CIE A Level Chemistry复习笔记7.3.1 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

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 methly group is said to be 2,3-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

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

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