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High Yield Topic

Haloalkanes and Haloarenes

Chemistry Unit 12
30 min read
IAT Advanced
High Weightage

1. Classification

Halogen compounds are classified based on the number of halogen atoms (mono, di, tri) and the hybridization of the carbon attached to the halogen.

  • Alkyl Halides (sp3): Primary (1°), Secondary (2°), Tertiary (3°). Attached directly to an alkyl carbon.
  • Allylic Halides (sp3): Halogen bonded to an sp3 hybridized carbon next to a C=C double bond.
  • Benzylic Halides (sp3): Halogen bonded to an sp3 hybridized carbon next to an aromatic ring.
  • Vinylic Halides (sp2): Halogen bonded directly to a C=C double bond carbon.
  • Aryl Halides (sp2): Halogen bonded directly to an aromatic ring (Haloarenes).

2. Nomenclature

IUPAC Name: Haloalkane (e.g., 2-Chloropropane). Halogen is always treated as a substituent, lowest locant rule applies.

Common Name: Alkyl halide (e.g., Isopropyl chloride).

Dihalides:

  • Geminal dihalide (Gem-dihalide): Both halogens on the SAME carbon (e.g., Ethylidene chloride).
  • Vicinal dihalide (Vic-dihalide): Halogens on ADJACENT carbons (e.g., Ethylene dichloride).

3. Nature of C–X Bond

Halogens are more electronegative than carbon, making the C–X bond highly polarized. The carbon becomes partially positive (electrophilic center) and the halogen partially negative.

Dipole Moment Order:

CH3Cl > CH3F > CH3Br > CH3I

Note: CH3Cl has a higher dipole moment than CH3F despite Fluorine's higher electronegativity because the longer C-Cl bond length compensates for the lower charge difference (Dipole Moment = Charge × Distance). This is a frequent IAT trap!

4. Preparation of Haloalkanes

From Alcohols:

  • Reaction with HX: R-OH + HX → R-X + H2O (Needs ZnCl2 catalyst for 1° and 2° alcohols - Lucas Reagent).
  • Reaction with PCl3/PCl5: Produces alkyl chlorides.
  • Thionyl Chloride (SOCl2): R-OH + SOCl2 → R-Cl + SO2↑ + HCl↑. Best method because byproducts are escapable gases.

From Hydrocarbons:

  • Free Radical Halogenation: Alkanes + Cl2 (in UV light) → Mixture of mono/poly haloalkanes.
  • Electrophilic Addition: Alkenes + HX → Markovnikov addition. Addition of HBr in presence of peroxide gives Anti-Markovnikov product.

Halogen Exchange (Name Reactions):

  • Finkelstein Reaction (For Alkyl Iodides): R-X + NaI → R-I + NaX↓ (in dry acetone). Le Chatelier's principle drives it as NaX precipitates.
  • Swarts Reaction (For Alkyl Fluorides): R-X + AgF/Hg2F2/CoF2 → R-F.

5. Preparation of Haloarenes

From Hydrocarbons:

Electrophilic Substitution: Benzene + X2 (with anhydrous FeCl3/AlCl3 in dark). It directs ortho and para if an activating group is present.

From Amines (Sandmeyer Reaction):

  1. Aniline reacts with NaNO2 + HCl at 0-5°C to form Benzene Diazonium Chloride (C6H5N2+Cl-).
  2. Diazonium salt reacts with Cu2X2/HX (where X = Cl, Br) to form Haloarene + N2 gas.
  3. Gatterman Reaction: Variation using Cu powder + HX instead of cuprous salts.

6. Physical Properties

  • Boiling Point: Higher than parent hydrocarbon due to polarity and stronger Van der Waals forces. Increases with molecular mass (R-I > R-Br > R-Cl > R-F). Note: Branching decreases boiling point (spherical shape reduces surface area).
  • Solubility: Haloalkanes are polar but insoluble in water because they cannot form hydrogen bonds with water nor break existing water-water H-bonds. Soluble in organic solvents.
  • Density: Bromo, iodo, and polychloro derivatives are heavier than water. Density order: I > Br > Cl.

7. Chemical Reactions

Nucleophilic Substitution

Nucleophile (electron rich) attacks the partial positive carbon, kicking off the halide leaving group. Haloarenes are extremely unreactive towards nucleophilic substitution compared to haloalkanes due to resonance stabilization of the C-X bond (partial double bond character).

Feature SN1 Mechanism SN2 Mechanism
Kinetics Unimolecular, Rate = k[R-X] Bimolecular, Rate = k[R-X][Nu-]
Steps Two steps (Carbocation intermediate) One step (Transition state)
Reactivity Order 3° > 2° > 1° > CH3X (Carbocation stability) CH3X > 1° > 2° > 3° (Steric hindrance)
Stereochemistry Racemization (Inversion + Retention) Inversion of Configuration (Walden Inversion)
Solvent Polar protic solvents (H2O, alcohols) Polar aprotic solvents (Acetone, DMSO)

Elimination Reactions

When heated with alcoholic KOH (or strong bases), haloalkanes undergo β-elimination (dehydrohalogenation) to form alkenes.

Saytzeff (Zaitsev) Rule: If multiple β-hydrogens exist, the hydrogen is removed from the β-carbon that results in the more highly substituted (more stable) alkene as the major product. (Eliminate from the carbon that already has fewer hydrogens).

Reaction with Metals

  • Grignard Reagent: Alkyl halides react with Magnesium in dry ether to form Alkyl Magnesium Halide (R-Mg-X). Highly reactive and acts as a strong nucleophile/base. Must be kept perfectly dry to avoid forming alkanes (RMgX + H2O → RH).
  • Wurtz Reaction: 2RX + 2Na(ether) → R-R. Useful for preparing symmetrical alkanes.

Reactions of Haloarenes

  • Electrophilic Substitution: Halogens are deactivating but ortho/para directing (due to resonance electron donation by lone pairs outweighing the inductive electron withdrawal).
  • Wurtz-Fittig Reaction: Alkyl Halide + Aryl Halide + 2Na (ether) → Alkylbenzene.
  • Fittig Reaction: 2 Aryl Halide + 2Na (ether) → Diphenyl (Biphenyl).

8. Polyhalogen Compounds

  • Dichloromethane (Methylene Chloride, CH2Cl2): Used as a solvent. Harms the human central nervous system.
  • Trichloromethane (Chloroform, CHCl3): Formerly an anesthetic. Slowly oxidizes in air/light to form poisonous phosgene gas (COCl2). Kept in dark, full bottles.
  • Triiodomethane (Iodoform, CHI3): Antiseptic properties due to the liberation of free iodine, not the compound itself.
  • Freons (CFCs): E.g., Freon-12 (CCl2F2). Harmful to the ozone layer (initiates radical chain reactions in the stratosphere).
  • p,p’-Dichlorodiphenyltrichloroethane (DDT): First chlorinated organic insecticide. Highly stable and fat-soluble; bioaccumulates in food chains causing toxicity.

9. Practice Mock Test

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Haloalkanes and Haloarenes

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