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.
Out of chlorobenzene and chloromethane which is more reactive towards nucleophilic substitution reaction?
Electrophilic Substitution Reactions of HaloarenesElectrophilic 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.
Electrophiles get attracted to ortho and para positions
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.