Nitro compound

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The structure of an organic nitro compound Nitro-group.svg
The structure of an organic nitro compound

In organic 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]

Contents

Synthesis

Preparation of aromatic nitro compounds

Structural details of nitrobenzene, distances in picometers. PhNO2&metric.png
Structural details of nitrobenzene, distances in picometers.

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:

  Benzol.svg + Nitronium ion vert.svg
 
 H+
Biochem reaction arrow forward NYNN horiz med.svg
Nitrobenzol.svg

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.

Preparation of aliphatic nitro compounds

Aliphatic nitro compounds can be synthesized by various methods; notable examples include:

Ter Meer Reaction

In nucleophilic 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:

Ter Meer Reaction.svg

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]

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. [17] 2-Nitrophenol is an aggregation pheromone of ticks.

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. [18] Nitrophenylethane is also found in members of the Annonaceae, Lauraceae and Papaveraceae. [19]

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. [20]

Reactions

Nitro compounds participate in several organic reactions, the most important being reduction of nitro compounds to the corresponding amines:

RNO2 + 3 H2 → RNH2 + 2 H2O

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: [21]

Nitro compound hydrogenation Nitrohydrogenation.svg
Nitro compound hydrogenation

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. [22] 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. [23] [24] Nitrosating a nitronate gives a nitrolic acid. [25]

Nitronates are also key intermediates in the Nef reaction: when exposed to acids or oxidants, a nitronate hydrolyzes to a carbonyl and azanone. [26]

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. [27]

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. [28]

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 (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.

See also

Related Research Articles

Nitromethane, sometimes shortened to simply "nitro", is an organic compound with the chemical formula CH
3NO
2. It is the simplest organic nitro compound. It is a polar liquid commonly used as a solvent in a variety of industrial applications such as in extractions, as a reaction medium, and as a cleaning solvent. As an intermediate in organic synthesis, it is used widely in the manufacture of pesticides, explosives, fibers, and coatings. Nitromethane is used as a fuel additive in various motorsports and hobbies, e.g. Top Fuel drag racing and miniature internal combustion engines in radio control, control line and free flight model aircraft.

<span class="mw-page-title-main">Oxime</span> Organic compounds of the form >C=N–OH

In organic chemistry, an oxime is an organic compound belonging to the imines, with the general formula RR’C=N−OH, where R is an organic side-chain and R' may be hydrogen, forming an aldoxime, or another organic group, forming a ketoxime. O-substituted oximes form a closely related family of compounds. Amidoximes are oximes of amides with general structure R1C(=NOH)NR2R3.

The nitrite ion has the chemical formula NO−
2. Nitrite is widely used throughout chemical and pharmaceutical industries. The nitrite anion is a pervasive intermediate in the nitrogen cycle in nature. The name nitrite also refers to organic compounds having the –ONO group, which are esters of nitrous acid.

In organic chemistry, a nitrile is any organic compound that has a −C≡N functional group. The name of the compound is composed of a base, which includes the carbon of the −C≡N, suffixed with "nitrile", so for example CH3CH2C≡N is called "propionitrile". The prefix cyano- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds, including methyl cyanoacrylate, used in super glue, and nitrile rubber, a nitrile-containing polymer used in latex-free laboratory and medical gloves. Nitrile rubber is also widely used as automotive and other seals since it is resistant to fuels and oils. Organic compounds containing multiple nitrile groups are known as cyanocarbons.

<span class="mw-page-title-main">Acyl halide</span> Oxoacid compound with an –OH group replaced by a halogen

In organic chemistry, an acyl halide is a chemical compound derived from an oxoacid by replacing a hydroxyl group with a halide group.

<span class="mw-page-title-main">Nitration</span> Chemical reaction which adds a nitro (–NO₂) group onto a molecule

In organic chemistry, nitration is a general class of chemical processes for the introduction of a nitro group into an organic compound. The term also is applied incorrectly to the different process of forming nitrate esters between alcohols and nitric acid. The difference between the resulting molecular structures of nitro compounds and nitrates is that the nitrogen atom in nitro compounds is directly bonded to a non-oxygen atom, whereas in nitrate esters, the nitrogen is bonded to an oxygen atom that in turn usually is bonded to a carbon atom.

In organic chemistry, the diazo group is an organic moiety consisting of two linked nitrogen atoms at the terminal position. Overall charge-neutral organic compounds containing the diazo group bound to a carbon atom are called diazo compounds or diazoalkanes and are described by the general structural formula R2C=N+=N−. The simplest example of a diazo compound is diazomethane, CH2N2. Diazo compounds should not be confused with azo compounds or with diazonium compounds.

<span class="mw-page-title-main">Michael addition reaction</span> Reaction in organic chemistry

In organic chemistry, the Michael reaction or Michael 1,4 addition is a reaction between a Michael donor and a Michael acceptor to produce a Michael adduct by creating a carbon-carbon bond at the acceptor's β-carbon. It belongs to the larger class of conjugate additions and is widely used for the mild formation of carbon-carbon bonds.

In organic chemistry, the Knoevenagel condensation reaction is a type of chemical reaction named after German chemist Emil Knoevenagel. It is a modification of the aldol condensation.

