7.3.2 Reactivity of 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

Halogenoalkanes readily undergo nucleophilic substitution reactions

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

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

• The unreactivity of halogenoarenes can therefore 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