Overview:
Halogen compounds include halogenoalkanes (alkyl halides) and halogenoarenes (aryl halides) — organic compounds where one or more hydrogen atoms have been replaced by halogen atoms (F, Cl, Br, or I). This chapter explores their naming, preparation, structure, reactivity, and differences in chemical behavior.
🔸 1. Preparation of Halogenoarenes
👉 Reaction Type: Electrophilic Substitution
- Halogenoarenes are formed by electrophilic substitution of aromatic rings.
👉 General Reaction:
- Benzene + Cl₂/Br₂ → Halogenoarene (e.g., chlorobenzene, bromobenzene)
- Requires anhydrous AlCl₃ or AlBr₃ as Lewis acid catalyst
👉 Example 1: Formation of Chlorobenzene
C6H6+Cl2→AlCl3C6H5Cl+HCl
👉 Mechanism (Simplified):
- Formation of electrophile:
Cl2+AlCl3→Cl++AlCl4− - Attack on benzene ring by Cl⁺ forming an intermediate arenium ion.
- Loss of H⁺ restores aromaticity and forms chlorobenzene.
👉 Example 2: Substitution on Methylbenzene (Toluene)
- Methyl group is an electron-donating group that activates the ortho and para positions.
- Product(s):
- 2-chloromethylbenzene (ortho)
- 4-chloromethylbenzene (para)
🔸 2. Difference in Reactivity Between Halogenoalkanes and Halogenoarenes
| Feature | Halogenoalkane (e.g., Chloroethane) | Halogenoarene (e.g., Chlorobenzene) |
|---|---|---|
| Bond Type | C–Cl is a polar single bond | C–Cl is partially double-bonded (resonance) |
| Electron Delocalization | No delocalization | Lone pair on Cl interacts with π-system |
| Bond Strength | Weaker (C–Cl ~346 kJ/mol) | Stronger due to partial double bond (~399 kJ/mol) |
| Reactivity with Nucleophiles | Undergoes nucleophilic substitution easily | Very resistant to nucleophilic attack |
| Mechanism | SN1 / SN2 | Rarely reacts unless under harsh conditions |
👉 Explanation:
- In halogenoalkanes, the C–X bond is polar, and the carbon is electrophilic, making it susceptible to nucleophilic attack (e.g., hydrolysis by OH⁻).
- In halogenoarenes, the lone pair on halogen overlaps with the π-system of the benzene ring, creating resonance structures that stabilize the C–Cl bond and reduce the δ⁺ charge on carbon. Thus, chlorobenzene does not undergo nucleophilic substitution easily.
🔬 Example Comparison:
- Chloroethane + NaOH (aqueous, heat) → Ethanol + NaCl ✅ (occurs)
- Chlorobenzene + NaOH (aqueous, heat) → No reaction ❌
🔸 Summary of Learning Outcomes
| Learning Outcome | Covered Concepts |
|---|---|
| 1. Recall the reactions by which halogenoarenes are produced | Electrophilic substitution of benzene and methylbenzene using Cl₂/Br₂ and AlCl₃/AlBr₃ |
| 2. Explain the difference in reactivity between halogenoalkane and halogenoarene | Bond strength, resonance stabilization, polarity, SN reactions, nucleophilicity |
🔸 Important Notes:
- Halogenoalkanes are used in synthesis due to their reactivity with nucleophiles → useful in preparing alcohols, amines, etc.
- Halogenoarenes require more severe conditions (e.g., strong bases and high temperatures) to react via nucleophilic aromatic substitution.
- Knowledge of electronic effects and reaction mechanisms is key to understanding their behavior.
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