Nitro compound

Source: Wikipedia, the free encyclopedia.
Not to be confused with Nitrate ester.
See also: Transition metal nitrite complex

The structure of an organic nitro compound

In

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]

Synthesis

Preparation of aromatic nitro compounds

Structural details of nitrobenzene, distances in picometers.[2]

Aromatic nitro compounds are typically synthesized by nitration. Nitration is achieved using a mixture of

nitronium
ion (NO+2), which is the electrophile:

 Benzene + Nitronium ion
 
 H+
Rightward reaction arrow with minor product(s) to top right
Nitrobenzene

The nitration product produced on the largest scale, by far, is

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

alkyl halide. In the so-called Ter Meer reaction (1876) named after Edmund ter Meer,[14]
the reactant is a 1,1-halonitroalkane:

The ter Meer reaction

The

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

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

catalytic hydrogenation. A variation is formation of a dimethylaminoarene with palladium on carbon and formaldehyde:[21]

Nitro compound hydrogenation
Nitro compound hydrogenation

The

Michael donor. Conversely, a nitroalkene reacts with enols as a Michael acceptor.[23][24]

Nitronates are also key intermediates in the

azanone.[25]

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.[26]

Dye syntheses

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.[27]

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

References

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  14. doi:10.1002/jlac.18761810102
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  15. doi:10.1021/ja01600a048
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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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  20. S2CID 52931949
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  21. doi:10.15227/orgsyn.047.0069
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  22. doi:10.1021/ja00099a004
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  25. ^ Smith (2020)), March's Organic Chemistry, rxn. 16-3.
  26. doi:10.1021/jo00048a012
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  27. PMID 16430210
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