Halo Arenes

Halo arenes are obtained by replacement of hydrogen atom of aromatic hydrocarbon by halogen.

Nature of C-X Bond

Haloarenes are less reactive than that of haloalkanes because

1. In haloarenes, halogen atom is attached to sp² hybridized carbon of arene but in haloalkanes, halogen is attached to sp³ carbon. sp² orbital has more ‘s’ character than sp³ orbital. Hence C-X bond in haloarenes has shorter distance (169 pm) than in halo alkanes (177 pm ). So in halo arenes C-X is stronger bond. Therefore, it is less reactive towards nucelophilic substitution reactions.

2. Due to resonance effect, C-X bond in halo arenes acquires a partial double bond character and the cleavage of C-X bond becomes more difficult than in halo alkanes.

3. Phenyl cation is unstable as it can not be stabilized through resonance. iv) Benzene has more electron density. Therefore a stronger nucleophile can not approach it easily.

Chloro Benzene

Preparation

• From Benzene By Electrophilic Substitution
• Sandmeyer's Reaction (Lab Method)

Electrophilic Substitution Reactions

Halogen atom on benzene ring is deactivating but ortho and para directing. Halogen atom on benzene ring increases the electron density at ortho and para positions than at meta position due to +M effect. Again halogen atom has -I effect and has tendency to withdraw electrons from the ring. So the ring gets slightly deactivated when compared to benzene. Therefore, electrophilic substitution reactions are slow in chloro benzene than in benzene.

Reaction with Metals

• Wurtz - Fittig reaction
• Fittig reaction