How does aromatic electrophilic substitution work?
First, let's have a look at all the clues in the name. Aromatic - this type of reaction is characteristic for aromatic compounds because it involves the aromatic ring. It is the only type of electrophilic substitution you should be concerned about on this level. Substitution - the general classification of these reaction, this tells us that there must be some kind of a replacemement going on, one functional group is substituted with a different one. Electrophilic - The main drive of this reaction is that an electrophilic (electron-liking) compound looks for another electron-rich one to bind with it and compensate for its own deficit. Putting it all together with benzene as the example - Benzene is the simplest aromatic compound and like all aromatic rings has delocalised electrons and is very electron-rich, it means there must be some other compound that acts as an electrophile wanting to attach to it. There are plenty of electrophiles but Cl+ is the easiest example, it wants to be attached and, knowing it's a substitution, something else must be replaced. The only possibility is a hydrogen as there's nothing else in benzene that could be a leaving group. That all gives us the general idea: first, a catalyst such as FeCl3 must be used to break Cl2. Cl- attaches to the catalyst and Cl+ is free to attack the ring, it bonds to one of the carbons, creating a transition state when both Cl and H are attached to one carbon, disturbing the delocalised electron structure but quickly the negatively charged FeCl4 draws the hydrogen to itself, breaking the C-H bond and breaking down back to FeCl3 and newly formed HCl. The products of such a reaction are chlorobenzene (a benzene with one hydrogen replaced by Cl) and HCl.
