How does electrophilic aromatic substitution occur?
The delocalised ring in aromatic compounds (such as benzene) is very stable and therefore requires a very strong electrophile to be created before it will react. An example of this would be the formation of the nitronium ion (NO2+) from sulfuric acid and nitric acid, where the nitric acid acts as a base by accepting a proton in the following reaction:H2SO4 + HNO3 -> HSO4- + NO2+ + H2OUsing the whiteboard I can then outline the mechanism for this reaction, showing how the nitronium ion breaks the aromatic delocalised ring, then the ring reforms by eliminating a hydrogen atom. This hydrogen atom then goes on to react with the hydrogen sulfate HSO4- ion to reform sulfuric acid, thus making sulfuric acid a catalyst in this reaction.We can draw parallels with this reaction to many other electrophilic substitution reactions such as chlorination, bromination and Friedel-Crafts alkylation reactions as they are all very similar in principle.
