Quick Notes Nitration of Benzene
- The nitration of a benzene is an electrophilic substitution reaction, with a nitronium ion, NO+2, acting as the electrophile.
- The nitronium ion is produced by reacting concentrated nitric acid with concentrated sulfuric acid.
- The reaction is carried out at 55oC and produces a nitroarene.
Full Notes Nitration of Benzene
Arenes are cyclic hydrocarbons that contain a delocalised electron bonding system – like in benzene with a ring of delocalised electrons (benzene is an arene).
At A-Level, nitration of an arene usually just means the substitution of a NO2 group onto a benzene ring. This reaction requires a temperature of approximately 55oC, concentrated nitric acid and concentrated sulfuric acid.
The first thing needed is an electrophile, in this case NO+2 (nitronium ion). This is produced by the reaction of concentrated nitric acid in the presence of concentrated sulfuric acid, which happens in two stages.
The nitronium ion (NO+2) is very strong electrophile that will react with benzene. Remember the basic outline for an electrophilic substitution reaction with benzene. See Benzene Reactions.
Nitration of Benzene Mechanism
An electrophile accepts electrons from the ring of delocalised electrons, forming a new bond with a carbon atom.
This removes one electron from the delocalised ring, making it positively charged.
The delocalised ring needs another electron to become stable again. To get this electron, the carbon-hydrogen bond breaks and the electrons go to the ring of delocalised electrons.
A positive hydrogen ion is released (proton), and the original electrophile is now bonded onto the benzene ring.
The hydrogen has been substituted for an electrophile, making the reaction an electrophilic substitution.
The hydrogen ion (H+) that is ‘kicked off’ the benzene ring reacts with the HSO-4 ion formed at the start, reforming H2SO4. This means H2SO4 is a catalyst in this reaction, because it’s reformed at the end of the reaction.