Unit 14. Hydrocarbons : 8 teaching hours

Chemistry – Class 11

14.1 Saturated Hydrocarbons (Alkanes): Preparation from haloalkanes (Reduction and Wurtz reaction), Decarboxylation, Catalytic hydrogenation of alkene and alkyne ; Chemical properties: Substitution reactions (halogenation, nitration & sulphonation only), oxidation of ethane ; Unsaturated hydrocarbons (Alkenes & Alkynes) .

No MCQ questions available for this chapter.

Class 11 Chemistry Nepal: Hydrocarbons Notes

Unit 14: Hydrocarbons (8 Teaching Hours)

  1. Saturated Hydrocarbons (Alkanes)

    • Preparation of Alkanes
      • From Haloalkanes
        • Reduction: Haloalkanes reduced with Zn/HCl or H₂/Pd (e.g., CH₃Cl + H₂ → CH₄ + HCl).
        • Wurtz Reaction: Two haloalkanes with Na in dry ether (e.g., 2CH₃Br + 2Na → C₂H₆ + 2NaBr).
      • Decarboxylation
        • Sodium salt of carboxylic acid with soda lime (NaOH + CaO) (e.g., CH₃COONa + NaOH → CH₄ + Na₂CO₃).
      • Catalytic Hydrogenation of Alkene and Alkyne
        • Alkenes/alkynes with H₂ using Ni/Pt/Pd catalyst (e.g., C₂H₄ + H₂ → C₂H₆, C₂H₂ + 2H₂ → C₂H₆).
        • Figure 1: Catalytic Hydrogenation (Diagram showing hydrogenation of ethene to ethane).
    • Chemical Properties of Alkanes
      • Substitution Reactions
        • Halogenation: With halogens (e.g., Cl₂) in UV light (e.g., CH₄ + Cl₂ → CH₃Cl + HCl).
        • Nitration: With conc. HNO₃ at high temperature (e.g., C₂H₆ + HNO₃ → C₂H₅NO₂ + H₂O).
        • Sulphonation: With conc. H₂SO₄ or SO₃ (e.g., C₂H₆ + SO₃ → C₂H₅HSO₃).
        • Figure 2: Halogenation of Methane (Diagram showing substitution of CH₄ with Cl₂).
      • Oxidation of Ethane
        • Complete combustion: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.
        • Controlled oxidation: With MoO₃ catalyst, forms acetaldehyde or ethanol.

  2. Alkenes

    • Preparation of Alkenes
      • Dehydration of Alcohol
        • Alcohol with conc. H₂SO₄ at 140–180°C (e.g., C₂H₅OH → C₂H₄ + H₂O).
      • Dehydrohalogenation
        • Haloalkane with alcoholic KOH (e.g., CH₃CH₂Br + KOH(alc) → C₂H₄ + KBr + H₂O).
      • Catalytic Hydrogenation of Alkyne
        • Alkyne with H₂ and Lindlar’s catalyst (Pd/CaCO₃) (e.g., C₂H₂ + H₂ → C₂H₄).
        • Figure 3: Dehydration of Ethanol (Diagram showing dehydration of ethanol to ethene).
    • Chemical Properties of Alkenes
      • Addition Reactions
        • With HX (Markovnikov’s Addition): Adds H to carbon with more H atoms (e.g., C₃H₆ + HBr → CH₃CHBrCH₃).
        • Peroxide Effect: In presence of peroxide, anti-Markovnikov addition (e.g., C₃H₆ + HBr(peroxide) → CH₃CH₂CH₂Br).
        • With H₂O: Acid-catalyzed hydration forms alcohol (e.g., C₂H₄ + H₂O → C₂H₅OH).
        • With O₃: Ozonolysis forms carbonyl compounds (e.g., C₂H₄ + O₃ → 2HCHO).
        • With H₂SO₄: Forms alkyl hydrogen sulphate (e.g., C₂H₄ + H₂SO₄ → C₂H₅HSO₄).
        • Figure 4: Markovnikov’s Addition (Diagram showing addition of HBr to propene).

  3. Alkynes

    • Preparation of Alkynes
      • From Carbon and Hydrogen
        • Electric arc between carbon electrodes in H₂ atmosphere (e.g., 2C + H₂ → C₂H₂).
      • From 1,2-Dibromoethane
        • Dehydrohalogenation with alcoholic KOH (e.g., CH₂BrCH₂Br + 2KOH(alc) → C₂H₂ + 2KBr + 2H₂O).
      • From Chloroform/Iodoform
        • Reaction with Ag powder (e.g., 2CHI₃ + 6Ag → C₂H₂ + 6AgI).
        • Figure 5: Preparation of Ethyne (Diagram showing dehydrohalogenation of 1,2-dibromoethane).
    • Chemical Properties of Alkynes
      • Addition Reactions
        • With H₂: Forms alkene, then alkane (e.g., C₂H₂ + H₂ → C₂H₄, C₂H₄ + H₂ → C₂H₆).
        • With HX: Forms vinyl halide, then gem-dihalide (e.g., C₂H₂ + HBr → CH₂=CHBr, CH₂=CHBr + HBr → CH₃CHBr₂).
        • With H₂O: Forms aldehyde/ketone (e.g., C₂H₂ + H₂O → CH₃CHO with HgSO₄/H₂SO₄).
      • Acidic Nature
        • Action with Sodium: Forms sodium acetylide (e.g., C₂H₂ + 2Na → Na₂C₂ + H₂).
        • Action with Ammoniacal AgNO₃: Forms white precipitate (e.g., C₂H₂ + 2AgNO₃ → Ag₂C₂↓ + 2HNO₃).
        • Action with Ammoniacal Cu₂Cl₂: Forms red precipitate (e.g., C₂H₂ + Cu₂Cl₂ → Cu₂C₂↓ + 2HCl).
        • Figure 6: Acidic Nature of Ethyne (Diagram showing precipitate formation with AgNO₃).

  4. Test of Unsaturation (Ethene & Ethyne)

    • Bromine Water Test
      • Ethene/Ethyne decolorizes red-brown Br₂ water by addition (e.g., C₂H₄ + Br₂ → C₂H₄Br₂).
    • Baeyer’s Test
      • Ethene/Ethyne decolorizes alkaline KMnO₄ (purple) forming glycols (e.g., C₂H₄ + H₂O + [O] → C₂H₆O₂).
      • Figure 7: Bromine Water and Baeyer’s Test (Diagram showing decolorization of Br₂ and KMnO₄).

  5. Comparative Studies of Physical Properties of Alkane, Alkene, and Alkyne

    • Physical Properties
      • Boiling/Melting Points: Increase with molecular weight; alkanes > alkenes > alkynes (due to stronger van der Waals in alkanes).
      • Solubility: Insoluble in water, soluble in non-polar solvents; alkanes least reactive, alkynes most polar.
      • Density: Increases with carbon chain length; alkanes > alkenes > alkynes.
      • Figure 8: Physical Properties Comparison (Diagram comparing boiling points of C₂H₆, C₂H₄, C₂H₂).

  6. Kolbe’s Electrolysis Methods for Preparation of Alkane, Alkene, and Alkyne

    • Kolbe’s Electrolysis
      • Electrolysis of sodium/potassium salts of carboxylic acids.
      • Alkane: From sodium acetate (e.g., 2CH₃COONa + 2H₂O → C₂H₆ + 2CO₂ + 2NaOH + H₂).
      • Alkene: From salts of unsaturated acids (e.g., fumarate yields C₂H₄).
      • Alkyne: From salts of dicarboxylic acids (e.g., oxalate yields C₂H₂).
      • Figure 9: Kolbe’s Electrolysis (Diagram showing electrolysis setup for ethane preparation).
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