Hydrocarbons

🔹 Alkanes

🔸 1. Production of Alkanes

• (a) Hydrogenation of Alkenes
  • Reaction: Alkene + H₂ → Alkane
  • Conditions: Ni or Pt catalyst, heat (~150°C)
  • Type of Reaction: Addition
  • Example: Ethene + H₂ → Ethane (in presence of Ni catalyst)
• (b) Cracking of Alkanes
  • Thermal Cracking:
    • High temperature (600–700°C), no catalyst
    • Produces alkenes and short-chain alkanes
  • Catalytic Cracking:
    • Moderate temperature (~500°C)
    • Catalyst: Al₂O₃ (alumina) or zeolite
    • More selective for branched or cyclic hydrocarbons

🔸 2. Reactions of Alkanes

• (a) Combustion
  • Complete Combustion:
    • CH₄ + 2O₂ → CO₂ + 2H₂O
  • Incomplete Combustion (limited oxygen):
    • CH₄ + 1½O₂ → CO + 2H₂O
    • CH₄ + O₂ → C (soot) + 2H₂O
• (b) Free-radical Substitution
  • Example: Ethane + Cl₂ → Chloroethane + HCl
  • Requires UV light or sunlight
  • Chlorine or Bromine reacts in this process

🔸 3. Mechanism of Free-Radical Substitution

• Initiation:
  • Cl₂ → 2Cl• (UV light breaks bond homolytically)
• Propagation:
  • Cl• + CH₄ → CH₃• + HCl
  • CH₃• + Cl₂ → CH₃Cl + Cl•
• Termination:
  • Cl• + Cl• → Cl₂
  • CH₃• + Cl• → CH₃Cl
  • CH₃• + CH₃• → C₂H₆

🔸 4. Use of Cracking

• Converts long-chain, less useful hydrocarbons into shorter, more valuable alkanes and alkenes
• Important in petroleum refining to meet fuel demand

🔸 5. Unreactivity of Alkanes

• Reasons:
  • Strong C–H and C–C σ-bonds
  • Non-polar molecules: not attracted to electrophiles or nucleophiles
  • Hence, alkanes are generally inert under normal conditions

🔸 6. Environmental Consequences

• From Internal Combustion Engines:
  • CO (Carbon Monoxide): toxic, binds to hemoglobin
  • NOx (Oxides of Nitrogen): cause acid rain and respiratory problems
  • Unburnt Hydrocarbons: contribute to photochemical smog
• Catalytic Converters:
  • Convert harmful gases into less harmful:
    • CO → CO₂
    • NO → N₂
    • Hydrocarbons → CO₂ + H₂O

🔹 Alkenes

🔸 1. Production of Alkenes

• (a) Elimination of HX from Halogenoalkanes
  • Reagent: Ethanolic NaOH
  • Condition: Heat
  • Example: C₂H₅Br + NaOH (ethanol, heat) → C₂H₄ + NaBr + H₂O
• (b) Dehydration of Alcohols
  • Catalyst: Hot Al₂O₃ or conc. H₂SO₄
  • Example: C₂H₅OH → C₂H₄ + H₂O
• (c) Cracking of Alkanes
  • Similar to alkanes: produces alkenes + alkanes

🔸 2. Reactions of Alkenes

• (a) Electrophilic Addition
  • (i) Hydrogenation:
    • Alkene + H₂ → Alkane
    • Catalyst: Pt or Ni, heat
  • (ii) Steam Addition:
    • Alkene + H₂O (g) → Alcohol
    • Catalyst: H₃PO₄ on silica, 300°C, high pressure
  • (iii) Addition of Hydrogen Halide (HX):
    • Room temperature
    • Markovnikov’s rule applies
    • Example: Propene + HBr → 2-bromopropane
  • (iv) Addition of Halogen (X₂):
    • Test for unsaturation (e.g. Br₂ decolorized)
    • Example: Ethene + Br₂ → 1,2-dibromoethane
• (b) Oxidation by Cold Dilute Acidified KMnO₄:
  • Forms diols
  • Example: Ethene → Ethane-1,2-diol (purple KMnO₄ decolorized)
• (c) Oxidation by Hot Conc. Acidified KMnO₄:
  • Cleaves the C=C bond
  • Products depend on substitution:
    • CH₂ = CH₂ → CO₂
    • CH₂ = CR₂ → carboxylic acid, ketones
  • Used to locate alkene positions in molecules
• (d) Addition Polymerisation
  • Monomers: Alkenes like ethene, propene
  • Example:
    • n(CH₂=CH₂) → –[CH₂–CH₂]–ₙ (polyethene)

🔸 3. Test for Alkenes

• Add aqueous bromine (Br₂ water):
  • Orange/brown color decolorizes if C=C is present

🔸 4. Mechanism of Electrophilic Addition

• Example: Br₂ + Ethene
  • Step 1: Polarization of Br₂ → Br⁺ + Br⁻
  • Step 2: Br⁺ attacks π bond → carbocation formed
  • Step 3: Br⁻ adds to carbocation → product
• Example: HBr + Propene
  • Follows Markovnikov’s rule:
    • H⁺ adds to carbon with more H
    • More stable carbocation forms

🔸 5. Inductive Effects and Carbocation Stability

• Carbocation Stability Order:
  • Tertiary > Secondary > Primary
• Reason:
  • Alkyl groups donate electron density (positive inductive effect)
  • Stabilizes the carbocation
• Explains Markovnikov’s Rule:
  • Electrophile (H⁺) adds where most stable carbocation forms

✅ Summary Table:

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