How do benzene rings react in a nitration reaction?

In a nitration reaction, benzene rings react with nitric acid in the presence of a catalyst to form nitrobenzene.

Nitration of benzene is an example of an electrophilic substitution reaction where a hydrogen atom of the benzene ring is replaced by a nitro group (-NO2). This reaction is facilitated by a strong acid, typically sulphuric acid, which acts as a catalyst. The process begins with the formation of the electrophile, the nitronium ion (NO2+), which is produced when nitric acid reacts with sulphuric acid.

The nitronium ion then attacks the benzene ring, which is rich in electron density due to the delocalised π electrons. This attack forms a sigma complex, an intermediate stage where the benzene ring loses its aromaticity temporarily. The hydrogen atom attached to the carbon where the nitronium ion has attached is then removed by the sulphuric acid, regenerating the catalyst and restoring the aromaticity of the benzene ring. The final product is nitrobenzene, a molecule where one hydrogen atom of the benzene has been replaced by a nitro group.

It's important to note that the nitration of benzene is a highly exothermic reaction, releasing a significant amount of energy. This is because the formation of the nitrobenzene product is more stable than the reactants due to the delocalisation of electrons in the benzene ring.

The reaction conditions, particularly the temperature, can influence the rate and outcome of the reaction. Lower temperatures favour the formation of mononitro compounds, while higher temperatures can lead to the formation of dinitro or even trinitro compounds. Therefore, careful control of the reaction conditions is crucial in the nitration of benzene.

In summary, the nitration of benzene involves the substitution of a hydrogen atom with a nitro group, facilitated by the action of a strong acid catalyst. The reaction is exothermic and the conditions can be manipulated to control the degree of nitration.