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Concept
Nitro (chimie)
Résumé
In organic chemistry, nitro compounds are organic compounds that contain one or more nitro functional groups (). 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, 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.
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 (), 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).
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:
Free radical nitration of alkanes. 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 °C and 8–12 atm).
Nucleophilic substitution reactions between halocarbons or organosulfates with silver or alkali nitrite salts.
Nitromethane can be produced in the laboratory by treating sodium chloroacetate with sodium nitrite.
Oxidation of oximes or primary amines.
Reduction of β-nitro alcohols or nitroalkenes.
By decarboxylation of α-nitro carboxylic acids formed from nitriles and ethyl nitrate.
In nucleophilic aliphatic substitution, sodium nitrite (NaNO2) replaces an alkyl halide.
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