{{Short description|Organic compound containing an –NO2 group}} {{Distinguish|Nitrate ester}} {{see also|Transition metal nitrite complex}} {{Use dmy dates|date=December 2022}} class=skin-invert-image|thumb|right|150px|The structure of an organic nitro compound

In organic chemistry, '''nitro compounds''' are organic compounds that contain one or more '''nitro''' functional groups ({{chem2|\sNO2}}). The nitro group is one of the most common explosophores (functional group that makes a compound explosive) used globally. The nitro group is also strongly electron-withdrawing. Because of this property, {{chem2|C\sH}} bonds alpha (adjacent) to the nitro group can be acidic. For similar reasons, the presence of nitro groups in aromatic compounds retards electrophilic aromatic substitution but facilitates nucleophilic aromatic substitution. Nitro groups are rarely found in nature. They are almost invariably produced by nitration reactions starting with nitric acid.<ref>{{cite book |title=Nitro and Nitroso Groups: Part 2, Volume 2 |year=1970 |editor=Henry Feuer |isbn=978-0-470-77117-4 |doi=10.1002/9780470771174 |publisher=John Wiley & Sons Ltd. |series=PATAI'S Chemistry of Functional Groups|volume=2 }}{{cite book |title=Nitro and Nitroso Groups: Supplement F: Part 2, Volume 2 |year=1982 |editor=Saul Patai |isbn=978-0-470-77167-9 |doi=10.1002/9780470771679 |publisher=John Wiley & Sons Ltd. |series=PATAI'S Chemistry of Functional Groups}}{{cite book |title=Amino, Nitroso and Nitro Compounds and Their Derivatives: Supplement F: Part 1, Volume 1 |year=1982 |editor=Saul Patai |isbn=978-0-470-77166-2 |doi=10.1002/9780470771662 |publisher=John Wiley & Sons Ltd. |series=PATAI'S Chemistry of Functional Groups}}</ref>

==Synthesis== ===Preparation of aromatic nitro compounds === [[File:PhNO2&metric.png|class=skin-invert-image|thumb|144px|Structural details of nitrobenzene, distances in picometers.<ref>{{cite journal |journal=Structural Chemistry |year=2007 |volume=18 |issue=6 |pages=739–753 |title=Molecular Structure and Conformation of Nitrobenzene Reinvestigated by Combined Analysis of Gas-Phase Electron Diffraction, Rotational Constants, and Theoretical Calculations |author=Olga V. Dorofeeva |author2=Yuriy V. Vishnevskiy |author3=Natalja Vogt |author4=Jürgen Vogt |author5=Lyudmila V. Khristenko |author6=Sergey V. Krasnoshchekov |author7=Igor F. Shishkov |author8=István Hargittai |author9=Lev V. Vilkov |doi=10.1007/s11224-007-9186-6 |s2cid=98746905}}</ref>]]

Aromatic nitro compounds are typically synthesized by nitration with nitric acid and sulfuric acid. The active intermediate is the nitronium ion ({{chem2|NO2+}}), an electrophile: <div>{{pad|1em}}class=skin-invert-image|x60px|Benzene + class=skin-invert-image|x60px|Nitronium ion {{Biochem reaction subunit|direction=forward|for_prod={{H+|nolink=y}}|imagesize=60px|container_style=vertical-align:middle}} class=skin-invert-image|x100px|Nitrobenzene</div> The nitration product produced on largest scale, by far, is nitrobenzene. Many explosives are produced by nitration including trinitrophenol (picric acid), trinitrotoluene (TNT), and trinitroresorcinol (styphnic acid).<ref>{{Ullmann|last=Gerald|first=Booth|title=Nitro Compounds, Aromatic|doi=10.1002/14356007.a17_411}}</ref>

In some cases, nitroarenes are produced through nucleophilic substitution, as in the Zinke nitration of phenols.

