Benzenes do not undergo electrophilic addition reactions due to the stable delocalised ring of π-electrons. However they do undergo electrophilic substitution. The hydrogen atom(s) in benzene is replaced by an electrophile, via a two step mechanism.
General Mechanism
Good electrophiles
As the electron density in the benzene ring is spread out, in order for a reaction to occur there must be a strong electrophile.
Halogen carriers, which are a type of catalyst, can produce strong electrophiles via polarisation. Common halogen carriers are aluminium halides and iron halides. The halogen carrier accepts a pair of electrons from the halogen on an electrophile. The difference in electron density in the carbon-halogen bond is increased whereby at times ionic species are produced.
R−X:→AlX3polarisationR+AlX4−
Example
R−Cl:→AlCl3polarisationR+AlCl4−
Halogenation
Benzene can react with a halogen in the presence of a catalyst. The catalyst polarises the electrophile so it is strong enough to react with the benzene.
C6H6+X2halogencarriercatalystC6H5X+HX
Example
C6H6+Br2FeBr3C6H5Br+HBr
Mechanism
Br−Br+FeBr3→Br++FeBr4−
Nitration
Benzene can be warmed with concentrated nitric acid and concentrated sulfuric acid to form a nitrobenzene. Nitronium ion is produced in situ.
In order for mononitration to occur the temperature must be below 50°C. A temperature above this will lead to multiple substitutions. The reaction is as follows:
