Class 11 Notes

Class 11 Chemistry Notes Chapter 6 (Hydrocarbons) – Chemistry Part-II Book

Philoid

Philoid

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Chemistry Part-II
Detailed Notes with MCQs of Chapter 6, Hydrocarbons. This is a fundamental chapter in organic chemistry, crucial not just for your Class 11 understanding but also forms the basis for many questions in competitive government exams. Pay close attention to the reactions, rules, and concepts.

Chapter 6: Hydrocarbons - Detailed Notes for Exam Preparation

1. Introduction

  • Definition: Organic compounds containing only carbon and hydrogen are called hydrocarbons. They are the simplest organic compounds and are considered the parent organic compounds from which others are derived.
  • Sources: Major sources include petroleum (crude oil), natural gas, coal, and plants.
  • Classification: Hydrocarbons are broadly classified into:
    • Acyclic or Open Chain Hydrocarbons (Aliphatic):
      • Saturated: Contain only carbon-carbon single bonds (Alkanes).
      • Unsaturated: Contain carbon-carbon multiple bonds (Alkenes - double bonds, Alkynes - triple bonds).
    • Cyclic or Closed Chain/Ring Hydrocarbons:
      • Alicyclic: Properties similar to aliphatic hydrocarbons (e.g., Cycloalkanes, Cycloalkenes).
      • Aromatic: Special properties, contain at least one benzene ring or follow Hückel's rule (e.g., Benzene, Toluene).

2. Alkanes (Paraffins)

  • General Formula: CnH2n+2 (where n = 1, 2, 3...)
  • Structure: Carbon atoms are sp³ hybridized, forming sigma (σ) bonds. Bond angle ≈ 109.5°. Freely rotating C-C single bonds.
  • Nomenclature: Follows IUPAC rules (Root word + 'ane').
  • Isomerism:
    • Chain Isomerism: Same molecular formula but different carbon chain skeletons (e.g., n-butane and isobutane).
    • Conformational Isomerism: Different spatial arrangements of atoms arising due to rotation around C-C single bonds. Studied using Newman and Sawhorse projections.
      • Ethane Example:
        • Staggered Conformation: Hydrogen atoms on adjacent carbons are maximum distance apart. Most stable (minimum repulsion). Dihedral angle = 60°.
        • Eclipsed Conformation: Hydrogen atoms on adjacent carbons are closest. Least stable (maximum repulsion). Dihedral angle = 0°.
        • Skew/Gauche: Any conformation between staggered and eclipsed.
  • Preparation Methods:
    • Hydrogenation of Alkenes/Alkynes (Sabatier-Senderens Reaction): Alkene/Alkyne + H₂ (in presence of Ni, Pt, or Pd catalyst) → Alkane.
    • Reduction of Alkyl Halides:
      • R-X + H₂ (Ni/Pt/Pd) → R-H + HX
      • R-X + Zn + H⁺ → R-H + ZnX₂
      • R-X + Zn-Cu couple + C₂H₅OH → R-H
    • Wurtz Reaction: 2R-X + 2Na (dry ether) → R-R + 2NaX (Used to prepare alkanes with an even number of carbon atoms; good for symmetrical alkanes).
    • Decarboxylation of Carboxylic Acids: R-COONa + NaOH (CaO, heat - Soda lime) → R-H + Na₂CO₃ (Produces alkane with one carbon less).
    • Kolbe's Electrolytic Method: 2R-COONa (aq) --Electrolysis--> R-R (at anode) + 2CO₂ + H₂ (at cathode) + 2NaOH (Produces symmetrical alkanes with even number of carbons).
  • Physical Properties:
    • Non-polar, insoluble in water, soluble in organic solvents.
    • Boiling Point: Increases with increasing molar mass. Branched alkanes have lower boiling points than corresponding straight-chain alkanes (less surface area).
    • First four members (C1-C4) are gases, C5-C17 are liquids, C18 onwards are solids at room temperature.
  • Chemical Properties: Generally unreactive due to strong C-C and C-H sigma bonds. Undergo substitution reactions.
    • Halogenation (Free Radical Substitution): Alkane + X₂ (UV light or high temp) → Alkyl Halide + HX.
      • Mechanism: Involves three steps:
        1. Initiation: Homolytic cleavage of halogen molecule (e.g., Cl₂ → 2Cl•).
        2. Propagation: Cl• + CH₄ → •CH₃ + HCl; •CH₃ + Cl₂ → CH₃Cl + Cl• (Chain reaction).
        3. Termination: Radicals combine (e.g., Cl• + Cl• → Cl₂; •CH₃ + •CH₃ → C₂H₆; •CH₃ + Cl• → CH₃Cl).
      • Reactivity order of halogens: F₂ > Cl₂ > Br₂ > I₂. Reactivity order of H atoms: 3° > 2° > 1°.
    • Combustion: Alkane + O₂ (excess) → CO₂ + H₂O + Heat (Exothermic, used as fuel).
    • Controlled Oxidation: Depending on catalyst and conditions, alkanes yield alcohols, aldehydes, or carboxylic acids.
      • CH₄ + O₂ (Cu/523K/100atm) → CH₃OH (Methanol)
      • CH₄ + O₂ (Mo₂O₃/heat) → HCHO (Methanal)
    • Isomerisation: n-Alkane (Anhydrous AlCl₃/HCl, heat) → Branched Alkane.
    • Aromatization: n-Alkanes (with ≥ 6 carbons) (Cr₂O₃ or V₂O₅ or Mo₂O₃ / 773K / 10-20 atm) → Benzene or its derivatives. (e.g., n-hexane → benzene).
    • Reaction with Steam: CH₄ + H₂O (Ni/1273K) → CO + 3H₂ (Synthesis gas).
    • Pyrolysis (Cracking): Decomposition of higher alkanes into lower alkanes, alkenes etc., at high temperature. Free radical mechanism.

