Chapter 12

• Alkanes


  Saturated hydrocarbons
• Bonding in alkanes


  • Electronegativity of C & H is very similar, sigma sigma (σ) bonds are non-polar
  • No polarity, no partial charge
  • No electron-deficient areas to attract nucleophiles
  • No electron-rich areas to attract electrophiles
  • Carbon forms 4 bonds in a tetrahedral shape (109.5°)
  • Non-polar, only induced dipole-dipole interaction
  • As bond angle increases, the boiling point increases

  
• Shape of alkanes


  • No lone pair of electrons, tetrahedral shape

  
• Boiling points of alkanes


  Vary based on chain length and branching
• Chain length


  Longer chains have higher boiling points due to more London dipole-dipole interactions
• Branching


  Branched alkanes have lower boiling points than unbranched ones with the same number of carbons due to fewer surface points of contact and fewer London bonds
• Fractional distillation separates crude oil into smaller compounds based on their different boiling points
• Straight chain alkanes have more London forces between molecules, leading to higher boiling points compared to branched alkanes
• Boiling point increases down the alkane homologous series as the number of carbon atoms increases, resulting in more London forces

  
• Sigma (σ) bonds


  Stronger covalent bonds formed by the overlap of orbitals, one from each bonding atom
• Tetrahedral shape


  109.5° bond angle

  
• Fractional distillation


  Separates crude oil into simpler, more useful mixtures based on boiling point
• Reactivity of alkanes


  • High bond enthalpies of C-C and C-H bonds
  • Very low polarity

  
• Bond enthalpy


  1. C = 346 kJ mol-1, C-H = 411 kJ mol-1 (C-H is stronger due to shorter bond length)
• Alkanes are combusted (burnt) on a large scale due to their use as fuels
• Complete combustion of alkanes


  Alkanes are burnt in excess oxygen, producing carbon dioxide and water

  
• Incomplete combustion of alkanes


  Reduced supply of oxygen leads to the production of carbon (soot) and toxic gases like carbon monoxide

  
• Carbon monoxide


  Toxic, odourless gas that binds to haemoglobin, preventing oxygen transport to organs

  
• Free radical substitution of alkanes requires ultraviolet light (sunlight)

  
• Free radical substitution of alkanes

  1. Initiation step: Cl-Cl or Br-Br bond is broken by UV energy

  2. Propagation step: Free radicals attack alkanes, creating more free radicals in a chain reaction

  3. Termination step: Two free radicals react together, forming a single unreactive molecule
• Free radical substitution is not suitable for preparing specific halogenoalkanes due to a mixture of substitution products

  
• Further substitution can occur during free radical substitution, leading to the formation of unwanted impurities