N2O4

N2O4, or dinitrogen tetroxide, is a chemical compound with the formula N2O4. It is the dimer of nitrogen dioxide (NO2) and exists in a temperature-dependent equilibrium with NO2: N2O4 ⇌ 2 NO2. At lower temperatures the dimer is favored, while higher temperatures shift the equilibrium toward NO2.

In terms of appearance, N2O4 is typically described as colorless in its pure form, whereas NO2 is a brown gas. The visible color of an N2O4/NO2 mixture depends on temperature and concentration, reflecting the dynamic balance between the two species. N2O4 is a relatively strong oxidizer and is more stable than NO2 under many conditions, but it readily dissociates to NO2 upon heating or under photochemical conditions. It is soluble in various organic solvents and is highly reactive with reducing agents and with organic materials.

N2O4 is formed by the dimerization of NO2: 2 NO2 ⇌ N2O4. It can be generated in situ by cooling NO2 gas or by controlled oxidation processes. Industrially, it has been used as an oxidizer in rocket propulsion systems, notably in combination with certain fuels in bipropellant engines. Its role as a source of NO2+ equivalents in nitration chemistry is a historical consideration, though handling and safety concerns limit its routine use in synthesis.

Safety and handling are important due to its toxicity and oxidizing nature. N2O4 is corrosive and can cause irritation to the eyes, skin, and respiratory tract. It can contribute to the formation of nitrogen oxides (NOx) upon decomposition or interaction with moisture, and must be stored and used with appropriate containment, ventilation, and protective equipment.