3. Alkenes (Olefins)

  • General Formula: CnH2n (where n = 2, 3...)
  • Structure: Contain at least one C=C double bond. Carbon atoms in the double bond are sp² hybridized (trigonal planar geometry, bond angle ≈ 120°). Double bond consists of one strong sigma (σ) bond and one weak pi (π) bond. Restricted rotation around C=C bond.
  • Nomenclature: Follows IUPAC rules (Root word + 'ene'). Position of double bond indicated.
  • Isomerism:
    • Chain and Position Isomerism: Similar to alkanes.
    • Geometrical (cis-trans) Isomerism: Arises due to restricted rotation around C=C bond. Requires each doubly bonded carbon to be attached to two different groups.
      • cis-isomer: Similar groups on the same side of the double bond.
      • trans-isomer: Similar groups on opposite sides of the double bond. (Generally more stable due to less steric hindrance).
  • Preparation Methods:
    • From Alkynes (Partial Reduction):
      • Alkyne + H₂ (Pd/C, quinoline or sulphur - Lindlar's catalyst) → cis-Alkene.
      • Alkyne + Na in liquid NH₃ (Birch Reduction) → trans-Alkene.
    • From Alkyl Halides (Dehydrohalogenation): Alkyl Halide + Alcoholic KOH (heat) → Alkene + KX + H₂O. Follows Saytzeff's Rule (Zaitsev's Rule): In dehydrohalogenation, the preferred product is the alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms (more substituted alkene is more stable).
    • From Vicinal Dihalides (Dehalogenation): CH₂X-CH₂X + Zn (Methanol, heat) → CH₂=CH₂ + ZnX₂.
    • From Alcohols (Acid Catalysed Dehydration): Alcohol + Conc. H₂SO₄ (or H₃PO₄, heat) → Alkene + H₂O. Follows Saytzeff's rule. Ease of dehydration: 3° > 2° > 1° alcohol.
  • Physical Properties: Similar to alkanes, but slightly more soluble in water due to π-electron cloud. Boiling points increase with mass. Cis isomers generally have higher boiling points than trans isomers (due to polarity).
  • Chemical Properties: More reactive than alkanes due to the presence of the weak π-bond. Undergo electrophilic addition reactions.
    • Addition of Dihydrogen (H₂): Alkene + H₂ (Ni/Pt/Pd) → Alkane.
    • Addition of Halogens (X₂): Alkene + X₂ (in CCl₄) → Vicinal Dihalide. (Test for unsaturation: Reddish-brown colour of Br₂ in CCl₄ is discharged).
    • Addition of Hydrogen Halides (HX): Alkene + HX → Alkyl Halide. Follows Markovnikov's Rule: The negative part of the addendum (adding molecule, e.g., X⁻ in HX) gets attached to that carbon atom of the double bond which possesses lesser number of hydrogen atoms.
      • Mechanism: Involves formation of a more stable carbocation intermediate (3° > 2° > 1°).
      • Anti-Markovnikov Addition (Peroxide Effect/Kharasch Effect): In the presence of peroxide (e.g., Benzoyl peroxide), addition of HBr (only HBr) occurs contrary to Markovnikov's rule. Free radical mechanism.
    • Addition of Sulphuric Acid: Alkene + Conc. H₂SO₄ → Alkyl Hydrogen Sulphate. Follows Markovnikov's rule.
    • Addition of Water (Hydration): Alkene + H₂O (H⁺, heat) → Alcohol. Follows Markovnikov's rule.
    • Oxidation:
      • With Cold, Dilute, Alkaline KMnO₄ (Baeyer's Reagent): Alkene → Vicinal Diol (Glycol). Purple colour of KMnO₄ is discharged (Test for unsaturation).
      • With Acidic KMnO₄ or K₂Cr₂O₇ (Strong Oxidation): Cleavage of double bond occurs, forming ketones and/or carboxylic acids depending on the alkene structure.
    • Ozonolysis: Alkene + O₃ (in CCl₄ or CH₂Cl₂) → Ozonide --(Zn/H₂O)--> Aldehydes and/or Ketones. Used to locate the position of the double bond.
    • Polymerisation: Alkenes undergo addition polymerisation at high temperature/pressure with catalysts (e.g., Ethene → Polythene).

