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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.
Although certain energetic/explosive materials, such as nitrocellulose or nitroglycerin have the prefix "nitro" in their name, they are not nitro compounds, but rather nitrate esters or various alcohols/polyols, and the name "nitro" comes from the old designation of the preparation method from the fact that the alcohol has been nitrated with nitric acid/nitrating mixture, just like the classical esters are prepared from an alcohol and an acid.
The −NO2 functional group is attached to an aliphatic carbon, either primary, secondary or tertiary.
The −NO2 functional group is attached to an amine. The parent inorganic compound, is nitramide, H2N–NO2.
The −NO2 functional group is attached directly to the aromatic group.
Nitromethane is sold as nitro fuel and can be purchased from hobby stores or online. Many countries limit the concentration of NM from nitro fuels due to its potential misuse in manufacturing of explosive materials. In EU, the limit is 30%.
Nitroethane is sometimes sold as nail polish remover or as solvent in many countries. However, due to its potential use in the manufacturing if amphetamine illicite drugs, it it classified as drug precursor and there may be limits on its sale in many countries. In the United States, nitroethane is a DEA List I chemical and individuals cannot purchase this compound. Other countries have similar restrictions.
Nitroaromatic and nitroamine compounds are extremely difficult to acquire since most are explosive materials, and their sale is prohibited to the general public, though some companies may acquire a permit for their acquisition and use. However, there are a few notable exceptions:
- 2,4-Dinitrophenol: Was originally used in the past as dieting aid, as DNP is a protonophore, which allows protons to leak across the inner mitochondrial membrane and thus bypass ATP synthase. As the dose increases and energy production is made more inefficient, metabolic rate increases (and more fat is burned) in order to compensate for the inefficiency and to meet energy demands. However, the side effects of DNP consumption are extremely dangerous to health: apart from the usual side effects usually associated with ingestion of similar aromatic compound (nausea, vomiting, sweating, dizziness, headache), DNP will cause cataracts, skin lesions as well as severe damage to the bone marrow, central nervous system and cardiovascular system. Because of its dangers, it was quickly banned, but recently it is becoming available again online, and can be acquired from online stores. In some countries however, like UK, it is still classified as explosive material and may not be acquired freely or easily.
- Nitrobenzene: used to mask unpleasant odors in shoe and floor polishes, leather dressings, paint solvents, and other materials, though due to its toxicity, in the last few decades it has largely been replaced, and it's getting harder to find lately.
- Nitroguanidine: commonly used in modern car airbags as propellant, NG has largely replaced the more toxic sodium azide.
The preparation of nitro compounds is usually different for each type of compound:
- 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 with peroxycarboxylic acids or DMDO.
- Decarboxylation of α-nitro carboxylic acids formed from nitriles and ethyl nitrate.
Nitroaromatics and nitroamines
Aromatic nitro compounds are typically synthesized by nitration. Nitration is achieved using a mixture of nitric acid and sulfuric acid, also called nitrating mixture, which produces the nitronium ion (NO2+).
Another but more specialized method for making aryl-NO2 group starts from halogenated phenols, is the Zinke nitration.
For the preparation of RDX and HMX, acetic anhydride is used in the nitrating process.