Why does benzene only undergo electrophilic substitution reactions? This property can be attributed to the remarkable stability of benzene, due to the 6 delocalized electrons forming a cloud of ᴨ electrons. Instead of forming three stationary C==C bonds, the electrons form a delocalized ring that gives benzene greater stability, and this is seen in the enthalpy change when the delocalized ring of electrons in benzene breaks down. Comparing the structures of benzene and 1,3,5-cyclohexatriene: one would expect similar enthalpy changes for the breakdown of the delocalized ring of benzene and the 1,3,5 pi bonds of 1,3,5-cyclohexatriene, but in real life it is about 150 kJ/mol in excess of the amount of energy needed to break the three pi bonds in 1, 3, 5-cyclohexatriene. This is why addition reactions are unlikely to occur with benzene molecules. In order for a molecule to be added to benzene, the delocalized ring of electrons must be broken, and since the energy required for this to happen is so high, this is not an easy thing to do. In the case of electrophilic substitution reactions, the delocalized ring of electrons remains as it is, so it does not require a large amount of energy and therefore the reactions occur more easily. What are electrophiles? Why does benzene undergo only electrophilic but not nucleophilic substitution? By definition, a molecule that forms a covalent bond by accepting a pair of electrons is called electrophilic. Any molecule, ion, or atom that is electron deficient in some way can behave as an electrophile. In contrast, any molecule that forms a covalent bond by donating a pair of electrons is called a nucleophile. Nucleophiles are generally electron-rich and look for positive atoms or molecules, which are usually found in the nucleus of an atom - hence the name Nucleophile. If we look at the structure of benzene, we can see that although it possesses an overall neutral charge, the delocalized electron cloud forms an area of ​​negative charge that attracts positively charged electrophiles or the positive ends of polar molecules. Nucleophiles, which have a negative charge, are not attracted to this cloud of delocalized electrons. So how does an electrophilic reaction (reaction mechanism) happen? This is best explained with the help of diagrams.1. The electrophile is attracted to the benzene molecule.2. Two of the delocalized electrons are used to form a bond with the electrophile. The remaining two pi bonds in the benzene molecule are unaffected, so delocalization is present but not across the entire benzene molecule.