===Preparation of aliphatic nitro compounds === Aliphatic nitro compounds can be synthesized by various methods; notable examples include: *Free radical nitration of alkanes.<ref>{{cite journal|last1=Markofsky|first1=Sheldon|last2=Grace|first2=W.G.|title=Nitro Compounds, Aliphatic|journal=Ullmann's Encyclopedia of Industrial Chemistry|date=2000|doi=10.1002/14356007.a17_401|isbn=978-3-527-30673-2}}</ref> The reaction produces fragments from the parent alkane, creating a diverse mixture of products; for instance, nitromethane, nitroethane, 1-nitropropane, and 2-nitropropane are produced by treating propane with nitric acid in the gas phase (e.g. 350–450&nbsp;°C and 8–12 atm). *Nucleophilic substitution reactions of silver nitrite onto primary<ref>N. Kornblum, R. A. Smiley, H. E. Ungnade, A. M. White, B. Taub, S. A. Herbert, Jr., ''J. Am. Chem. Soc.'' 1955, 77, 5528 – 5533;</ref> halocarbons<ref>{{cite journal|last1=Kornblum|first1=N.|last2=Ungnade|first2=H. E.|title=1-Nitroöctane|journal=Organic Syntheses|date=1963|volume=4|page=724|doi=10.15227/orgsyn.038.0075}}</ref> or organosulfates<ref>{{cite journal|last1=Walden|first1=P.|title=Zur Darstellung aliphatischer Sulfocyanide, Cyanide und Nitrokörper|journal=Berichte der Deutschen Chemischen Gesellschaft|date=1907|volume=40|issue=3|pages=3214–3217|doi=10.1002/cber.19070400383|url=https://zenodo.org/record/1426247}}</ref> (the Meyer synthesis). *Oxidation of oximes<ref>{{cite journal|last1=Olah|first1=George A.|last2=Ramaiah|first2=Pichika|last3=Chang-Soo|first3=Lee|last4=Prakash|first4=Surya|title=Convenient Oxidation of Oximes to Nitro Compounds with Sodium Perborate in Glacial Acetic Acid|journal=Synlett|date=1992|volume=1992|issue=4|pages=337–339|doi=10.1055/s-1992-22006}}</ref> or primary amines.<ref>{{cite journal|last1=Ehud|first1=Keinan|last2=Yehuda|first2=Mazur|title=Dry ozonation of amines. Conversion of primary amines to nitro compounds|journal=The Journal of Organic Chemistry|date=1977|volume=42|issue=5|pages=844–847|doi=10.1021/jo00425a017}}</ref> *Reduction of β-nitro alcohols<ref>{{cite journal |last1=Chandrasekhar |first1=S. |last2=Shrinidhi |first2=A. |title=Useful Extensions of the Henry Reaction: Expeditious Routes to Nitroalkanes and Nitroalkenes in Aqueous Media |journal=Synthetic Communications |date=2014 |volume=44 |issue=20 |pages=3008–3018 |doi=10.1080/00397911.2014.926373|s2cid=98439096 |url=https://figshare.com/articles/journal_contribution/1053153 }}</ref> or nitroalkenes.<ref>{{cite journal |last1=Shrinidhi |first1=A. |title=Microwave-assisted chemoselective reduction of conjugated nitroalkenes to nitroalkanes with aqueous tri-n-butyltin hydride |journal=Cogent Chemistry |date=2015 |volume=1 |issue=1 |article-number=1061412 |doi=10.1080/23312009.2015.1061412|doi-access=free }}</ref> *By decarboxylation of α-nitro carboxylic acids (a variant of the Krapcho decarboxylation). The latter can be formed via α-nitration, e.g. between nitriles and ethyl nitrate;<ref>{{cite journal|last1=Wislicenus|first1=Wilhelm|last2=Endres|first2=Anton|title=Ueber Nitrirung mittels Aethylnitrat [Nitrification by means of ethyl nitrate]|journal=Berichte der Deutschen Chemischen Gesellschaft|date=1902|volume=35|issue=2|pages=1755–1762|doi=10.1002/cber.190203502106|url=https://zenodo.org/record/1426046}}</ref><ref>{{cite book|last1=Weygand|first1=Conrad|editor1-last=Hilgetag|editor1-first=G.|editor2-last=Martini|editor2-first=A.|title=Weygand/Hilgetag Preparative Organic Chemistry|date=1972|publisher=John Wiley & Sons, Inc.|location=New York|isbn=978-0-471-93749-4|page=1007|edition=4th}}</ref> or a Meyer-type reaction with a halocarboxylic acid. For example, nitromethane can be produced in the laboratory by treating sodium chloroacetate with sodium nitrite.<ref>{{cite journal|last1=Whitmore|first1=F. C.|last2=Whitmore|first2=Marion G.|title=Nitromethane|journal=Organic Syntheses|date=1923|volume=1|page=401|doi=10.15227/orgsyn.003.0083}}</ref> (In general, alkali nitrites are unsuitable for Meyer reactions, as they give an equilibrium of mostly nitrite esters; but decarboxylation drives the initial equilibrium to the nitro product.)

