Why does Benzene undergo only electrophilic substitution reactions?
This property can be attributed to the remarkable stability of Benzene, due to the 6 delocalised electrons forming a ᴨ cloud of electrons. Instead of the electrons forming three stationary C==C bonds, they form a delocalized ring which gives benzene greater stability, and this is seen in the enthalpy change when breaking the delocalized ring of electrons in benzene.
Comparing the structures of benzene and 1, 3, 5-cyclohexatriene:
One would expect to have similar enthalpy changes for breaking the delocalized ring of benzene and the 1, 3, 5 pi bonds of the 1, 3, 5-cyclohexatriene, but in real life it is ...view middle of the document...
Nucleophiles, possessing a negative charge, are not attracted to this delocalized electron cloud.
So how does an electrophilic reaction occur (reaction mechanism)?
This is best explained with the help of diagrams.
1. 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 whole benzene molecule. This is called an intermediate (also known as an Arenium ion), and it is a positive ion because the benzene molecule was neutral and a positive electrophile was added to it. The positive charge is dispersed all over the delocalized part of the ring.
3. It is not possible to get a positive ion (the electrophile) on its own in a chemical system, and now the negative ion (nucleophile) that was associated with the positive ion makes an appearance. This nucleophile possesses a lone pair of electrons, which it donates to the hydrogen atom on the ring. Hence, the pair of electrons which adjoin the hydrogen atom to the carbon atom on the ring are not necessary any longer, and are returned to the ring to reform the delocalized electron cloud.
So, overall, a Hydrogen atom on the benzene ring has been substituted by an electrophile.
Different electrophilic substitution reactions undergone by benzenes
Benzenes react with chlorine and bromine and undergo electrophilic substitution reactions, but the presence of a catalyst (Either FeBr3, FeCl3 or AlCl3) is necessary. Even Fe can be added, but it is not completely a catalyst, as it reacts with the bromine or chlorine to form:
a. 2Fe + 3Cl2 2FeCl3
b. 2Fe +3Br2 2FeBr3
These compounds subsequently act as the catalyst.
Reaction mechanism for halogenation:
1. The bromine or chlorine molecule approaches the benzene ring. It becomes polar as it gets close because the delocalized electrons repel the electron pair between the two Br or Cl atoms. The positive end of the Br2 or Cl2 molecule (electrophile) comes closer to the benzene ring.
2. The Br2 or Cl2 molecule splits into an anion and a cation. The positive ion (cation) comes close to the benzene ring and a pair of electrons from one of the pi bonds is used to make a covalent bond with it. This is the intermediate.
3. The anion forms a bond with the catalyst to form either:
AlCl4- or FeCl4- or FeBr4-
This molecule approaches the intermediate. The negative ion donates a pair of electrons to form a...