State that the bonding in alkanes is single covalent and that alkanes are saturated hydrocarbons

Organic Chemistry – Alkanes

Learning Objective (Cambridge IGCSE 0620)

State that the bonding in alkanes consists only of single covalent (σ) bonds and that alkanes are saturated hydrocarbons.

Core Content (required for the syllabus)

Definition & General Formula

• Alkanes are hydrocarbons in which every carbon atom is bonded only by single σ‑bonds to other carbon atoms and to hydrogen atoms.
• Because no C=C or C≡C bonds are present, alkanes contain the maximum possible number of hydrogen atoms – they are therefore called saturated hydrocarbons.
• General molecular formula:
    CₙH₂ₙ₊₂  (where n = number of carbon atoms)

Bonding & Molecular Geometry

• Each carbon forms four single σ‑bonds (C–C or C–H).
• The four σ‑bonds adopt a tetrahedral arrangement with a bond angle of ≈ 109.5°.
• All C–H and C–C bonds are non‑polar; because the molecule is symmetrical the individual bond dipoles cancel, giving the alkane an overall non‑polar character.

Why Alkanes Are Saturated

“Saturated” means that every carbon already carries the maximum number of hydrogen atoms (four bonds per carbon). Adding more hydrogen would require breaking an existing C–C bond, which does not occur under normal conditions.

Physical Properties – Trends

Trend explanation (syllabus wording): As the carbon chain length increases, the surface area of the molecule increases. This enlarges the instantaneous dipoles that give rise to London dispersion forces, so more energy (higher temperature) is required to separate molecules – boiling and melting points therefore rise.

Reactivity – Combustion

• Alkanes burn in excess oxygen to give carbon dioxide and water.
• The reaction is exothermic (heat is released).
• General balanced equation:
    CₙH₂ₙ₊₂ + (n + ½) O₂ → n CO₂ + (n + 1) H₂O
• Example – combustion of propane:
    C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O (ΔH < 0)

Reactivity – Free‑Radical Substitution (chlorination)

• Alkanes react with halogens (Cl₂, Br₂) in the presence of ultraviolet (UV) light.
• UV light supplies the activation energy needed to break the Cl–Cl bond and generate radicals.
• Overall (core) equation:
    CₙH₂ₙ₊₂ + Cl₂ → CₙH₂ₙ₊₁Cl + HCl
• Example – chlorination of methane:
    CH₄ + Cl₂ →[hv] CH₃Cl + HCl
• Safety reminder (useful for practical work): Chlorine gas is toxic and UV light can damage eyes; carry out the reaction in a fume‑hood and wear appropriate protective equipment.

Naming Straight‑Chain Alkanes (IUPAC – Core)

1. Identify the longest continuous carbon chain – this gives the base name (meth‑, eth‑, prop‑, but‑, pent‑, hex‑, …).
2. Number the chain from the end nearest a substituent (if any).
3. For the core syllabus you only need to name the unbranched alkanes from methane to butane (or up to hexane if required by the teacher).
4. Combine the prefix with the suffix “‑ane”.
    Examples: CH₄ → methane; C₂H₆ → ethane; C₃H₈ → propane; C₄H₁₀ → butane.

Key Points to Remember (Core)

• All bonds in alkanes are single σ‑bonds (C–C and C–H).
• General formula: CₙH₂ₙ₊₂.
• Alkanes are saturated – they contain the maximum possible number of hydrogen atoms.
• Each carbon is tetrahedral (≈ 109.5°) → molecules are essentially non‑polar.
• Boiling and melting points increase with chain length because London dispersion forces become stronger.
• Reactions required by the syllabus:
  • Complete combustion – produces CO₂ and H₂O and releases heat.
  • Free‑radical halogenation – overall equation shown above; UV light provides the activation energy.

Supplementary Material (optional for extended work)

Radical‑Chain Mechanism (chlorination)

1. Initiation: Cl₂ →[hv] 2 Cl·
2. Propagation:
   • Cl· + R‑H → HCl + R·
   • R· + Cl₂ → R‑Cl + Cl·
3. Termination: Cl· + Cl· → Cl₂, R· + Cl· → R‑Cl, R· + R· → R‑R

Branch‑Chain Naming (beyond the core)

For extended study, include substituent naming (e.g., 2‑methylpropane = isobutane) and the rules for multiple substituents, prefixes (di‑, tri‑), and numbering to give the lowest‑set of locants.

Additional Examples of Alkanes (n = 5–10)

Check Your Understanding (Core)

1. Write the molecular formula for the alkane that contains five carbon atoms.
2. Explain why alkanes are called saturated hydrocarbons.
3. Identify the type of bond between the carbon atoms in propane.
4. Write the balanced combustion equation for butane and state whether the reaction is exothermic or endothermic.
5. Describe what happens to methane when it reacts with chlorine under UV light, including the role of UV radiation.

Suggested Diagram

Include a 3‑D structural diagram of methane showing the central carbon atom with four single σ‑bonds to hydrogen atoms arranged tetrahedrally (bond angle ≈ 109.5°).