In contract to alkenes and alkynes, aromatic compounds undergo substitution not addition due to very stable of nature of benzene ring and its resistance to break the double bond. Substitution reactions are characterized by different atoms or groups of atoms replacing hydrogen atoms in a hydrocarbon molecule.

Specific reactions of benzene are considered as alkylation, halogenation, nitration and sulfonation.

Alkylation: An alkyl group (R-) from an alkyl chloride (R-Cl) substitutes for a hydrogen atom on the benzene ring. A catalyst, AlCl3 is needed for alkylation.

2. Halogenation: ( bromination or chlorination) A hydrogen atom on a benzene ring can be replaced by bromine or chlorine if benzene is treated with Br2 or Cl2. in presence of a catalyst. The catalyst is actually FeBr3 in case of bromination and or FeCl3 in case of chlorination.

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