Organic Chemistry - Hydrocarbons

This topic covers the classification, structures, properties, and reactions of hydrocarbons, including alkanes and alkenes. It explains concepts such as homologous series, isomerism, cracking, polymerisation, and industrial applications like hydrogenation.

1. Homologous Series
  • πŸ”Ή Definition:

    πŸ”Ή A family of organic compounds with the same general formula, similar chemical properties, and a gradual change in physical properties.

  • πŸ”Ή Key Points:
    • πŸ”Ή Members differ by a –CH₂– group.
    • πŸ”Ή Examples: Alkanes, Alkenes, Alcohols.
2. Alkanes
  • πŸ”Ή Definition:

    πŸ”Ή Saturated hydrocarbons containing only single covalent bonds between carbon atoms.

  • πŸ”Ή General Formula:

    πŸ”Ή Cβ‚™Hβ‚‚β‚™β‚Šβ‚‚

  • πŸ”Ή Properties:
    • πŸ”Ή Non-polar molecules β€” insoluble in water, soluble in organic solvents.
    • πŸ”Ή Generally unreactive due to strong C–C and C–H bonds.
  • πŸ”Ή Structures & Naming (C₁–Cβ‚„):
    • πŸ”Ή Unbranched:
      • πŸ”Ή Methane (CHβ‚„)
      • πŸ”Ή Ethane (Cβ‚‚H₆)
      • πŸ”Ή Propane (C₃Hβ‚ˆ)
      • πŸ”Ή Butane (Cβ‚„H₁₀)
    • πŸ”Ή Branched Example:

      πŸ”Ή 2-methylpropane (Cβ‚„H₁₀)

  • πŸ”Ή Reactivity:
    • πŸ”Ή Combustion:
      • πŸ”Ή Complete:

        πŸ”Ή Alkane + Oβ‚‚ β†’ COβ‚‚ + Hβ‚‚O

      • πŸ”Ή Incomplete:

        πŸ”Ή Produces CO or carbon (soot).

    • πŸ”Ή Substitution with Chlorine:

      πŸ”Ή Occurs in UV light, e.g., CHβ‚„ + Clβ‚‚ β†’ CH₃Cl + HCl.

3. Isomerism
  • πŸ”Ή Definition:

    πŸ”Ή Compounds with the same molecular formula but different structural arrangements.

  • πŸ”Ή Example:

    πŸ”Ή Butane and 2-methylpropane (Cβ‚„H₁₀).

4. Alkenes
  • πŸ”Ή Definition:

    πŸ”Ή Unsaturated hydrocarbons containing at least one C=C double bond.

  • πŸ”Ή General Formula:

    πŸ”Ή Cβ‚™Hβ‚‚β‚™

  • πŸ”Ή Properties:
    • πŸ”Ή More reactive than alkanes due to double bond.
  • πŸ”Ή Structures & Naming (C₂–Cβ‚„):
    • πŸ”Ή Unbranched:
      • πŸ”Ή Ethene (CHβ‚‚=CHβ‚‚)
      • πŸ”Ή Propene (CHβ‚‚=CH–CH₃)
      • πŸ”Ή But-1-ene (CHβ‚‚=CH–CH₂–CH₃)
      • πŸ”Ή But-2-ene (CH₃–CH=CH–CH₃)
    • πŸ”Ή Branched Example:

      πŸ”Ή 2-methylpropene

  • πŸ”Ή Manufacture:
    • πŸ”Ή Cracking:
      • πŸ”Ή Definition:

        πŸ”Ή Breaking down large alkanes into smaller alkanes and alkenes.

      • πŸ”Ή Conditions:

        πŸ”Ή High temperature + catalyst.

      • πŸ”Ή Example:

        πŸ”Ή C₁₀Hβ‚‚β‚‚ β†’ Cβ‚ˆHβ‚β‚ˆ + Cβ‚‚Hβ‚„

      • πŸ”Ή By-product:

        πŸ”Ή Hydrogen gas.

  • πŸ”Ή Differences from Alkanes:
    • πŸ”Ή Bond Type:

      πŸ”Ή Single bonds only (alkanes) vs at least one double bond (alkenes).

