Introduction to nitration of benzene
Nitration of benzene is an example of elctrophilic aromatic substitution reaction. Here nitronium ion (NO2+) acts as an electrophile and reacts with benzene to form nitrobenzene. As for example Benzene reacts with concentrated nitric acid in presence of concentrated sulphuric acid as a catalyst, and form nitrobenzene.
Nitration of benzene
When benzene is treated with concentrated nitric acid and concentrated sulphuric acid at below 55o C temperature, nitrobenzene is formed. If we increase the temperature there is a greater chance of entering more than one nitro group in the benzene ring.
The elctrophilic substitution reaction mechanism for nitration of benzene
The formation of the electrophile
Substitution of hydrogen of a benzene ring with a nitro group can only be done using nitronium ion or nitryl cation (NO2+) as an electrophile. This ion is formed by the reaction between the concentrated nitric acid and concentrated sulphuric acid.
HNO3 + 2H2SO4 → NO2+ + 2HSO4– + H3O+
Here the hydronium ion (H3O+) is not involved in any further steps of the reaction whereas the hydrogensulphate ion (HSO4–) is.
The elctrophilic substitution of benzene is occur in few steps, which are as follows:
Step 1: Addition of nitronium ion or nitryl cation (NO2+)
When nitronium ion (NO2+) approaches towards the benzene ring 2a, the electropositive nitronium ion attracts by the electron rich benzene ring. Two electrons of one of the double bond inside the benzene ring breaks down to form a bond between the nitronium ion (NO2+) and the benzene ring 2b. Thus the delocalisation of double bond partly broken and positive charge from the nitronium ion (NO2+) stays inside the ring. This positive charge is then delocalised all through the benzene ring just like it shows in above scheme. The hydrogen attached to the same carbon where the nitro group is attached is also shown in figure which will be removed in next step.
Step 2: Removal of hydrogen ion (H+)
The hydrogensulphate ion (HSO4–) which was formed during the formation of electrophile- nitronium ion, removes the hydrogen attached with the nitro group containing carbon. This hydrogen (H+) leaves the electron to the benzene ring. Thus the positive charge of the ring neutralized and delocalisation reestablished. By taking hydrogen ion, hydrogensulphate ion (HSO4–) becomes sulphuric acid as it was before (catalyst).
HSO4– + H+ → H2SO4
Further nitration of nitrobenzene
Nitrobenzene is very reactive and it can produce dinitrobenzene in presence of same reagent. The directing effect of nitrobenzene is 93% meta, 6% ortho and 1% para. The formation of ortho, meta and para-dinitrobenzene is as follows:
Nitrobenzene is very toxic and it can readily absorb through the skin. So precaution must need to be taken during and after the reaction. Nitrobenzene may cause:
- liver/kidney damage
- lung irritation
- nervous system damage
- impair vision
- weakness in arms and legs
- increase heart rate
- gastrointestinal irritation
- internal bleeding
- and rarely death.
In United states it is classified as a extremely hazardous substance and likely human carcinogenic.