<span class="mw-page-title-main">Henry reaction</span> Chemical reaction

The Henry reaction is a classic carbon–carbon bond formation reaction in organic chemistry. Discovered in 1895 by the Belgian chemist Louis Henry (1834–1913), it is the combination of a nitroalkane and an aldehyde or ketone in the presence of a base to form β-nitro alcohols. This type of reaction is also referred to as a nitroaldol reaction. It is nearly analogous to the aldol reaction that had been discovered 23 years prior that couples two carbonyl compounds to form β-hydroxy carbonyl compounds known as "aldols". The Henry reaction is a useful technique in the area of organic chemistry due to the synthetic utility of its corresponding products, as they can be easily converted to other useful synthetic intermediates. These conversions include subsequent dehydration to yield nitroalkenes, oxidation of the secondary alcohol to yield α-nitro ketones, or reduction of the nitro group to yield β-amino alcohols.

<span class="mw-page-title-main">Nitrosation and nitrosylation</span> Process of converting organic compounds into nitroso derivatives

Nitrosation and nitrosylation are two names for the process of converting organic compounds or metal complexes into nitroso derivatives, i.e., compounds containing the R−NO functionality. The synonymy arises because the R-NO functionality can be interpreted two different ways, depending on the physico-chemical environment:

<span class="mw-page-title-main">Alkyl nitrite</span> Organic compounds of the form R–O–N=O

In organic chemistry, alkyl nitrites are a group of organic compounds based upon the molecular structure R−O−N=O, where R represents an alkyl group. Formally they are alkyl esters of nitrous acid. They are distinct from nitro compounds.

<span class="mw-page-title-main">Nitroethane</span> Chemical compound

Nitroethane is an organic compound having the chemical formula C2H5NO2. Similar in many regards to nitromethane, nitroethane is an oily liquid at standard temperature and pressure. Pure nitroethane is colorless and has a fruity odor.

In organic chemistry, the Nef reaction is an organic reaction describing the acid hydrolysis of a salt of a primary or secondary nitroalkane to an aldehyde or a ketone and nitrous oxide. The reaction has been the subject of several literature reviews.

The reduction of nitro compounds are chemical reactions of wide interest in organic chemistry. The conversion can be effected by many reagents. The nitro group was one of the first functional groups to be reduced. Alkyl and aryl nitro compounds behave differently. Most useful is the reduction of aryl nitro compounds.

A nitroalkene, or nitro olefin, is a functional group combining the functionality of its constituent parts, an alkene and nitro group, while displaying its own chemical properties through alkene activation, making the functional group useful in specialty reactions such as the Michael reaction or Diels-Alder additions.

β-Nitrostyrene Chemical compound

β-Nitrostyrene is an aromatic compound and a nitroalkene used in the synthesis of indigo dye and the slimicide bromo-nitrostyrene.

<span class="mw-page-title-main">Nitrolic acid</span>

Nitrolic acids are organic compounds with the functional group RC(NO2)=NOH. They are prepared by the reaction of nitroalkanes with base and nitrite sources:

<span class="mw-page-title-main">Nitro-Mannich reaction</span> Chemical reaction

The nitro-Mannich reaction is the nucleophilic addition of a nitroalkane to an imine, resulting in the formation of a beta-nitroamine. With the reaction involving the addition of an acidic carbon nucleophile to a carbon-heteroatom double bond, the nitro-Mannich reaction is related to some of the most fundamental carbon-carbon bond forming reactions in organic chemistry, including the aldol reaction, Henry reaction and Mannich reaction.

An organic azide is an organic compound that contains an azide functional group. Because of the hazards associated with their use, few azides are used commercially although they exhibit interesting reactivity for researchers. Low molecular weight azides are considered especially hazardous and are avoided. In the research laboratory, azides are precursors to amines. They are also popular for their participation in the "click reaction" between an azide and an alkyne and in Staudinger ligation. These two reactions are generally quite reliable, lending themselves to combinatorial chemistry.

References

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  2. ↑ Olga V. Dorofeeva; Yuriy V. Vishnevskiy; Natalja Vogt; Jürgen Vogt; Lyudmila V. Khristenko; Sergey V. Krasnoshchekov; Igor F. Shishkov; István Hargittai; Lev V. Vilkov (2007). "Molecular Structure and Conformation of Nitrobenzene Reinvestigated by Combined Analysis of Gas-Phase Electron Diffraction, Rotational Constants, and Theoretical Calculations". Structural Chemistry. 18 (6): 739–753. doi:10.1007/s11224-007-9186-6. S2CID   98746905.
  3. ↑ Gerald, Booth. "Nitro Compounds, Aromatic". Ullmann's Encyclopedia of Industrial Chemistry . Weinheim: Wiley-VCH. doi:10.1002/14356007.a17_411. ISBN   978-3527306732.
  4. ↑ Markofsky, Sheldon; Grace, W.G. (2000). "Nitro Compounds, Aliphatic". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a17_401. ISBN   978-3-527-30673-2.
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  6. ↑ Walden, P. (1907). "Zur Darstellung aliphatischer Sulfocyanide, Cyanide und Nitrokörper". Berichte der Deutschen Chemischen Gesellschaft. 40 (3): 3214–3217. doi:10.1002/cber.19070400383.
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  16. ↑ 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). (Article)
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  26. ↑ Smith (2020)), March's Organic Chemistry, rxn. 16-3.
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  28. ↑ Nagpal, Akanksha; Valley, Michael P.; Fitzpatrick, Paul F.; Orville, Allen M. (2006). "Crystal Structures of Nitroalkane Oxidase: Insights into the Reaction Mechanism from a Covalent Complex of the Flavoenzyme Trapped during Turnover". Biochemistry. 45 (4): 1138–50. doi:10.1021/bi051966w. PMC   1855086 . PMID   16430210.
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