Nitrogen dioxide is achemical compound with the formulaNO2. One of severalnitrogen oxides, nitrogen dioxide is a reddish-brown gas. It is aparamagnetic, bent molecule with C2vpoint group symmetry. Industrially,NO2 is an intermediate in the synthesis ofnitric acid, millions of tons of which are produced each year, primarily for the production offertilizers.
Nitrogen dioxide is poisonous and can be fatal if inhaled in large quantities.[8] Cooking with agas stove produces nitrogen dioxide which causes poorerindoor air quality. Combustion of gas can lead to increased concentrations of nitrogen dioxide throughout the home environment which is linked torespiratory issues and diseases.[9][10] The LC50 (median lethal dose) for humans has been estimated to be 174 ppm for a 1-hour exposure.[11] It is also included in theNOx family ofatmospheric pollutants.
Nitrogen dioxide is a reddish-brown gas with a pungent, acrid odor above 21.2 °C (70.2 °F; 294.3 K) and becomes a yellowish-brown liquid below 21.2 °C (70.2 °F; 294.3 K). It forms anequilibrium with itsdimer,dinitrogen tetroxide (N2O4), and converts almost entirely toN2O4 below −11.2 °C (11.8 °F; 261.9 K).[6]
Thebond length between thenitrogen atom and the oxygen atom is 119.7 pm. This bond length is consistent with abond order between one and two.
Unlikeozone (O3) thegroundelectronic state of nitrogen dioxide is adoublet state, since nitrogen has one unpaired electron,[12] which decreases thealpha effect compared withnitrite and creates a weak bonding interaction with the oxygen lone pairs. The lone electron inNO2 also means that this compound is afree radical, so the formula for nitrogen dioxide is often written as•NO2.
The reddish-brown color is a consequence of preferential absorption of light in the blue region of the spectrum (400–500 nm), although the absorption extends throughout the visible (at shorter wavelengths) and into the infrared (at longer wavelengths). Absorption of light at wavelengths shorter than about 400 nm results in photolysis (to formNO + O, atomic oxygen); in the atmosphere the addition of the oxygen atom so formed toO2 results in ozone.
Industrially, nitrogen dioxide is produced and transported as its cryogenic liquid dimer,dinitrogen tetroxide. It is produced industrially by the oxidation of ammonia, theOstwald Process. This reaction is the first step in the production of nitric acid:[13]
Instead, most laboratory syntheses stabilize and then heat the nitric acid to accelerate the decomposition. For example, the thermal decomposition of some metal nitrates generatesNO2:[14]
Nitric acid decomposes slowly to nitrogen dioxide by the overall reaction:
4HNO3 → 4NO2 + 2H2O +O2
The nitrogen dioxide so formed confers the characteristic yellow color often exhibited by this acid. However, the reaction is too slow to be a practical source ofNO2.
As suggested by the weakness of the N–O bond,NO2 is a good oxidizer. Consequently, it will combust, sometimes explosively, in the presence ofhydrocarbons.[16]
This reaction is one of the steps in theOstwald process for the industrial production of nitric acid from ammonia.[13] This reaction is negligibly slow at low concentrations of NO2 characteristic of the ambient atmosphere, although it does proceed upon NO2 uptake to surfaces. Such surface reaction is thought to produce gaseousHNO2 (often written asHONO) in outdoor and indoor environments.[17]
The reactivity of nitrogen dioxide towardorganic compounds has long been known.[18] For example, it reacts with amides to give N-nitroso derivatives.[19] It is used for nitrations under anhydrous conditions.[20]
NO2 is introduced into the environment by natural causes, including entry from thestratosphere, bacterial respiration, volcanos, and lightning. These sources makeNO2 atrace gas in theatmosphere of Earth, where it plays a role in absorbingsunlight and regulating the chemistry of thetroposphere, especially in determiningozone concentrations.[24]
Nitrogen dioxide also forms in mostcombustion processes. At elevated temperaturesnitrogen combines withoxygen to form nitrogen dioxide:
N2 + 2 O2 → 2 NO2
For the general public, the most prominent sources ofNO2 areinternal combustion engines, as combustion temperatures are high enough to thermally combine some of the nitrogen and oxygen in the air to formNO2.[8] Nitrogen dioxide accounts for a small fraction (generally well under 0.1) ofNOx auto emissions.[25]
Outdoors,NO2 can be a result of traffic from motor vehicles.[26] Indoors, exposure arises from cigarette smoke,[27] andbutane andkerosene heaters and stoves.[28] Indoor exposure levels ofNO2 are, on average, at least three times higher in homes with gas stoves compared to electric stoves.[29][30]
