Nitration

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

Aromatic nitration

Aromatic nitration occurs with aromatic organic compounds via an electrophilic aromatic substitution mechanism involving the attack of the electron-rich benzene ring by the (nitryl) nitronium ion.

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.

Reaction mechanism

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, Fluorenone nitration is selective and yields a tri-nitro compound ^[1] or tetra-nitro compound ^[1] by tweaking reaction conditions just slightly. Another example of trinitration can be found in the synthesis of phloroglucinol.

Other nitration reagents include nitronium tetrafluoroborate which is a true nitronium salt. This compound can be prepared from hydrogen fluoride, nitric acid and boron trifluoride ^[1]. Aromatic nitro compounds are important intermediates to anilines by action of a reducing agent.

Further elucidation of selectivity can be found in examining ring substituents and the effect they have on the reaction rate of this electrophilic aromatic substitution. Deactivating groups such as other nitro groups have an electron pair withdrawing effect which deactivates the reaction (creating difficulty in the formation of poly-nitro products) and directs the electrophilic nitronium ion to attack the aromatic meta position.

Deactivating meta-directoring substituents include thionyl, cyano groups, keto, can be esters or carboxylic derivatives. Nitration can be activated 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 ^[1] 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 ^[1] 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 is reacted with nitric acid and mercury nitrate to picric acid.

References