====ter Meer reaction==== In nucleophilic aliphatic substitution, sodium nitrite (NaNO<sub>2</sub>) replaces an alkyl halide. In the '''ter Meer reaction''', named after Edmund ter Meer, who first reported it in 1876,<ref>{{cite journal | author = Edmund ter Meer | title = Ueber Dinitroverbindungen der Fettreihe | journal = Justus Liebigs Annalen der Chemie | volume = 181 | issue = 1 | pages = 1–22 | year = 1876 | doi = 10.1002/jlac.18761810102| url = https://zenodo.org/record/1427353 | author-link = Edmund ter Meer }}</ref> the reactant is a 1,1-halonitroalkane: :class=skin-invert-image|The ter Meer reaction

The reaction mechanism is proposed in which in the first slow step a proton is abstracted from nitroalkane '''1''' to a carbanion '''2''' followed by protonation to an aci-nitro '''3''' and finally nucleophilic displacement of chlorine by an acyl substitution-like process.<ref>{{cite journal |doi=10.1021/ja01600a048 |title=Aci-Nitroalkanes. I. The Mechanism of the ter Meer Reaction1 |journal=Journal of the American Chemical Society |volume=78 |issue=19 |pages=4980–4984 |year=1956 |last1=Hawthorne |first1=M. Frederick}}</ref> When the same reactant is reacted with potassium hydroxide the reaction product is the 1,2-dinitro dimer.<ref>''3-Hexene, 3,4-dinitro-'' D. E. Bisgrove, J. F. Brown, Jr., and L. B. Clapp. ''Organic Syntheses'', Coll. Vol. 4, p. 372 (1963); Vol. 37, p. 23 (1957). ([http://www.orgsynth.org/orgsyn/pdfs/CV4P0372.pdf Article])</ref>

==Occurrence== === In nature === Chloramphenicol is a rare example of a naturally occurring nitro compound. At least some naturally occurring nitro groups arose by the oxidation of amino groups.<ref>{{cite journal |doi=10.1016/j.jmb.2007.06.014 |pmid=17765264 |title=Structure and Action of the N-oxygenase AurF from Streptomyces thioluteus |journal=Journal of Molecular Biology |volume=373 |issue=1 |pages=65–74 |year=2007 |last1=Zocher |first1=Georg |last2=Winkler |first2=Robert |last3=Hertweck |first3=Christian |last4=Schulz |first4=Georg E}}</ref> 2-Nitrophenol is an aggregation pheromone of ticks.{{cn|date=February 2026}}