4. Alkynes

  • General Formula: CnH2n-2 (where n = 2, 3...)
  • Structure: Contain at least one C≡C triple bond. Carbon atoms in the triple bond are sp hybridized (linear geometry, bond angle = 180°). Triple bond consists of one strong sigma (σ) bond and two weak pi (π) bonds.
  • Nomenclature: Follows IUPAC rules (Root word + 'yne'). Position of triple bond indicated.
  • Isomerism: Chain and Position isomerism. No geometrical isomerism around the triple bond due to linear structure.
  • Preparation Methods:
    • From Calcium Carbide: CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂ (Ethyne/Acetylene).
    • From Vicinal Dihalides: CHX₂-CHX₂ + 2 Alcoholic KOH (heat) → Alkyne + 2KX + 2H₂O. (Stronger base like NaNH₂ may be needed for terminal alkynes).
  • Physical Properties: Similar to alkanes and alkenes. First three members are gases, next eight are liquids, higher ones are solids. Insoluble in water, soluble in organic solvents.
  • Acidity of Alkynes: Hydrogen atoms attached to triply bonded carbons (terminal alkynes) are acidic in nature (due to 50% s-character of sp hybrid orbital, making it more electronegative). They react with strong bases like NaNH₂ or metals like Na.
    • HC≡CH + NaNH₂ → HC≡C⁻Na⁺ + NH₃ (Sodium acetylide)
    • This property distinguishes terminal alkynes from non-terminal alkynes and alkenes/alkanes.
  • Chemical Properties: Undergo electrophilic addition reactions, similar to alkenes, but can add two molecules of the reagent.
    • Addition of Dihydrogen (H₂): Alkyne + H₂ (Ni/Pt/Pd) → Alkene → Alkane. (Can be stopped at alkene stage using Lindlar's catalyst or Na/liq NH₃).
    • Addition of Halogens (X₂): Alkyne + X₂ → Dihaloalkene + X₂ → Tetrahaloalkane.
    • Addition of Hydrogen Halides (HX): Alkyne + HX → Haloalkene + HX → Geminal Dihalide (Halogens on the same carbon). Follows Markovnikov's rule.
    • Addition of Water (Hydration - Kucherov's Reaction): Alkyne + H₂O (dil. H₂SO₄, HgSO₄) → Enol (unstable) → Carbonyl Compound (Aldehyde or Ketone). Ethyne gives ethanal; other alkynes give ketones (Markovnikov addition).
    • Polymerisation:
      • Linear Polymerisation: Ethyne → Polyacetylene.
      • Cyclic Polymerisation: Ethyne (Red hot iron tube, 873K) → Benzene.

