space Shuttle, it is used as a station-keeping propellant on most geostationary satellites and many deep-space probes. It is also the primary oxidizer for Russia’s Proton rocket. It is used as a station-keeping propellant on most geostationary satellites and many deep-space probes. It is also the primary oxidizer for Russia’s Proton rocket. It is used as a station-keeping propellant on most geostationary satellites and many deep-space probes. It is also the primary oxidizer for Russia’s Proton rocket.
When used as a propellant, dinitrogen tetroxide is commonly referred to as nitrogen tetroxide and the abbreviation NTO is widely used. Additionally, NTO is often used with the addition of a small percentage of nitric oxide, which prevents stress-corrosion cracking of titanium alloys, and as such, propellant-grade NTO is converted to mixed oxides of nitrogen ( MoN). ) is referred to as . Most spacecraft now use MON instead of NTO; For example, the Space Shuttle Reaction Control System used MON3 (NTO containing 3% NO by weight).
On July 24, 1975, NTO poisoning affected three American astronauts on their 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 thrusters to fire after the fresh air intake in the cabin was opened, allowing NTO smoke to enter the cabin. One crew member lost consciousness during the descent. Upon landing, the crew was hospitalized for five days for chemical-induced pneumonia and edema.  
power generation using N 2 O 4
The tendency of N2O4 to reversibly break down into NO2 has led to research on its use as a so-called dissipative gas in advanced power generation systems.  The “cool” dinitrogen tetroxide is compressed and heated, causing it to dissociate into nitrogen dioxide at half its molecular weight. This heated nitrogen dioxide is expanded by means of a turbine, it is cooled and pressurized, and then further cooled in a heat sink, allowing it to recombine into nitrogen tetroxide at the original molecular weight goes. Then it is much easier to compress to start the whole cycle again. Such dissipative gas Breton cycles have the potential to significantly increase the efficiency of power conversion equipment.
Intermediate in the manufacture of nitric acid
Nitric acid is largely manufactured through N2O4 . This species reacts with water to give both nitrous acid and nitric acid:
N 2 O 4 + H 2 O → HNO 2 + HNO 3
On heating the product HNO 2 does not disproportionate to NO and more nitric acid. Upon exposure to oxygen, NO is converted back to nitrogen dioxide:
2 NO + O 2 → 2 NO 2
The resulting NO 2 and N 2 O 4 can then be returned to the cycle to give a mixture of nitrous and nitric acid.
Synthesis of Metal Nitrates
N 2 O 4 behaves as a salt [NO + ] [NO 3 – ], the former being a strong oxidant:
2 N 2 O 4 + M → 2 NO + M (NO 3 ) 2
where M = Cu , Zn , or Sn .
If the metal nitrate is completely under anhydrous conditions N 2 O 4are prepared from, a series of covalent metal nitrates can be formed with many transition metals. This is because there is a thermodynamic preference for covalent bonds with such metals rather than forming an ionic structure for the nitrate ion. Such compounds must be prepared under anhydrous conditions, as the nitrate ion is a much weaker ligand than water, and simple hydrated nitrate will form if water is present. The related anhydrous nitrates themselves are covalent, and many, such as anhydrous copper nitrate, are unstable at room temperature. Anhydrous titanium nitrate sublimes only in vacuum at 40 °C. Many anhydrous transition metal nitrates have striking colours. This branch of chemistry was developed by Cliff Edison and Norman Logan at the University of Nottingham in the UK during the 1960s and 1970s,