Nitration - Scope

Scope

Selectivity can be a challenge in nitrations because as a rule more than one compound may result but only one is desired, so alternative products act as contaminants or are simply wasted. Accordingly, it is desirable to design syntheses with suitable selectivity; for example, by controlling the reaction conditions, fluorenone can be selectively trinitrated or tetranitrated.

Another example of selective trinitration is when benzene is symmetrically trinitrated to 1,3,5 trinitrobenzene to produce a precursor for the synthesis of phloroglucinol.

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 results in a 50/50 mixture of para and meta nitroaniline. In this reaction the fast-reacting and activating aniline (ArNH₂) exists in equilibrium with the more abundant but less reactive (deactivated) anilinium ion (ArNH₃+), which may explain this reaction product distribution. According to another source 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.