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Nitration

 
Sci-Tech Dictionary: nitration
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(nī′trā·shən)

(organic chemistry) Introduction of an NO2- group into an organic compound.

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Sci-Tech Encyclopedia: Nitration
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A process in which a nitro group (NO2) becomes chemically attached to a carbon, oxygen, or nitrogen atom in an organic compound. A hydrogen or halogen atom is often replaced by the nitro group. Three general reactions summarize nitration chemistry:

  1. C nitration, in which the nitro group attaches itself to a carbon atom [reaction (1)].


  2. O nitration (an esterification reaction), in which an ON bond is formed to produce a nitrate [reaction (2)].


  3. N nitration, in which a NN bond is formed [reaction (3)].


Aromatics, alcohols, glycols, and amines are generally nitrated with mixed acids via an ionic reactions. The mixture includes nitric acid, a strong acid such as sulfuric acid which acts as a catalyst, and a small amount of water. With sulfuric acid, nitric acid is ionized to the nitronium ion, NO2+, which is the nitrating agent.

Propane is commercially nitrated in relatively large amounts using nitric acid in gas-phase free-radical reactions at temperatures of about 380–420°C. Nitric acid decomposes at these temperatures to produce nitrogen dioxide radicals (actually a mixture of ·NO2 and ·ONO) and a hydroxy radical (·OH). In the free-radical reaction, about 35–40% of the nitric acid reacts to form four C1-C3 nitroparaffins; CC bonds are broken during the nitration. The remaining nitric acid acts mainly as an oxidizing agent to form aldehydes, alcohols, carbon monoxide, carbon dioxide, water, and small amounts of other oxidized materials. Commercially, an adiabatic reactor is used, and the heat of reaction is employed to preheat and vaporize the nitric acid feed (containing water).

The product stream from free-radical nitrations is a condensation of mixed nitroparaffins. This liquid mixture is washed to remove the aldehydes, and then is distilled to recover each of the four nitroparafins—nitromethane, nitroethane, 1-nitropropane, and 2-nitropropane. The unreacted propane is recovered, combined with the feed propane, and returned to the reactor. The oxides of nitrogen are converted back to nitric acid; carbon monoxide, carbon dioxide, and water are discarded.

In the classical Victor Meyer process, an organic halide (often a bromide) is reacted with silver nitrite to produce a nitrohydrocarbon and silver halide. In a modified process, sodium nitrite, dissolved in a suitable solvent, is substituted for the more expensive silver nitrite. The desired nitroalkanes are produced in high yields by these processes, whereas they are produced in rather low yields in free-radical nitrations.

Nitrations can also often be performed by addition reactions using unsaturated hydrocarbons with nitric acid or nitrogen dioxide.

Wikipedia: Nitration
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Nitration is a general chemical process for the introduction of a nitro group into a chemical compound. Examples of nitrations are the conversion of glycerin to nitroglycerin and the conversion of toluene to trinitrotoluene. Both of these conversions use nitric acid and sulfuric acid [1].

Aromatic nitration

In aromatic nitration, aromatic organic compounds are nitrated via an electrophilic aromatic substitution mechanism involving the attack of the electron-rich benzene ring by the nitronium ion.

Nitration of benzene

Aromatic nitro compounds are important intermediates to anilines by action of a reducing agent. Benzene is nitrated by refluxing with concentrated sulfuric acid and concentrated nitric acid at 50 °C.The sulfuric acid is regenerated and hence acts as a catalyst. It also absorbs water.

The formation of a nitronium ion (the electrophile) from nitric acid and sulfuric acid and subsequent reaction of the ion with benzene is shown below:

Aromatic nitration mechanism

Alternative mechanisms have also been proposed, as the one involving single electron transfer (SET).[2][3]

Scope

Selectivity is always a challenge in nitrations. The nitration of fluorenone is selective and yields a tri-nitro compound [4] or tetra-nitro compound[5] by modifying reaction conditions. Another example of trinitration can be found in the synthesis of phloroglucinol.

Other nitration reagents include nitronium tetrafluoroborate, a nitronium salt. This compound can be prepared from hydrogen fluoride, nitric acid, and boron trifluoride.[6]

The substituents on aromatic rings affect the rate of this electrophilic aromatic substitution. Deactivating groups such as other nitro groups have an electron-withdrawing effect. Such groups deactivate (slow) the reaction and directs the electrophilic nitronium ion to attack the aromatic meta position. Deactivating meta-directoring substituents include sulfonyl, cyano groups, keto, esters, and carboxylates. Nitration can be accelerated by activating groups such as amino, hydroxy and methyl groups also amides and ethers resulting in para and ortho isomers.

The direct nitration of aniline with nitric acid and sulfuric acid, according to one source [7] results in a 50/50 mixture of para and meta nitroaniline. In this reaction the fast-reacting and activating aniline (ArNH2) is in equilibrium with the more abundant but less reactive and deactivating anilinium ion (ArNH3+), which may explain this reaction product distribution. According to another source [8] a more controlled nitration of aniline starts with the formation of acetanilide by reaction with acetic anhydride followed by the actual nitration. Because the amide is a regular activating group the products formed are the para and ortho isomers. Heating the reaction mixture is sufficient to hydrolyze the nitroamide back to the nitroamine.

In the Wolfenstein-Boters reaction, benzene reacts with nitric acid and mercury nitrate to give picric acid.

References

  1. ^ John McMurry Organic Chemistry 2nd Ed.
  2. ^ Esteves, P. M.; Carneiro, J. W. M.; Cardoso, S. P.; Barbosa, A. G. H.; Laali, K. K.; Rasul, G.; Prakash, G. K. S.; e Olah, G. A. (2003). "Unified Mechanism Concept of Electrophilic Aromatic Nitration Revisited: Convergence of Computational Results and Experimental Data". J. Am. Chem. Soc. 125: 4836. doi:10.1021/ja021307w. 
  3. ^ Queiroz, J. F.; Carneiro, J. W. M.; Sabino A. A.; Sparapan, R.; Eberlin, M. N.; Esteves, P. M. (2006). "Electrophilic Aromatic Nitration: Understanding Its Mechanism and Substituent Effects". J. Org. Chem. 71: 6192. doi:10.1021/jo0609475. 
  4. ^ E. O. Woolfolk and Milton Orchin, "2,4,7-Trinitrofluorenone", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv3p0837 ; Coll. Vol. 3: 837 
  5. ^ Melvin S. Newman and H. Boden, "2,4,5,7-Tetranitrofluorenone", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv5p1029 ; Coll. Vol. 5: 1029 
  6. ^ George A. Olah and Stephen J. Kuhn, "Benzonitrile, 2-methyl-3,5-dinitro-", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv5p0480 ; Coll. Vol. 5: 480 
  7. ^ Web resource: Link
  8. ^ Web source: Link

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