    • πŸ”Ή Reactivity:

      πŸ”Ή Less reactive (alkanes) vs more reactive (alkenes).

    • πŸ”Ή Test:

      πŸ”Ή No reaction with bromine water (alkanes) vs decolourises bromine water (alkenes).

  • πŸ”Ή Reactions:
    • πŸ”Ή Combustion:

      πŸ”Ή Burns with a smokier flame than alkanes.

    • πŸ”Ή Polymerisation:

      πŸ”Ή Forms addition polymers.

    • πŸ”Ή Addition Reactions:
      • πŸ”Ή With hydrogen (hydrogenation).
      • πŸ”Ή With halogens (bromine water decolourises).
      • πŸ”Ή With steam (forms alcohols).
5. Polyunsaturated in Food
  • πŸ”Ή Definition:

    πŸ”Ή Contain multiple C=C double bonds in their hydrocarbon chains.

  • πŸ”Ή Source:

    πŸ”Ή Oils from plants and fish β€” healthier than saturated fats.

6. Hydrogenation in Margarine Production
  • πŸ”Ή Definition:

    πŸ”Ή Adding hydrogen to unsaturated vegetable oils in the presence of a nickel catalyst.

  • πŸ”Ή Effect:

    πŸ”Ή Converts C=C bonds to single bonds, turning oil into semi-solid margarine.

  • πŸ”Ή Benefits:

    πŸ”Ή Improves shelf life and texture.

  • πŸ”Ή Complete combustion of methane: CHβ‚„ + 2Oβ‚‚ β†’ COβ‚‚ + 2Hβ‚‚O
  • πŸ”Ή Complete combustion of ethane: 2Cβ‚‚H₆ + 7Oβ‚‚ β†’ 4COβ‚‚ + 6Hβ‚‚O
  • πŸ”Ή Incomplete combustion of propane: C₃Hβ‚ˆ + 3Oβ‚‚ β†’ 3CO + 4Hβ‚‚O
  • πŸ”Ή Substitution of methane with chlorine: CHβ‚„ + Clβ‚‚ β†’ CH₃Cl + HCl (UV light)
  • πŸ”Ή Cracking of decane: C₁₀Hβ‚‚β‚‚ β†’ Cβ‚ˆHβ‚β‚ˆ + Cβ‚‚Hβ‚„
  • πŸ”Ή Hydrogenation of ethene: Cβ‚‚Hβ‚„ + Hβ‚‚ β†’ Cβ‚‚H₆ (Ni catalyst)
  • πŸ”Ή Addition of bromine to ethene: Cβ‚‚Hβ‚„ + Brβ‚‚ β†’ Cβ‚‚Hβ‚„Brβ‚‚
  • πŸ”Ή Hydration of ethene to ethanol: Cβ‚‚Hβ‚„ + Hβ‚‚O β†’ Cβ‚‚Hβ‚…OH (acid catalyst)

  • ⚠️ Alkanes are completely unreactive (they can react under specific conditions like UV light).
  • ⚠️ All hydrocarbons react the same way with bromine water (only alkenes decolourise it without heating).
  • ⚠️ Cracking produces only alkenes (it produces both alkanes and alkenes, sometimes hydrogen).
  • ⚠️ Polyunsaturated fats are always solid at room temperature (they are usually oils).

  • πŸ‘‰ Memorise general formulas for alkanes (Cβ‚™Hβ‚‚β‚™β‚Šβ‚‚) and alkenes (Cβ‚™Hβ‚‚β‚™).
  • πŸ‘‰ Be able to identify isomers by drawing different structures for the same molecular formula.
  • πŸ‘‰ Know the bromine water test for unsaturation and how to describe it in words.
  • πŸ‘‰ Remember conditions for cracking: high temperature + catalyst.
  • πŸ‘‰ Draw and name branched hydrocarbons accurately according to IUPAC rules.
  • πŸ‘‰ Explain why alkenes are more reactive using the double bond as the reason.

πŸ“š Further Understanding