Dinitrogen tetroxide

Nitrogen tetroxide (dinitrogen tetroxide or nitrogen peroxide) is the chemical compound N2O4. It is a powerful oxidizer, and is highly toxic and corrosive. N2O4 has received much attention as a rocket propellant. It is a useful reagent in chemical synthesis.

Structure and properties
The molecule is planar with an N-N bond distance of 1.78 Å and N-O distances of 1.19 Å. Unlike NO2, N2O4 is diamagnetic. It is also colorless but can appear brownish yellow liquid due to the presence of NO2 according to the following equilibrium:


 * N2O4 2NO2

Higher temperatures push the equilibrium towards nitrogen dioxide. Inevitably, some nitrogen tetroxide is a component of smog containing nitrogen dioxide.

Production
Nitrogen dioxide is made by the catalytic oxidation of ammonia: steam is used as a diluent to reduce the combustion temperature. Most of the water is condensed out, and the gases are further cooled; the nitric oxide that was produced is oxidised to nitrogen dioxide, and the remainder of the water is removed as nitric acid. The gas is essentially pure nitrogen tetroxide, which is condensed in a brine-cooled liquefier.

Use as a rocket propellant
Dinitrogen tetroxide is one of the most important rocket propellants ever developed, and by the late 1950s it became the storable oxidizer of choice for rockets in both the USA and USSR. It is a hypergolic propellant often used in combination with a hydrazine-based rocket fuel. One of the earliest uses of this combination was on the Titan rockets used originally as ICBM's and then as launch-vehicles for many spacecraft. Used on the U.S. Gemini and Apollo spacecraft, it continues to be used on the Space Shuttle, most geo-stationary satellites, and many deep-space probes. It now seems likely that NASA will continue to use this oxidiser in the next-generation 'crew-vehicles' which will replace the shuttle. It is also the primary oxidizer for Russia's Proton rocket and China's Long March rockets.

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 nitric oxide, which inhibits stress-corrosion cracking of titanium alloys, and in this form, propellant-grade NTO is referred to as "Mixed Oxides of Nitrogen" or "MON". Most spacecraft now use MON instead of NTO, for example, the Space Shuttle reaction control system uses MON3 (NTO containing 3wt%NO). 

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" nitrogen tetroxide is compressed and heated, causing it to dissociate into nitrogen dioxide 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 cycles have the potential to considerably increase efficiencies of power conversion equipment.

Chemical reactions
N2O4 has a very rich chemistry.

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 and nitric acid:
 * N2O4 + H2O →  HNO2  +  HNO3

The coproduct HNO2 upon heating disproportionates to NO and more nitric acid.

Synthesis of metal nitrates
N2O4 behaves as the salt [NO+][NO3−], the former being a strong oxidant:
 * 2 N2O4 +  M  →  2 NO  +  M(NO3)2

(M = Cu, Zn, Sn). N2O4 see: NOBF4