5. Aromatic Hydrocarbons (Arenes)

  • Introduction: Cyclic hydrocarbons containing alternating double and single bonds with special stability (aromaticity). Benzene (C₆H₆) is the parent compound.
  • Nomenclature: Based on benzene ring. Monosubstituted (Toluene, Phenol), Disubstituted (ortho-, meta-, para- isomers).
  • Structure of Benzene:
    • Kekulé Structures: Proposed two resonance structures with alternating double bonds. Explains equivalence of all C-C bonds.
    • Resonance Hybrid: Actual structure is a hybrid of contributing structures. All C-C bond lengths are equal (139 pm), intermediate between C-C single (154 pm) and C=C double (134 pm) bonds.
    • Orbital Picture: Each carbon is sp² hybridized. Forms sigma framework (planar hexagon). Each carbon has one unhybridized p-orbital perpendicular to the plane, containing one electron. These p-orbitals overlap sideways above and below the ring, forming delocalized pi electron clouds.
  • Aromaticity (Hückel's Rule): For a compound to be aromatic, it must:
    1. Be planar.
    2. Be cyclic.
    3. Have complete delocalization of pi electrons in the ring.
    4. Possess (4n + 2) pi electrons, where n is an integer (0, 1, 2...). Benzene has 6 pi electrons (n=1).
  • Preparation of Benzene:
    • Cyclic Polymerisation of Ethyne: (Red hot iron tube, 873K).
    • Decarboxylation of Sodium Benzoate: C₆H₅COONa + NaOH (CaO, heat) → C₆H₆ + Na₂CO₃.
    • Reduction of Phenol: C₆H₅OH + Zn (dust, heat) → C₆H₆ + ZnO.
  • Physical Properties: Non-polar, immiscible with water, soluble in organic solvents. Characteristic odour.
  • Chemical Properties: Despite unsaturation, benzene is highly stable due to resonance/delocalization. It primarily undergoes Electrophilic Aromatic Substitution (EAS) reactions, where the aromatic character is retained.
    • Mechanism of EAS:
      1. Generation of electrophile (E⁺).
      2. Attack of electrophile on benzene ring to form a resonance-stabilized carbocation intermediate (arenium ion or sigma complex).
      3. Loss of proton (H⁺) from the sigma complex to restore aromaticity.
    • Nitration: C₆H₆ + Conc. HNO₃ + Conc. H₂SO₄ (Nitrating mixture, <333K) → C₆H₅NO₂ (Nitrobenzene). Electrophile: NO₂⁺ (nitronium ion).
    • Halogenation: C₆H₆ + X₂ (Anhydrous AlX₃ or FeX₃ - Lewis acid catalyst) → C₆H₅X (Halobenzene). Electrophile: X⁺ (halonium ion).
    • Sulphonation: C₆H₆ + Conc. H₂SO₄ (or Oleum, heat) → C₆H₅SO₃H (Benzenesulphonic acid). Electrophile: SO₃. Reversible reaction.
    • Friedel-Crafts Alkylation: C₆H₆ + R-X (Anhydrous AlCl₃) → C₆H₅R (Alkylbenzene). Electrophile: R⁺ (carbocation). Limitations: Polyalkylation, rearrangement of carbocation possible. Ring must not be strongly deactivated.
    • Friedel-Crafts Acylation: C₆H₆ + R-COCl or (RCO)₂O (Anhydrous AlCl₃) → C₆H₅COR (Acylbenzene/Ketone). Electrophile: RCO⁺ (acylium ion). Advantage: No polyacylation or rearrangement. Deactivating, so only one acyl group adds.
    • Addition Reactions (Under Drastic Conditions):
      • Hydrogenation: C₆H₆ + 3H₂ (Ni/Pt, high temp/pressure) → C₆H₁₂ (Cyclohexane).
      • Halogenation: C₆H₆ + 3Cl₂ (UV light, 500K) → C₆H₆Cl₆ (Benzene hexachloride, BHC - an insecticide).
    • Combustion: C₆H₆ + O₂ → CO₂ + H₂O. Burns with a sooty flame due to high carbon content.
  • Directive Influence of Functional Groups in Monosubstituted Benzene: The group already present directs the incoming electrophile to specific positions (ortho, meta, para).
    • Activating Groups (Electron Donating): Increase electron density in the ring, make EAS faster. Direct incoming group to ortho- and para- positions. Examples: -OH, -NH₂, -NHR, -NR₂, -OCH₃, -CH₃, -C₂H₅ etc. (Halogens are weakly deactivating but o,p-directing due to opposing inductive and resonance effects).
    • Deactivating Groups (Electron Withdrawing): Decrease electron density in the ring, make EAS slower. Direct incoming group to meta- position. Examples: -NO₂, -CN, -SO₃H, -CHO, -COR, -COOH, -COOR, -NR₃⁺ etc.
  • Carcinogenicity and Toxicity: Benzene and polynuclear hydrocarbons (containing more than two fused benzene rings, e.g., anthracene, phenanthrene) are toxic and carcinogenic (cancer-causing).

