Skip to content
SmartScience
All Chemistry chapters

ICSE Class 10 · Chemistry · Chapter 11

Organic Chemistry – Hydrocarbons

8 topics · 23 questions

Smart summary

Everything you need, in order

Introduction to Organic Chemistry

  • Organic Chemistry: Study of carbon compounds (except CO, CO₂, carbonates, cyanides).
  • Why Special Study: Carbon forms millions of compounds due to catenation (self-linking), tetravalency, and ability to form multiple bonds.
  • Catenation: Property of carbon to form long chains by bonding with other carbon atoms. C-C bonds are strong.
  • Tetravalency: Carbon has valency 4, forms four covalent bonds (sp³, sp², sp hybridization).
  • Hydrocarbons: Organic compounds containing only carbon and hydrogen. General formula: CₓHᵧ.
  • Classification: Saturated (alkanes - single bonds) and Unsaturated (alkenes - double bonds, alkynes - triple bonds).

Alkanes (Saturated Hydrocarbons)

  • General Formula: CₙH₂ₙ₊₂. Single bonds only (saturated).
  • Homologous Series: CH₄ (methane), C₂H₆ (ethane), C₃H₈ (propane), C₄H₁₀ (butane), C₅H₁₂ (pentane).
  • Nomenclature: -ane suffix. Meth- (1C), Eth- (2C), Prop- (3C), But- (4C), Pent- (5C).
  • Preparation of Alkanes:
  • - Methane (CH₄): From sodium ethanoate (sodium acetate) - CH₃COONa + NaOH → CH₄ + Na₂CO₃ (soda lime, heat).
  • - Ethane (C₂H₆): From sodium propanoate (sodium propionate) - C₂H₅COONa + NaOH → C₂H₆ + Na₂CO₃ (soda lime, heat).
  • - From alkyl halides: CH₃I + 2[H] → CH₄ + HI (iodomethane reduction), C₂H₅Br + 2[H] → C₂H₆ + HBr (bromoethane reduction).
  • Properties: Non-polar, insoluble in water, undergo substitution reactions.
  • Combustion: CH₄ + 2O₂ → CO₂ + 2H₂O + Heat (complete). CH₄ + O₂ → CO + H₂O + Heat (incomplete, toxic CO).
  • Substitution: CH₄ + Cl₂ → CH₃Cl + HCl (in sunlight, chloromethane). Further substitution gives CH₂Cl₂, CHCl₃, CCl₄.
  • Cracking/Pyrolysis: Large alkanes break into smaller ones. C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (high temperature, catalyst).
  • Controlled Oxidation: Incomplete oxidation of alkanes gives alcohols, aldehydes, carboxylic acids.

Alkenes (Unsaturated Hydrocarbons)

  • General Formula: CₙH₂ₙ. Contain C=C double bond (unsaturated).
  • Homologous Series: C₂H₄ (ethene), C₃H₆ (propene), C₄H₈ (butene), C₅H₁₀ (pentene).
  • Nomenclature: -ene suffix. Ethene (ethylene), propene (propylene).
  • Preparation of Alkenes:
  • - Ethene (C₂H₄): From ethyl alcohol (ethanol) - C₂H₅OH → C₂H₄ + H₂O (dehydration with conc. H₂SO₄ at 170°C).
  • - Dehydrohalogenation: CH₃CH₂Cl + KOH (alc.) → CH₂=CH₂ + KCl + H₂O (from alkyl halides).
  • Test for Unsaturation: Decolorize bromine water (Br₂). C₂H₄ + Br₂ → C₂H₄Br₂ (colorless). Turns purple KMnO₄ colorless.
  • Properties: Unsaturated, undergo addition reactions due to reactive double bond.
  • Combustion: C₂H₄ + 3O₂ → 2CO₂ + 2H₂O + Heat (complete combustion).
  • Hydrogenation (Addition): C₂H₄ + H₂ → C₂H₆ (Ni catalyst, 200°C, converts alkene to alkane).
  • Halogenation (Addition): C₂H₄ + Cl₂ → C₂H₄Cl₂ (1,2-dichloroethane), C₂H₄ + Br₂ → C₂H₄Br₂ (colorless from orange).
  • Addition Reactions: H₂, Cl₂, Br₂, HCl, H₂O can add across double bond following Markovnikov's rule.
  • Polymerization: nC₂H₄ → (C₂H₄)ₙ (polyethylene/polythene). Used in plastics, bottles, bags.

