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Nitro compound
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{{Short description|Organic compound containing an −NO₂ group}} {{Distinguish|Nitrate ester}} {{see also|Transition metal nitrite complex}} {{Use dmy dates|date=December 2022}} [[File:Nitro-group.svg|class=skin-invert-image|thumb|right|150px|The structure of an organic nitro compound]] In [[organic chemistry]], '''nitro compounds''' are [[organic compound]]s that contain one or more '''nitro''' [[functional group]]s ({{chem2|\sNO2}}). The nitro group is one of the most common [[explosophore]]s (functional group that makes a compound explosive) used globally. The nitro group is also strongly [[electron-withdrawing group|electron-withdrawing]]. Because of this property, {{chem2|[[Carbon–hydrogen bond|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. Nitration is achieved using a mixture of [[nitric acid]] and [[sulfuric acid]], which produce the [[nitronium]] ion ({{chem2|NO2+}}), which is the electrophile: <div>{{pad|1em}}[[File:Benzol.svg|class=skin-invert-image|x60px|Benzene]] + [[File:Nitronium ion vert.svg|class=skin-invert-image|x60px|Nitronium ion]] {{Biochem reaction subunit|direction=forward|for_prod={{H+|nolink=y}}|imagesize=60px|container_style=vertical-align:middle}} [[File:Nitrobenzol.svg|class=skin-invert-image|x100px|Nitrobenzene]]</div> The nitration product produced on the 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> Another but more specialized method for making aryl–NO<sub>2</sub> group starts from halogenated phenols, is the [[Zinke nitration]]. ===Preparation of aliphatic nitro compounds === Aliphatic nitro compounds can be synthesized by various methods; notable examples include: *[[Free radical]] [[nitration]] of [[alkane]]s.<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|1-nitropropane]], and [[2-Nitropropane|2-nitropropane]] are produced by treating [[propane]] with [[nitric acid]] in the gas phase (e.g. 350–450 °C and 8–12 [[Atmosphere (unit)|atm]]). *[[Nucleophilic substitution]] reactions between [[halocarbon]]s<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 [[organosulfate]]s<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> with [[Silver nitrite|silver]] or [[Alkali metal|alkali]] [[nitrite]] salts. *Nitromethane can be produced in the laboratory by treating [[chloroacetic acid|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> *[[Organic redox reaction|Oxidation]] of [[oxime]]s<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 (chemistry)|primary]] [[amine]]s.<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 [[Henry reaction|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 [[nitroalkene]]s.<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 |page=1061412 |doi=10.1080/23312009.2015.1061412|doi-access=free }}</ref> *By [[decarboxylation]] of [[alpha and beta carbon|α]]-nitro [[carboxylic acid]]s formed from [[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> ====Ter Meer Reaction==== In [[nucleophilic substitution|nucleophilic aliphatic substitution]], [[sodium nitrite]] (NaNO<sub>2</sub>) replaces an [[alkyl halide]]. In the so-called Ter Meer reaction (1876) named after [[Edmund ter Meer]],<ref>{{cite journal | author = Edmund ter Meer | title = Ueber Dinitroverbindungen der Fettreihe | journal = [[Liebigs Annalen|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: :[[File:Ter Meer Reaction.svg|class=skin-invert-image|The ter Meer reaction]] The [[reaction mechanism]] is proposed in which in the first slow step a [[Hydron (chemistry)|proton]] is abstracted from nitroalkane '''1''' to a [[carbanion]] '''2''' followed by [[protonation]] to an aci-nitro '''3''' and finally [[nucleophilic displacement]] of chlorine based on an experimentally observed hydrogen [[kinetic isotope effect]] of 3.3.<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 [[natural product|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 [[tick]]s. Examples of nitro compounds are rare in nature. [[3-Nitropropionic acid]] found in [[fungus|fungi]] and plants (''[[Indigofera]]''). [[Nitropentadecene]] is a defense compound found in [[termite]]s. [[Aristolochic acid|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 reaction]]s, 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 [[arylamine|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> [[File:Nitrohydrogenation.svg|class=skin-invert-image|500px|center|Nitro compound hydrogenation]] The [[locant|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 acid]]s can be deprotonated in aqueous solution. The conjugate base is called a [[nitronate]], and behaves similar to an [[enolate]]. In the [[nitroaldol reaction]], it [[direct addition|adds directly]] to [[aldehyde]]s, and, with [[enone]]s, can serve as a [[Michael reaction|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> [[Nitrosation|Nitrosating]] a nitronate gives a [[nitrolic acid]].<ref>{{cite book|title=Nitrosation|first=D. L. H.|last=Williams|publisher=[[Cambridge University Press|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 group|carbonyl]] and [[azanone]].<ref>Smith (2020)), ''March's Organic Chemistry'', rxn. 16-3.</ref> [[Grignard reagent]]s 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> ===Dye syntheses=== The [[Leimgruber-Batcho indole synthesis|Leimgruber–Batcho]], [[Bartoli indole synthesis|Bartoli]] and [[Baeyer-Emmerling indole synthesis|Baeyer–Emmerling]] indole syntheses begin with aromatic nitro compounds. [[Indigo dye|Indigo]] can be synthesized in a condensation reaction from [[nitrobenzaldehyde|''ortho''-nitrobenzaldehyde]] and [[acetone]] in strongly basic conditions in a reaction known as the [[Baeyer–Drewson indigo synthesis]]. ===Biochemical reactions=== Many [[Flavin group|flavin]]-dependent [[enzyme]]s 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 explosive]]s 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}} {{Functional groups}} {{Nitrogen compounds}} {{Authority control}} [[Category:Nitro compounds| ]] [[Category:Functional groups]]
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