Class 12 Chemistry Unit 6: Haloalkanes and Haloarenes – Practice Questions (2025-26)
Unit 6 moves beyond basic hydrocarbons to explore functional group reactivity. By replacing hydrogen atoms with halogens, we generate compounds with distinct properties driven by the polarized Carbon-Halogen bond.
This guide examines the logic behind Nucleophilic Substitution, clarifying the differences between SN1 and SN2 pathways, while also breaking down the stereochemical principles of chiral molecules. It covers essential preparation methods, chemical properties, and the environmental effects of polyhalogen compounds like DDT and Freons.
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Class XII Unit 6: Haloalkanes & Haloarenes
Haloalkanes & Haloarenes: Reactivity & Structure
Unit 6 bridges hydrocarbon chemistry with functional group reactivity. This guide explores how the polarized Carbon-Halogen bond drives reactions like Nucleophilic Substitution ($S_N1/S_N2$) and Elimination, while also addressing the environmental impact of polyhalogen compounds.
Concept Check
Pop-out quiz to test your understanding of mechanisms and reagents.
Organic Chemistry Challenge
Ready to test your mastery? Identify reagents, predict products, and distinguish between SN1 and SN2 pathways.
Nature of C-X Bond
Halogens are more electronegative than carbon, creating a permanent dipole. The carbon acquires a partial positive charge ($\delta+$), making it an electrophilic center susceptible to nucleophilic attack.
- Bond Polarity: C-F > C-Cl > C-Br > C-I
- Bond Length: C-F < C-Cl < C-Br < C-I
- Reactivity: R-I > R-Br > R-Cl > R-F (Weakest bond breaks first)
Bond Enthalpy (kJ/mol)
Methods of Preparation
From Alcohols
Best Method-
Thionyl Chloride:
R-OH + SOCl2 → R-Cl + SO2↑ + HCl↑Byproducts are gases (pure product).
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Lucas Reagent:
R-OH + HCl (ZnCl2) → R-ClOrder: 3° > 2° > 1°
From Hydrocarbons
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Free Radical Halogenation:
Alkane + Cl2 (UV) → MixturePoor yield of single isomer.
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Electrophilic Addition:
Alkene + HX → Alkyl HalideFollows Markovnikov’s Rule.
Halogen Exchange
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Finkelstein (Iodides):
R-X + NaI → R-I + NaXSolvent: Dry Acetone (ppt forms).
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Swarts (Fluorides):
R-Br + AgF → R-F + AgBrReagents: AgF, Hg2F2, SbF3.
Reaction Mechanisms: SN1 vs SN2
SN2 (Bimolecular)
- Kinetics: Rate = k[Substrate][Nu]
- Step: Single concerted step.
- Stereochem: Inversion (Walden).
- Order: Methyl > 1° > 2° > 3°
SN1 (Unimolecular)
- Kinetics: Rate = k[Substrate]
- Step: Two steps (Carbocation).
- Stereochem: Racemization.
- Order: 3° > 2° > 1° > Methyl
Stereochemistry Essentials
Chirality
Objects non-superimposable on their mirror image. Usually implies an asymmetric carbon bonded to 4 different groups.
Enantiomers
Stereoisomers that are mirror images. They have identical physical properties but rotate light in opposite directions.
Racemic Mixture
A 50:50 mixture of two enantiomers. Optically inactive due to external compensation.
Left Hand
Right Hand (Mirror)
Reactivity of Haloarenes
Why Low Reactivity?
Aryl halides are extremely less reactive towards Nucleophilic Substitution due to:
- Resonance Effect: Partial double bond character in C-X bond makes cleavage difficult.
- Hybridization: sp2 carbon holds the halogen more tightly than sp3.
- Instability: Phenyl cation is unstable (rules out SN1).
Dow’s Process (Forcing conditions)
Chlorobenzene can be converted to Phenol using NaOH at 623 K and 300 atm.
Effect of -NO2
Electron withdrawing groups at ortho/para positions increase reactivity by stabilizing the intermediate carbanion.
Polyhalogen Compounds
DDT
First chlorinated organic insecticide. Banned due to bio-magnification.
Freons (CFCs)
Refrigerants. Cause ozone layer depletion in stratosphere.
Chloroform
Stored in dark bottles to prevent formation of poisonous Phosgene (COCl2).
Iodoform
Earlier used as antiseptic but replaced due to smell. Activity due to free Iodine.
CCl4
Carbon Tetrachloride. Used as feedstock for refrigerants. Exposure causes liver cancer.