Alkynes (Unsaturated Hydrocarbons)

  • General Formula: CₙH₂ₙ₋₂. Contain C≡C triple bond (unsaturated).
  • Homologous Series: C₂H₂ (ethyne), C₃H₄ (propyne), C₄H₆ (butyne).
  • Nomenclature: -yne suffix. Ethyne (acetylene), propyne.
  • Preparation of Alkynes:
  • - Ethyne (C₂H₂): From calcium carbide - CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂↑ (laboratory preparation).
  • - From 1,2-dibromoethane: CH₂Br-CH₂Br + 2KOH (alc.) → HC≡CH + 2KBr + 2H₂O (dehydrohalogenation).
  • Properties: Unsaturated, more reactive than alkenes, decolorizes Br₂ water and KMnO₄. Burns with sooty flame.
  • Combustion: 2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O + Heat (complete). Produces more soot due to high C:H ratio.
  • Addition Reactions:
  • - Hydrogenation: C₂H₂ + H₂ → C₂H₄ (ethene, Ni catalyst), C₂H₄ + H₂ → C₂H₆ (ethane, more catalyst needed).
  • - Halogenation: C₂H₂ + Cl₂ → C₂H₂Cl₂ (1,1,2,2-tetrachloroethane), C₂H₂ + 2Br₂ → C₂H₂Br₄ (tetrabromoethane).
  • - Can add two molecules: First to alkene, then to alkane due to reactive triple bond.
  • Uses: Oxyacetylene welding (burns at 3000°C with O₂), manufacturing polymers, organic synthesis.

Alcohols (Ethanol)

  • General Formula: R-OH (where R is alkyl group).
  • Ethanol (C₂H₅OH): Common alcohol, also called ethyl alcohol.
  • Preparation:
  • - Laboratory method: From ethyl bromide - C₂H₅Br + KOH (aq) → C₂H₅OH + KBr (hydrolysis of alkyl halide).
  • - Industrial method: Fermentation of sugars - C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (yeast catalyst, anaerobic).
  • Physical Properties:
  • - Colorless liquid, boiling point 78°C, miscible with water, characteristic odor.
  • - Density: 0.789 g/cm³, neutral to litmus, poisonous in large quantities.
  • Chemical Properties:
  • - Combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + Heat (burns with blue flame).
  • - Action with sodium: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂↑ (forms sodium ethoxide, hydrogen gas).
  • - Esterification: C₂H₅OH + CH₃COOH → CH₃COOC₂H₅ + H₂O (forms ethyl acetate, conc. H₂SO₄ catalyst).
  • - Dehydration: C₂H₅OH → C₂H₄ + H₂O (conc. H₂SO₄, 170°C, forms ethene).
  • Uses: Solvent, fuel (gasohol), alcoholic beverages, antiseptics, manufacturing chemicals.
  • Denatured Alcohol: Ethanol made unfit for drinking by adding methanol or pyridine (industrial use).

Carboxylic Acids

  • General Formula: R-COOH (where R is alkyl or H).
  • Examples: Methanoic acid (HCOOH), Ethanoic acid (CH₃COOH, acetic acid).
  • Physical Properties:
  • - Sour taste, characteristic odor, higher boiling points than alcohols.
  • - Soluble in water (lower members), form hydrogen bonds, act as weak acids.
  • Chemical Properties:
  • - Acidic nature: React with bases - CH₃COOH + NaOH → CH₃COONa + H₂O (sodium acetate).
  • - Esterification: CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O (ethyl acetate, conc. H₂SO₄ catalyst).
  • - Reaction with carbonates: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂↑.
  • - Reduction: To alcohols using LiAlH₄ or catalytic hydrogenation.
  • Uses: Food preservatives (acetic acid in vinegar), manufacturing plastics, dyes, pharmaceuticals.