Multiple Choice Questions (MCQs)

  1. Which of the following reactions is used to prepare alkanes with an even number of carbon atoms by coupling alkyl halides with sodium metal in dry ether?
    (a) Kolbe's electrolysis
    (b) Wurtz reaction
    (c) Decarboxylation
    (d) Sabatier-Senderens reaction

  2. The addition of HBr to propene in the presence of benzoyl peroxide follows:
    (a) Markovnikov's rule
    (b) Saytzeff's rule
    (c) Anti-Markovnikov's rule (Peroxide effect)
    (d) Hückel's rule

  3. Which catalyst is used to convert ethyne into benzene by cyclic polymerisation?
    (a) Ni/Pt/Pd
    (b) Red hot iron tube at 873 K
    (c) Anhydrous AlCl₃
    (d) Lindlar's catalyst

  4. Identify the most stable conformation of n-butane:
    (a) Eclipsed
    (b) Partially eclipsed
    (c) Gauche
    (d) Anti (fully staggered)

  5. Ozonolysis of 2-methylbut-2-ene (followed by Zn/H₂O treatment) yields:
    (a) Propanone and Ethanal
    (b) Propanal and Methanal
    (c) Propanone and Methanal
    (d) Butan-2-one and Methanal

  6. Which of the following compounds is aromatic according to Hückel's rule?
    (a) Cyclobutadiene
    (b) Cyclooctatetraene
    (c) Benzene
    (d) Cyclohexene

  7. In the Friedel-Crafts acylation of benzene, the electrophile is:
    (a) R⁺
    (b) Cl⁺
    (c) RCO⁺
    (d) AlCl₄⁻

  8. Terminal alkynes are acidic because the hydrogen is attached to:
    (a) sp³ hybridized carbon
    (b) sp² hybridized carbon
    (c) sp hybridized carbon
    (d) A carbon atom with high electron density

  9. Dehydration of ethanol with concentrated H₂SO₄ at 443 K yields:
    (a) Ethane
    (b) Ethene
    (c) Ethoxyethane (Diethyl ether)
    (d) Ethyne

  10. The nitration of toluene primarily yields:
    (a) m-Nitrotoluene
    (b) o-Nitrotoluene and p-Nitrotoluene
    (c) A mixture of o-, m-, and p-Nitrotoluene in equal amounts
    (d) Benzene

Answer Key for MCQs:

  1. (b)
  2. (c)
  3. (b)
  4. (d)
  5. (a) [Structure: CH₃-C(CH₃)=CH-CH₃. Ozonolysis breaks the double bond. Left part C(CH₃)₂=O (Propanone/Acetone), Right part CH(CH₃)=O (Ethanal/Acetaldehyde)] - Correction: The question stated 2-methylbut-2-ene. Structure is CH₃-C(CH₃)=CH-CH₃. Ozonolysis yields CH₃-C(=O)-CH₃ (Propanone) and O=CH-CH₃ (Ethanal). So (a) is correct.
  6. (c)
  7. (c)
  8. (c)
  9. (b)
  10. (b) [Methyl group (-CH₃) is an ortho-, para- directing group]

Remember to thoroughly revise these concepts, especially the reaction mechanisms and named reactions/rules, as they are frequently tested. Good luck with your preparation!

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