Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Haloarenes are significantly less reactive towards Nucleophilic Substitution () reactions compared to haloalkanes due to resonance effects, difference in hybridization of the carbon atom in the bond ( vs ), and instability of the phenyl cation.
Resonance Effect: The electron pairs on the halogen atom are in conjugation with electrons of the ring, resulting in a partial double bond character in the bond, making it difficult to break.
Nucleophilic substitution can occur under drastic conditions (high temperature and pressure), such as in the Dow's Process for the synthesis of Phenol.
Presence of electron-withdrawing groups (EWG) like at ortho and para positions increases the reactivity of haloarenes towards nucleophilic substitution.
Electrophilic Substitution Reactions (): The halogen atom is deactivating but ortho/para directing due to the effect (resonance) and effect (induction).
Major Electrophilic reactions include Halogenation, Nitration (), Sulphonation (), and Friedel-Crafts Alkylation/Acylation.
Wurtz-Fittig Reaction: A mixture of an alkyl halide and aryl halide treated with sodium in dry ether gives an alkylarene.
Fittig Reaction: Aryl halides when treated with sodium in dry ether give diphenyl (biphenyl).
📐Formulae
💡Examples
Problem 1:
Explain why -nitrochlorobenzene is more reactive than chlorobenzene towards nucleophilic substitution with .
Solution:
The presence of the group at the para position withdraws electrons from the benzene ring via the effect. This decreases the electron density at the bond and stabilizes the intermediate carbanion (Meisenheimer complex) formed during the reaction, making the nucleophilic attack by easier.
Explanation:
Electron-withdrawing groups at - and - positions facilitate reactions in haloarenes by dispersing the negative charge of the sigma-complex.
Problem 2:
Predict the products of the reaction:
Solution:
-Chloroacetophenone (Major) and -Chloroacetophenone (Minor).
Explanation:
This is a Friedel-Crafts Acylation reaction. The atom on the ring directs the incoming acyl group () to the ortho and para positions. The para product is major due to less steric hindrance.