Nitro compounds are organic compounds that contain one or more nitro functional groups (−NO2). They are often highly explosive, especially when the compound contains more than one nitro group and is impure. The nitro group is one of the most common explosophores (functional group that makes a compound explosive) used globally. This property of both nitro and nitrate groups is because their thermal decomposition yields molecular nitrogen (N2) gas plus considerable energy, due to the high strength of the bond in molecular nitrogen.
The nitro group is also strongly electron-withdrawing. This makes the C−H bonds alpha to the nitro group acidic. The presence of nitro groups retards electrophilic aromatic substitution but facilitates nucleophilic aromatic substitution because they are highly electron-withdrawing.
- 1 Production and occurrence
- 2 Reactions of aliphatic nitro compounds
- 3 Reactions of aromatic nitro compounds
- 4 See also
- 5 References
Production and occurrence
Preparation of aromatic nitro compounds
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 (NO2+), which the electrophile. The 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 method, starting from halogenated phenols, is the Zinke nitration.
Preparation of aliphatic nitro compounds
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.
Tar 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, 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 a nitronate 3 and finally nucleophilic displacement of chlorine based on an experimentally observed hydrogen kinetic isotope effect of 3.3. When the same reactant is reacted with potassium hydroxide the reaction product is the 1,2-dinitro dimer
Occurrence in nature
Chloramphenicol is a rare example of a naturally-occurring nitro compound. At least some naturally occurring nitro groups arise by the oxidation of amino groups. 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. Nitrophenylethane is found in Aniba canelilla. Nitrophenylethane is also found in members of the Annonaceae, Lauraceae and Papaveraceae.
Reactions of aliphatic nitro compounds
Nitro compounds participate in several organic reactions, the most important being their reduction to the corresponding amines:
- RNO2 + 3 H2 → RNH2 + 2 H2O
Nitroalkanes are somewhat acidic. The pKas of nitromethane and isopropyl nitrate, are 17.2 and 16.9 in DMSO solution. These values suggest aqueous pKas of around 11. In other words, these carbon acids can be deprotonated in aqueous solution. The conjugate base is called nitronate. Nitronates protonate at oxygen to give a tautomer of nitroalkyl precursor. This process is the start of a reaction that converts nitronates to aldehydes or ketones, called the Nef reaction.
Nitromethane undergoes base-catalyzed additions to aldehydes in 1,2-addition in the nitroaldol reaction. Similarly, it adds to alpha-beta unsaturated carbonyl compounds as a 1,4-addition in the Michael reaction as a Michael donor. Nitroalkenes are Michael acceptors in the Michael reaction with enolate compounds.
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.
Reactions of aromatic nitro compounds
Reduction of aromatic nitro compounds with hydrogen over a meta 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:
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.
- Functional group
- Reduction of nitro compounds
- Nitrite (also an NO2 group, but bonds differently)
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