• 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