The nitration of aromatic rings is an important reaction industrially since it produces a range of valuable compounds which are used mainly as explosives and dyes. Like all reactions involving aromatic rings this reaction is an example of an electrophilic substitution reaction. The electrophile which is used in this nitration reaction is the nitronium ion or nitryl cation (NO2+) ion. The nitronium ion (NO2+) is generated by the reaction of concentrated nitric acid with concentrated sulfuric acid. The sulfuric acid acts as a catalyst in this reaction. An equation for the nitration of benzene is shown below and as you can see it is a typical electrophilic substitution reaction.
The end result of this reaction is that the nitro group (-NO2) replaced or substitutes for one
of the hydrogen atoms on the benzene ring. The equation below
summaries this reaction, note that the sulfuric acid
is not included in the equation since it is present as a catalyst.
If benzene is warmed under reflux conditions (as shown opposite) to between 50-550C with a 3:1 mixture of
concentrated sulfuric acid and
nitric acids the main product is a yellow oily liquid, nitrobenzene. The mixture of
concentrated nitric and
sulfuric acid is needed because these two acids react together to produce the
nitronium ion (NO2+)
electrophile which will substitute for one of the hydrogen atoms on the benzene ring.
The three steps shown above can be combined as shown below to give the overall equation for the formation of the nitronium ion, the electrophile in a nitration reaction:
The nitration of benzene or any aromatic molecule is simply another example of an electrophilic substitution reaction. The mechanism for the nitration of benzene is shown below, but it should be obvious it is just another example of an electrophilic substitution reaction characteristic of aromatic rings.
If the temperature of the nitration mixture is not carefully monitored and it is allowed to rise above 650C during the reflux reaction then an additional nitro group will add to the benzene ring to produce 1,3-dinitrobenzene. An equation for this is shown below:
As mentioned above when the electrophile adds to the benzene ring the delocalisation of the pi(π) electrons is lost. In order to help stabilise this intermediate cation it will be resonance stabilised as shown below:
