A chlorobenzene (phenyl chloride) is formed if benzene is treated with chlorine in the presence of ferric chloride but the absence of water.
The reaction is driven to completion by the gentle heating of the reaction mixture. The overall equation of the reaction is:
Firstly, it is necessary for there to be an electrophile to attack the regions of high electron density within the benzene ring. This is achieved by the splitting of the single bond within the chlorine molecule, accompanied by the creation of a chloronium ion. It is this chloronium ion which acts as the electrophile in the reaction.
The pi electrons in the benzene ring are attacked by the chloronium ion and a proton is lost from the benzene molecule intermediate due to resonance stabilisation. The proton (which is chemically a hydrogen ion) regenerates the ferric chloride catalyst and the reaction continues for as long as there are reactants present.
The progress of the reaction can be tested by wet blue litmus paper. The hydrogen chloride fumes given off turn it red. This reaction can be summed up by the stoichiometric equation:
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