Isomerism and Structural Formulas

  • Isomerism: Compounds with same molecular formula but different structural formulas.
  • Structural Isomers: Different arrangement of atoms. Example: C₄H₁₀ has n-butane and isobutane.
  • n-Butane: CH₃-CH₂-CH₂-CH₃ (straight chain). Boiling point 0°C.
  • Isobutane: (CH₃)₃CH (branched chain). Boiling point -12°C.
  • Position Isomerism: In alkenes - C₄H₈ can be but-1-ene or but-2-ene (double bond position differs).
  • Importance: Isomers have different physical and chemical properties despite same formula.

Homologous Series and Important Reactions

  • Homologous Series: Group of organic compounds with same functional group, similar properties, differing by -CH₂- unit.
  • Characteristics: Same general formula, gradual change in properties, similar chemical reactions.
  • Difference: Each successive member differs by 14 u (CH₂ = 12 + 2 = 14).
  • Functional Groups: -OH (alcohols), -COOH (carboxylic acids), C=C (alkenes), C≡C (alkynes).
  • Cracking: Breaking large hydrocarbons into smaller ones. C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (high temp, catalyst).
  • Substitution: Saturated hydrocarbons (alkanes). Addition: Unsaturated hydrocarbons (alkenes, alkynes).
  • Laboratory Tests: Br₂ water test distinguishes saturated (no reaction) from unsaturated (decolorization).

Formulas

Keep these ready

Formula 01

Alkanes: CₙH₂ₙ₊₂

General formula for saturated hydrocarbons

Formula 02

Alkenes: CₙH₂ₙ

General formula for hydrocarbons with one double bond

Formula 03

Alkynes: CₙH₂ₙ₋₂

General formula for hydrocarbons with one triple bond

Quiz

Test your knowledge

Multiple choice

  1. Question 1 of 22Knowledge
    The general formula for alkanes is:
  2. Question 2 of 22Knowledge
    The property of carbon to form long chains is called:
  3. Question 3 of 22Knowledge
    Which hydrocarbon is known as acetylene?
  4. Question 4 of 22Knowledge
    The test for unsaturation in hydrocarbons uses:
  5. Question 5 of 22Knowledge
    Ethyne is prepared from:
  6. Question 6 of 22Application
    The molecular formula C₄H₁₀ represents:
  7. Question 7 of 22Knowledge
    Alkanes undergo which type of reaction?
  8. Question 8 of 22Knowledge
    The suffix used for alkenes is:
  9. Question 9 of 22Assertion-Reasoning
    Assertion (A): Methane is the simplest hydrocarbon. Reason (R): It has one carbon atom.
  10. Question 10 of 22Knowledge
    Compounds with same molecular formula but different structures are called:
  11. Question 11 of 22Knowledge
    The general formula CₙH₂ₙ represents:
  12. Question 12 of 22Knowledge
    Polythene is a polymer of:
  13. Question 13 of 22Knowledge
    In a homologous series, successive members differ by:
  14. Question 14 of 22Comprehension
    Complete combustion of methane produces:
  15. Question 15 of 22Knowledge
    The reaction CH₄ + Cl₂ → CH₃Cl + HCl is an example of:
  16. Question 16 of 22Knowledge
    Ethene is prepared by dehydration of:
  17. Question 17 of 22Knowledge
    The reaction C₂H₄ + H₂ → C₂H₆ requires:
  18. Question 18 of 22Knowledge
    Ethyne is prepared from calcium carbide by reaction with:
  19. Question 19 of 22Knowledge
    The reaction C₂H₅OH + CH₃COOH → CH₃COOC₂H₅ + H₂O is called:
  20. Question 20 of 22Knowledge
    Which gas is evolved when ethanoic acid reacts with sodium carbonate?
  21. Question 21 of 22Assertion-Reasoning
    Assertion (A): Alkanes are more reactive than alkenes. Reason (R): Alkanes contain only single bonds while alkenes contain double bonds.
  22. Question 22 of 22Application
    The cracking of decane (C₁₀H₂₂) produces hexane and butene. Write the balanced equation for this reaction.

Practice numericals

  1. Problem 1 of 1Application

0 of 23 answered