Workers in industries whereNO2 is used are also exposed and are at risk foroccupational lung diseases, andNIOSH has set exposure limits and safety standards.[6] Workers in high voltage areas especially those with spark or plasma creation are at risk.[citation needed]Agricultural workers can be exposed toNO2 arising from grain decomposing in silos; chronic exposure can lead to lung damage in a condition called "silo-filler's disease".[31][32]
NO2 diffuses into the epithelial lining fluid (ELF) of therespiratory epithelium and dissolves. There, it chemically reacts with antioxidant and lipid molecules in the ELF. The health effects ofNO2 are caused by the reaction products or their metabolites, which arereactive nitrogen species andreactive oxygen species that can drivebronchoconstriction, inflammation, reduced immune response, and may have effects on the heart.[33]
Acute harm due toNO2 exposure is rare. 100–200 ppm can cause mild irritation of the nose and throat, 250–500 ppm can causeedema, leading tobronchitis orpneumonia, and levels above 1000 ppm can cause death due to asphyxiation from fluid in the lungs. There are often no symptoms at the time of exposure other than transient cough, fatigue or nausea, but over hours inflammation in the lungs causes edema.[34][35]
For skin or eye exposure, the affected area is flushed with saline. For inhalation, oxygen is administered,bronchodilators may be administered, and if there are signs ofmethemoglobinemia, a condition that arises when nitrogen-based compounds affect thehemoglobin in red blood cells,methylene blue may be administered.[36][37]
Exposure to low levels ofNO2 over time can cause changes in lung function.[39] Cooking with agas stove is associated with poorerindoor air quality. Combustion of gas can lead to increased concentrations of nitrogen dioxide throughout the home environment which is linked torespiratory issues and diseases.[9][10] Children exposed toNO2 are more likely to be admitted to hospital withasthma.[40]
In 2019, theCourt of Justice of the EU, found thatFrance did not comply with the limit values of the EU air quality standards applicable to the concentrations of nitrogen dioxide (NO2) in 12 air quality zones.[41]
Interaction ofNO2 and otherNOx with water, oxygen and other chemicals in the atmosphere can formacid rain which harms sensitive ecosystems such as lakes and forests.[42] Elevated levels ofNO 2 can also harm vegetation, decreasing growth, and reduce crop yields.[43]
^"nitrogen dioxide (CHEBI:33101)".Chemical Entities of Biological Interest (ChEBI). UK: European Bioinformatics Institute. 13 January 2008. Main. Archived fromthe original on 4 March 2016. Retrieved4 October 2011.
^Mendiara, S. N.; Sagedahl, A.; Perissinotti, L. J. (2001). "An electron paramagnetic resonance study of nitrogen dioxide dissolved in water, carbon tetrachloride and some organic compounds".Applied Magnetic Resonance.20 (1–2):275–287.doi:10.1007/BF03162326.S2CID97875925.
^abcHolleman, A. F.; Wiberg, E. (2001)Inorganic Chemistry. Academic Press: San Diego.ISBN0-12-352651-5.
^Chan, Wai-To; Heck, Simone M.; Pritchard, Huw O. (2001). "Reaction of nitrogen dioxide with hydrocarbons and its influence on spontaneous ignition. A computational study".Physical Chemistry Chemical Physics.3 (1):56–62.Bibcode:2001PCCP....3...56C.doi:10.1039/b006088g.
^Riebsomer, J. L. (1945). "The Reactions of Nitrogen Tetroxide with Organic Compounds".Chemical Reviews.36 (2):157–233.doi:10.1021/cr60114a002.
^Emil White (1967). "Deamination of Amines. 2-Phenylethyl Benzoate Via the Nitrosoamide Decomposition".Organic Syntheses.47: 44.doi:10.15227/orgsyn.047.0044.
^Subcommittee on Emergency and Continuous Exposure Guidance Levels for Selected Submarine Contaminants; Committee on Toxicology; Board on Environmental Studies and Toxicology; Division on Earth and Life Studies; National Research Council.Chapter 12: Nitrogen Dioxide in Emergency and Continuous Exposure Guidance Levels for Selected Submarine Contaminants. National Academies Press, 2007.ISBN978-0-309-09225-8
^US Dept. of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry, Division of Toxicology. April 2002ATSDR Nitrous Oxides.
^abU.S. EPA. Integrated Science Assessment for Oxides of Nitrogen – Health Criteria (2016 Final Report). U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-15/068, 2016.Federal Register Notice Jan 28, 2016 Free download available atReport page at EPA website.
