alkanes

• Alkanes and cycloalkanes


  Saturated hydrocarbons
• Shape around carbon atom in saturated hydrocarbons

  • Tetrahedral
  • Bond angle 109.5°
• Sigma (σ) bond


  Single C-C bond formed by overlap of sp3 orbitals
• Rotation can occur around a sigma bond
• Reactivity of alkanes


  Low due to high bond enthalpies of C-C and C-H bonds and low polarity of σ-bonds
• Alkanes readily burn in the presence of oxygen
• Complete combustion of alkanes

  C8H18(g) + 12.5 O2(g) → 8CO2(g) + 9 H2O(l)
• Incomplete combustion of alkanes

  1. CH4(g) + 3/2 O2(g) → CO(g) + 2 H2O(l)

  2. CH4(g) + O2(g) → C(s) + 2 H2O(l)
• Carbon (soot) from incomplete combustion can cause global dimming
• Carbon monoxide is highly toxic as it forms a stronger bond with haemoglobin than oxygen
• Catalytic cracking


  1. Turns straight chain alkanes into branched and cyclic alkanes and aromatic hydrocarbons

  2. Used for making motor fuels
• Substitution reactions of alkanes with chlorine/bromine

  Alkane + Cl2/Br2 (UV light) → Haloalkane + HCl/HBr
• Mechanism of alkane substitution with chlorine

  1. Initiation: Cl2 → 2Cl·

  2. Propagation: CH4 + Cl· → CH3· + HCl, CH3· + Cl2 → CH3Cl + Cl·

  3. Termination: CH3· + Cl· → CH3Cl, CH3· + CH3· → CH3CH3
• Excess Cl2 can lead to further substitution to form CH2Cl2, CHCl3, CCl4
• Overall reaction equations


  • CH4 + 4 Cl2 → CCl4 + 4 HCl
  • CH3F + 3 Cl2 → CFCl3 + 3 HCl