Q. What is a halogenoarene?
An aromatic hydrocarbon in which one or more hydrogen atoms bonded to the aromatic ring are replaced by a halogen atom.
Q. How can you tell whether a substitution reaction by a halogen will occur in the benzene ring or on the side chain?
- If UV light is present and a catalyst is absent, substitution takes place on the side chain.
- If a catalyst is present and UV light is absent, substitution takes place in the benzene ring.
Q. Describe the halogenation of benzene.
Benzene only reacts with bromine if a halogen carrier is present.
Q. Name the mechanism for the bromination of benzene.
Electrophilic Substitution. The Br+ ion accepts a pair of electrons from the delocalized ring. The intermediate is so unstable that it breaks down, releasing a hydrogen ion and forming the product, bromobenzene. This process is similar to the nitration of benzene.
Q. Compare the reactivity of chlorobenzene to chloroalkanes.
The C-Cl bond in chlorobenzene is stronger than expected. One of the lone pairs on the chlorine atom interacts with the delocalised ring electrons, strengthening the bond. This means the bond is harder to break and hence requires more energy to overcome. Therefore, chlorobenzene is less reactive than chloroalkanes.
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