Chemical Properties of Haloarenes

Haloarenes, also known as aryl halides, undergo a number of reactions such as nucleophilic substitution reactions, electrophilic substitution reactions. Like haloalkanes, they also react with metals. Let us discuss them one by one :

Nucleophilic Substitution Reactions in Haloarenes

Haloarenes undergo nucleophilic substitution reactions. They are, however, far less reactive than haloalkanes towards nucleophilic substitution reactions.

Why are haloarenes less reactive towards nucleophilic substitution reactions?

Haloarenes are far less reactive than haloalkanes towards nucleophilic substitution reactions due to the following reasons :

• Resonance effect : Due to resonance in haloarenes, the C−X bond acquires some double bond character and is difficult to break.
  
• Hybridisation of carbon : In haloalkanes, the carbon atom attached to the halogen atom is sp³ hybridised but in haloarenes it is sp² hybridised . An sp² hybridised orbital, being closer to the nucleus, is shorter and hence stronger than an sp³ hybridised orbital. Hence, the C-X bond in haloarenes is shorter and stronger.
• Polarity of C-X bond : An sp² hybridised carbon is more electronegative than an sp³ hybridised carbon due to more s-character. Thus, the C-X bond in haloarene is less polar, which decreases its reactivity.

Electrophilic Substitution Reactions of Haloarenes

Electrophilic substitution reactions are prevalent in aromatic compounds, and, haloarenes are no exception. Halogen atom is slightly deactivating and o,p-directing; therefore, electrophilic substitution in haloarenes occurs at ortho and para positions with respect to the halogen atom.

Why are halogens deactivating?

Due to −I-effect, halogen atoms have some tendency to withdraw electrons from the benzene ring that reduces the overall electron density on the ring. As a result, the ring gets somewhat deactivated as compared to benzene.

Why does substitution occur only at ortho and para positions of haloarenes?

In haloarenes, electron density is more at ortho and para positions which is evident from the resonance structures of haloarenes given below :

Because of this reason, electrophiles (electron deficient species) get attracted towards ortho and para positions of haloarenes.

Reaction of Haloarenes with Metals

Aryl halides, like alkyl halides, react with metals to form a number of compounds.

We will learn more about nucleophilic substitution, electrophilic substitution and reaction with metals in case of haloarenes on next three pages.