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Variations on this method is one of the main techniques for [[arylamine]] production. |
Variations on this method is one of the main techniques for [[arylamine]] production. |
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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 [[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> |
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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> |
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Nitronates are also key intermediates in the [[Nef reaction]]: when exposed to pH extremes, a nitronate hydrolyzes to a [[carbonyl group|carbonyl]] and [[azanone]].<ref>Smith (2020)), ''March's Organic Chemistry''.</ref> |
Nitronates are also key intermediates in the [[Nef reaction]]: when exposed to pH extremes, a nitronate hydrolyzes to a [[carbonyl group|carbonyl]] and [[azanone]].<ref>Smith (2020)), ''March's Organic Chemistry''.</ref> |
Inorganic chemistry, nitro compounds are organic compounds that contain one or more nitro functional groups (−NO2). 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, C−H 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.[1]
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 (NO+2), which is the electrophile:
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).[3] Another but more specialized method for making aryl–NO2 group starts from halogenated phenols, is the Zinke nitration.
Aliphatic nitro compounds can be synthesized by various methods; notable examples include:
Innucleophilic aliphatic substitution, sodium nitrite (NaNO2) replaces an alkyl halide. In the so-called Ter Meer reaction (1876) named after Edmund ter Meer,[14] the reactant is a 1,1-halonitroalkane:
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 based on an experimentally observed hydrogen kinetic isotope effect of 3.3.[15] When the same reactant is reacted with potassium hydroxide the reaction product is the 1,2-dinitro dimer.[16]
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.[17] 2-Nitrophenol is an aggregation pheromoneofticks.
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. Nitrophenylethane is found in Aniba canelilla.[18] Nitrophenylethane is also found in members of the Annonaceae, Lauraceae and Papaveraceae.[19]
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.[20]
Nitro compounds participate in several organic reactions, the most important being their reduction to the corresponding amines:
Variations on this method is one of the main techniques for arylamine production.
The α-carbon of nitroalkanes is somewhat acidic. The pKa values of nitromethane and 2-nitropropane are respectively 17.2 and 16.9 in dimethyl sulfoxide (DMSO) solution, suggesting an aqueous pKa of around 11.[21] 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 directlytoaldehydes, and, with enones, can serve as a Michael donor. Conversely, a nitroalkene reacts with enols as a Michael acceptor.[22][23]
Nitronates are also key intermediates in the Nef reaction: when exposed to pH extremes, a nitronate hydrolyzes to a carbonyl and azanone.[24]
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.[25]
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.[26]
Reduction of aromatic nitro compounds with hydrogen over metal catalysts gives anilines. Virtually all aromatic amines (anilines) are derived from nitroaromatics. A variation is formation of a dimethylaminoarene with palladium on carbon and formaldehyde:[27]
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.
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 (N2), 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.
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Hydrocarbons (only C and H) |
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Only carbon, hydrogen, and oxygen (only C, H and O) |
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Only one element, not being carbon, hydrogen, or oxygen (one element, not C, H or O) |
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Other |
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Nitrogen species
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Hydrides |
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Organic |
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Oxides |
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Halides |
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Oxidation states |