Reactivity of Halogenoarenes (Cambridge (CIE) A Level Chemistry): Revision Note

Difference in Reactivity of Halogenoalkanes & Halogenoarenes

• Halogenoarenes are very unreactive compared to halogenoalkanes

• The difference in reactivity between the two compounds is because of the carbon-halogen bond strengths

Halogenoalkanes

• The halogenoalkane chloroethane can take part in nucleophilic substitution reactions

• A nucleophile, such as a hydroxide (OH^-) ion, will attack the slightly positive carbon atom

• A covalent bond is formed between that carbon atom and the nucleophile which causes the carbon-halogen bond to break

• Overall, the halogen is replaced by the nucleophile

Nucleophilic substitution of halogenoalkanes

Halogenoalkanes readily undergo nucleophilic substitution reactions due to the difference in electronegativity between the carbon and halogen

Halogenoarenes

• Halogenoarenes, such as chlorobenzene, do not readily undergo nucleophilic substitution reactions

  • Only under extremely harsh conditions, such as temperatures of 200 ^oC and a pressure of 200 atmospheres, will the chlorine in chlorobenzene get replaced by a nucleophile such as a hydroxide (OH^-) ion

• This is because the carbon-chlorine bond is very strong and cannot be easily broken

  • One of the lone pairs of electrons on the chlorine will interact with the π system of the ring

  • This causes the carbon-chlorine bond to have a partial double-bond character, which strengthens the bond

How the carbon-chlorine bond affects the π system

The carbon-chlorine bond is very strong, as it has partial double-bond character

• Therefore, the unreactivity of halogenoarenes can be explained by the delocalisation of a lone pair on the halogen over the benzene

• This causes additional stabilisation of the system and strengthens the carbon-halogen bond, which affects the reactions that halogenoarenes will undergo

• It gets harder to break the carbon-halogen bond in halogenoarenes, which decreases reactivity