Examples of nitro compounds are rare in nature. 3-Nitropropionic acid found in fungi and plants (''Indigofera''). Nitropentadecene is a defense compound found in termites. Aristolochic acids are found in the flowering plant family Aristolochiaceae. Nitrophenylethane is found in ''Aniba canelilla''.<ref>{{cite journal | last1=Maia | first1=José Guilherme S. | last2=Andrade | first2=Eloísa Helena A. | title=Database of the Amazon aromatic plants and their essential oils | journal=Química Nova | publisher=FapUNIFESP (SciELO) | volume=32 | issue=3 | year=2009 | issn=0100-4042 | doi=10.1590/s0100-40422009000300006 | pages=595–622 |url=http://www.scielo.br/pdf/qn/v32n3/a06v32n3.pdf| doi-access=free }}</ref> Nitrophenylethane is also found in members of the Annonaceae, Lauraceae and Papaveraceae.<ref>{{cite book | last1=Kramer | first1=K.U. | last2=Kubitzki | first2=K. | last3=Rohwer | first3=J.G. | last4=Bittrich | first4=V. | title=Flowering Plants, Dicotyledons: Magnoliid, Hamamelid, and Caryophyllid Families | publisher=Springer-Verlag, Berlin | series=Families and genera of vascular plants | year=1993 | isbn=978-3-540-55509-4 | url=https://books.google.com/books?id=K_pGAAAAYAAJ}}</ref>

=== In pharmaceuticals === Despite the occasional use in pharmaceuticals, the nitro group is associated with mutagenicity and genotoxicity and therefore is often regarded as a liability in the drug discovery process.<ref name="pmid30295477">{{cite journal |vauthors=Nepali K, Lee HY, Liou JP |title=Nitro-Group-Containing Drugs |journal=J. Med. Chem. |volume=62 |issue=6 |pages=2851–2893 |date=March 2019 |pmid=30295477 |doi=10.1021/acs.jmedchem.8b00147 |s2cid=52931949 }}</ref>

== Reactions== Nitro compounds participate in several organic reactions, the most important being reduction of nitro compounds to the corresponding amines: :RNO<sub>2</sub> + 3 H<sub>2</sub> → RNH<sub>2</sub> + 2 H<sub>2</sub>O Virtually all aromatic amines (e.g. aniline) are derived from nitroaromatics through such catalytic hydrogenation. A variation is formation of a dimethylaminoarene with palladium on carbon and formaldehyde:<ref>{{cite journal |title=ETHYL p-DIMETHYLAMINOPHENYLACETATE |journal= Organic Syntheses|year= 1967|volume= 47|page= 69|url=http://orgsyn.org/Content/pdfs/procedures/cv5p0552.pdf |doi=10.15227/orgsyn.047.0069}}</ref> class=skin-invert-image|500px|center|Nitro compound hydrogenation

The α-carbon of nitroalkanes is somewhat acidic. The p''K''<sub>a</sub> values of nitromethane and 2-nitropropane are respectively 17.2 and 16.9 in dimethyl sulfoxide (DMSO) solution, suggesting an aqueous p''K''<sub>a</sub> of around 11.<ref>{{cite journal | doi = 10.1021/ja00099a004| title = Is Resonance Important in Determining the Acidities of Weak Acids or the Homolytic Bond Dissociation Enthalpies (BDEs) of Their Acidic H-A Bonds?| journal = Journal of the American Chemical Society| volume = 116| issue = 20| page = 8885| year = 1994| last1 = Bordwell| first1 = Frederick G| last2 = Satish| first2 = A. V}}</ref> In other words, these carbon acids can be deprotonated in aqueous solution. The conjugate base is called a nitronate, and behaves similar to an enolate. In the nitroaldol reaction, it adds directly to aldehydes, and, with enones, can serve as a Michael donor. Conversely, a nitroalkene reacts with enols as a Michael acceptor.<ref>{{cite journal|author1=Ranganathan, Darshan |author2=Rao, Bhushan |author3=Ranganathan, Subramania |author4=Mehrotra, Ashok |author5=Iyengar, Radha |name-list-style=amp |title=Nitroethylene: a stable, clean, and reactive agent for organic synthesis|journal=The Journal of Organic Chemistry|year=1980|volume=45|issue=7|pages=1185–1189|doi=10.1021/jo01295a003}}</ref><ref>{{cite journal|author1=Jubert, Carole |author2=Knochel, Paul |name-list-style=amp |title=Preparation of polyfunctional nitro olefins and nitroalkanes using the copper-zinc reagents RCu(CN)ZnI|journal=The Journal of Organic Chemistry|year=1992|volume=57|issue=20|pages=5431–5438|doi=10.1021/jo00046a027}}</ref> Nitrosating a nitronate gives a nitrolic acid.<ref>{{cite book|title=Nitrosation|first=D.&nbsp;L.&nbsp;H.|last=Williams|publisher=Cambridge University|location=Cambridge, UK|year=1988|isbn=0-521-26796-X|url=https://archive.org/details/nitrosation0000will|url-access=registration|page=44}}</ref>

