4.2/4.3 Alkanes and Alkenes

• Alkanes


  Homologous series of saturated hydrocarbons containing only carbon-carbon single bonds
• Alkanes


  • General formula CnH2n+2
  • Nonpolar as carbon electronegativity is similar to hydrogen
  • Each carbon bonded to 4 other atoms with bond angle of 109.5°
  • Intermolecular forces are London dispersion forces which increase with increasing chain length
• Factors affecting boiling point of alkanes
• Reactions of alkanes


  • Unreactive due to high bond enthalpy of C-C and C-H bonds and low polarity of sigma bonds
  • Complete combustion to CO2 and H2O
  • Incomplete combustion to CO and soot
• Carbon monoxide


  Toxic odourless gas that binds to haemoglobin, preventing oxygen transport
• Free radical substitution of alkanes


  1. Initiation - UV light causes homolytic fission of C-H bond

  2. Propagation - chain reaction with radicals

  3. Termination - two radicals collide
• Limitations of free radical substitution
• Substitution can occur on middle or end carbon
• Alkenes


  Carbon has three sigma bonding pairs and one pi bond
• Alkenes


  • Planar arrangement with 120° bond angles
  • Cannot freely rotate around C=C bond
• Cis-trans isomers


  • Cis - same groups on same side of C=C
  • Trans - different groups on opposite sides of C=C
• E/Z isomers


  • E - higher priority groups on opposite sides
  • Z - higher priority groups on same side
• Reactivity of alkenes


  More reactive than alkanes due to weaker pi bond
• Hydrogenation of alkenes


  Alkene + H2 → Alkane, requires Ni catalyst, high temp and pressure
• Halogenation of alkenes


  Alkene + Y2 → Dihaloalkane, electrophilic addition
• Hydration of alkenes


  Alkene + H2O → Alcohol, requires steam, high temp and pressure, and acid catalyst
• Electrophilic addition mechanism


  Electrophile attacks C=C, forming carbocation intermediate, then nucleophile adds
• Markovnikov's rule


  Halogen ends up bonded to most substituted carbon, forming most stable carbocation
• Addition polymerisation


  Many monomers with C=C form long chain polymer
• Alternatives to non-biodegradable polymers

  • Recycling
  • Combustion for energy recovery
  • Feedstock recycling
  • Bioplastics from renewable sources
  • Biodegradable polymers