Nitration

Nitration is a a general chemical process for the introduction of a nitro group in a chemical compound. Examples of nitrations are the conversion of glycerin to nitroglycerin and the conversion of toluene to trinitrotoluene, both conversions use nitric acid and sulfuric acid.

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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. Alternative mechanisms have been proposed, as the one involving single electron transfer (SET)^[1][2]. 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.

(1) 2H₂SO₄ + HNO₃ → 2HSO₄^1- + NO₂⁺ + H₃O⁺

(2) C₆H₆ + NO₂⁺ → C₆H₅NO₂ + H⁺

(3) H⁺ + H₃O⁺ + 2HSO₄^1- → H₂O + 2H₂SO₄

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 is shown below:

Scope

Selectivity is always a challenge in nitrations, The nitration of fluorenone is selective and yields a tri-nitro compound^[3] or tetra-nitro compound ^[4] 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.^[5]

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 ^[6] results in a 50/50 mixture of ortho 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 ^[7] 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