Nitric oxide or nitrogen monoxide is a chemical compound with chemical formula NO. This gas is an important signaling molecule in the body of mammals, including humans, and is an extremely important intermediate in the chemical industry. It is also an air pollutant produced by cigarette smoke, automobile engines and power plants.

NO is an important messenger molecule involved in many physiological and pathological processes within the mammalian body both beneficial and detrimental. Appropriate levels of NO production are important in protecting an organ such as the liver from ischemic damage. However sustained levels of NO production result in direct tissue toxicity and contribute to the vascular collapse associated with septic shock, whereas chronic expression of NO is associated with various carcinomas and inflammatory conditions including juvenile diabetes, multiple sclerosis, arthritis and ulcerative colitis.

Nitric oxide should not be confused with nitrous oxide (N₂O), a general anaesthetic and greenhouse gas, or with nitrogen dioxide (NO₂) which is another air pollutant. The nitric oxide molecule is a free radical, which is relevant to understanding its high reactivity.

Despite being a simple molecule, NO is a fundamental player in the fields of neuroscience, physiology, and immunology, and was proclaimed “Molecule of the Year” in 1992

Reactions

• When exposed to oxygen, NO is converted into nitrogen dioxide.

  2 NO + O₂ → 2 NO₂

  This conversion has been speculated as occurring via the ONOONO intermediate. In water, NO reacts with oxygen and water to form HNO₂ or nitrous acid. The reaction is thought to proceed via the following stoichiometry:

  4 NO + O₂ + 2 H₂O → 4 HNO₂

  • NO will react with fluorine, chlorine, and bromine to form the XNO species, known as the nitrosyl halides, such as nitrosyl chloride. Nitrosyl iodide can form but is an extremely short lived species and tends to reform I₂.

    2 NO + Cl₂ → 2 NOCl

    • Nitric oxide reacts with acetone and an alkoxide to a diazeniumdiolate or nitrosohydroxylamine and Methyl acetate :

    This is a very old reaction (1898) but of interest today in NO prodrug research. Nitric oxide can also react directly with sodium methoxide, forming sodium formate and nitrous oxide .

    Preparation

    • Commercially, NO is produced by the oxidation of ammonia at 750°C to 900°C (normally at 850°C) in the presence of platinum as catalyst:

    4 NH₃ + 5 O₂ → 4 NO + 6 H₂O

    The uncatalyzed endothermic reaction of O₂ and N₂ which is performed at high temperature (>2000°C) with lightning has not been developed into a practical commercial synthesis (see Birkeland-Eyde process):