Nitronates are also key intermediates in the Nef reaction: when exposed to acids or oxidants, a nitronate hydrolyzes to a carbonyl and (respectively) azanone or nitric acid.<ref>Smith (2020), ''March's Organic Chemistry'', rxn.&nbsp;16-3.</ref>

Grignard reagents combine with nitro compounds to give a nitrone; but a Grignard reagent with an α hydrogen will then add again to the nitrone to give a hydroxylamine salt.<ref>{{cite journal|doi=10.1021/jo00048a012|title=Nitrones from addition of benzyl and allyl Grignard reagents to alkyl nitro compounds: chemo-, regio-, and stereoselectivity of the reaction|first1=Giuseppe|last1=Bartoli|first2=Enrico|last2=Marcantoni|first3=Marino|last3=Petrini|orig-date=14 Apr 1992|publisher=American Chemical Society|journal=Journal of Organic Chemistry|volume=57|number=22|year=1992|pages=5834–5840}}</ref>

The nitro moiety is a mild photosensitizer, and EUV irradiation of a nitroarene can lead to either oxidation of another compound (the nitroarene being itself reduced to a hydroxylamine) or radical-nucleophilic aromatic substitution.<ref>Döpp, Dietrich (1975). "Reactions of aromatic nitro compounds via excited triplet states", in Wild, U.P.; Döpp, Dietrich; Dürr, H. (eds.) ''Triplet States II''. Topics in Current Chemistry, vol 55. Springer, Berlin, Heidelberg. {{doi|10.1007/BFb0050594}}</ref>

===Dye syntheses=== The Leimgruber–Batcho, Bartoli and Baeyer–Emmerling indole syntheses begin with aromatic nitro compounds. Indigo can be synthesized in a condensation reaction from ''ortho''-nitrobenzaldehyde and acetone in strongly basic conditions in a reaction known as the Baeyer–Drewson indigo synthesis.

===Biochemical reactions=== Many flavin-dependent enzymes are capable of oxidizing aliphatic nitro compounds to less-toxic aldehydes and ketones. Nitroalkane oxidase and 3-nitropropionate oxidase oxidize aliphatic nitro compounds exclusively, whereas other enzymes such as glucose oxidase have other physiological substrates.<ref>{{cite journal|last1=Nagpal|first1=Akanksha|first2=Michael P. |last2=Valley |first3=Paul F. |last3=Fitzpatrick |first4=Allen M. |last4=Orville |date=2006|title=Crystal Structures of Nitroalkane Oxidase: Insights into the Reaction Mechanism from a Covalent Complex of the Flavoenzyme Trapped during Turnover|journal=Biochemistry|pmid=16430210|doi=10.1021/bi051966w|volume=45|issue=4|pmc=1855086|pages=1138–50}}</ref>

===Explosions=== Explosive decomposition of organo nitro compounds are redox reactions, wherein both the oxidant (nitro group) and the fuel (hydrocarbon substituent) are bound within the same molecule. The explosion process generates heat by forming highly stable products including molecular nitrogen (N<sub>2</sub>), carbon dioxide, and water. The explosive power of this redox reaction is enhanced because these stable products are gases at mild temperatures. Many contact explosives contain the nitro group.

==See also== * Functional group * Reduction of nitro compounds * Nitration * Nitrite (also an NO<sub>2</sub> group, but bonds differently) * Nitroalkene * Nitroglycerin

==References== {{Reflist}}

{{Commons category|Nitro compounds}} {{EB1911 poster|Nitro Compounds}}

{{Functional groups}}

{{Nitrogen compounds}} {{Authority control}}

Category:Nitro compounds Category:Functional groups