The nitration of benzene is another example of an electrophilic aromatic substitution reaction. In this reaction, a hydrogen atom on the benzene ring is substituted with a nitro group (-NO2). The typical reagents for the nitration reaction are nitric acid and sulfuric acid:
As with other electrophilic aromatic substitution reactions, the benzene ring acts as a nucleophile. However, nitric acid (HNO3) is not a very good electrophile (nor is benzene a very good nucleophile). Sulfuric acid (H2SO4) helps overcome this by converting nitric acid into nitronium ions (NO2+), which are much better electrophiles. The benzene ring then nucleophilically attacks a nitronium ion, and re-aromatization of the subsequent complex leads to the nitrobenzene product.
The first step of the nitration mechanism involves the formation of the nitronium ion from nitric acid. First, let’s look at the Lewis structures of nitric acid, sulfuric acid, and the nitronium ion:
Note that the structure of nitric acid is a bit unusual, with a positively charged nitrogen and a negatively charged oxygen, but a net neutral charge.
The first step of this mechanism involves the protonation of nitric acid by sulfuric acid. In other words, sulfuric acid acts as a Brønsted-Lowry acid by transferring a proton (H+) to nitric acid. For this kind of proton transfer mechanism, a lone pair on the nitric acid oxygen (the one with a hydrogen attached) attacks a hydrogen atom on sulfuric acid. This is accompanied by cleavage of the sulfuric acid H-O bond, with the electrons going onto the oxygen:
Now that the nitric acid oxygen is protonated (and positively charged), it is a good leaving group; oxygen, which is electronegative, doesn’t like to be positively charged, and wants to take the electrons from the N-O bond. The protonated oxygen is “pushed” off of the nitrogen atom by a lone pair of electrons on the negatively charged nitric acid oxygen, forming a N=O double bond. This leads to the cleavage of the bond between the nitrogen and the protonated oxygen, with the electrons going onto the oxygen (neutralizing the charge), and releasing water. This forms the electrophilic nitronium ion required for reaction with benzene:
With the nitronium ion electrophile formed, it can react with the benzene ring. The π electrons of the benzene ring (which are drawn as double bonds) nucleophilically attack the electrophilic nitrogen of the nitronium ion, cleaving one of the N=O π bonds, with the electrons going onto the oxygen (giving it a negative charge):
This step disrupts the aromaticity of the benzene ring, which is not very energetically favourable. As such, there is a strong drive to re-aromatize the benzene ring. This is accomplished by removing the hydrogen atom on the benzene carbon bonded to the new nitro group. The water molecule released earlier can act as a base, donating a lone pair to this hydrogen, breaking the C-H bond, and re-forming the double bond in the benzene ring (or more accurately, re-establishing the aromaticity of the benzene ring). Note the similarities between this step and the E1 unimolecular elimination mechanism seen with the alkyl halides.
The mechanism for the benzene nitration reaction is very similar to the mechanisms of the other electrophilic aromatic substitution reactions. The primary difference is the mechanism by which the electrophile is formed. Many of the electrophilic aromatic substitution reactions involve the activation of the electrophile through the use of a Lewis acid catalyst, such as in Friedel-Crafts alkylation, Friedel-Crafts acylation, and electrophilic aromatic halogenation. Instead, the nitration mechanism uses a strong Brønsted-Lowry acid (sulfuric acid) to convert nitric acid into a better electrophile, the nitronium ion.
Once the electrophile is formed, you can see that the mechanisms for the nucleophilic attack (and subsequent re-aromatization) are nearly identical:
The nitration of benzene is an electrophilic aromatic substitution reaction, in which a nitro group (-NO2) is introduced onto a benzene ring. This reaction proceeds via the formation of an electrophilic nitronium ion (NO2+) from nitric acid (HNO3), resulting from proton transfer from sulfuric acid (H2SO4). Following the nucleophilic attack of benzene onto the nitronium ion, re-aromatization leads to the nitrobenzene product.