Producing Halogenoarenes (Cambridge (CIE) A Level Chemistry): Revision Note

Production of Halogenoarenes

• Halogenoarenes are arenes which are bonded to halogen atoms

• They can be prepared from substitution reactions of arenes with chlorine or bromine in the presence of an anhydrous catalyst

Substitution of benzene to form halogenoarenes

• Chlorine gas is bubbled into benzene at room temperature and in the presence of an anhydrous AlCl₃ catalyst to form chlorobenzene

• The AlCl₃ catalyst is also called a halogen carrier and is required to generate the electrophile (Cl⁺)

• This electrophile attacks the electron-rich benzene ring in the first stage of the reaction which disrupts the delocalised π system in the ring

• To restore the aromatic stabilization, a hydrogen atom is removed in the second stage of the electrophilic substitution reaction to form chlorobenzene

• When this happens, the delocalised π system of the ring is restored

• The same reaction occurs with benzene and bromine in the presence of an AlBr₃ catalyst to form bromobenzene

Formation of chlorobenzene

Halogenoarenes can be formed from the electrophilic substitution reaction of arenes with halogens

Substitution of methylbenzene to form halogenoarenes

• The electrophilic substitution of methylbenzene with halogens results in the formation of multiple halogenoarenes as products

• This is because the methyl group (which is an alkyl group) in methylbenzene is electron-donating and pushes electron density into the benzene ring

• This makes the benzene ring more reactive towards electrophilic substitution reactions

• The methyl group is said to be 2,4-directing and as a result, the 2 and 4 positions are activated

• Electrophilic substitution of methylbenzene with chlorine and anhydrous AlCl₃ catalyst, therefore, gives 2-chloromethylbenzene and 4-chloromethylbenzene

• The reaction mechanism is the same as the substitution mechanism of benzene

Chlorination of methylbenzene

The methyl group on methylbenzene directs the incoming halogen on the 2 and 4 position

• In the presence of excess chlorine, substitution on the 6 position will also occur