Alkanes

• What is the general formula of alkanes?

  CnH2n+2
• What is a hydrocarbon?


  A compound made up of only carbon and hydrogen.
• Arrangement of bonds around each carbon atoms in an alkane.

  • Each carbon atom forms four single bonds.
  • 4 regions of election density
  • Equal electron pair repulsion
  • 109.5 degrees bond angles
  • A tetrahedral arrangement around the carbon atoms
• The effect of increasing chain length on the boiling point of alkanes

  • As the carbon chain increases means more electrons are present, creating stronger temporary induced dipoles, which increases the strength of London forces between molecules.
  • Also the surface contact between molecules increases, which increases the strength of London forces which require more energy to overcome.
• The effect of branching on the boiling point of alkanes

  • The branched alkanes cannot pack closely together which means less surface contact between molecules. This leads to weaker London forces between molecules, which require less energy to overcome.
• Why are alkanes described as being saturated?

  The carbon atoms in alkanes are joined together by single covalent bonds
• Why are alkanes used as fuels?

  • Release a large amount of energy when they undergo combustion
• What is combustion?

  Combustion is the reaction between a fuel and oxygen, resulting in the release of energy as heat.
• What is the difference between complete and incomplete combustion

  • In complete combustion, there is a plentiful supply of oxygen, the fuel reacts completely to form carbon dioxide and water vapour.
  • In incomplete combustion, there is an insufficient supply of oxygen, the fuel doesn't react completely, producing carbon monoxide and water vapour.
  • It also produces solid carbon (soot)
• Why does incomplete combustion occur?

  • When there is insufficient supply of oxygen for the fuel to react completely.
  • Longer chain alkanes need more oxygen compared to shorter chains.
• Word equation of the complete combustion of a hydrocarbon:

  Hydrocarbon + oxygen ----> carbon dioxide + water
• Products that can be formed during incomplete combustion of alkane:

  • Carbon Monoxide
  • Carbon
  • Water vapour
  • Unburnt hydrocarbon
• Why is Carbon Monoxide harmful?

  • Is poisonous as it can prevent oxygen from binding to haemoglobin in the blood.
• Why do alkanes have a low reactivity?

  • The strong C-C and C-H covalent bonds require a lot of energy to break.
  • The non-polar nature of C-C bonds make it difficult for alkanes to attract nucleophiles or electrophiles.
• What is a free radical?

  • A free radical is a species with an unpaired electron (represented by a dot)
  • The unpaired electron in free radicals makes them very reactive.
• Steps in a Free radical substitution mechanism:

  1. Initiation (forming the radicals)
  2. Propagation (repeated reactions that regenerates the free radical)
  3. Termination (when two free radicals combine to form a stable product)
• General equation for the reaction between an alkane and a halogen:

  Alkane + halogen -----> Haloalkane + Hydrogen Halide
• The conditions that are required for alkanes molecules to undergo reactions with halogens (S/E)

  • Alkanes react with halogens when exposed to UV light.
  • The energy from the UV light is enough to breaks the halogen - halogen bond which forms the radicals, which initiate the reaction.
• What type of reaction occurs when alkanes react with halogens?

  Free Radical Substitution
• E.G. What is the equation for the initiation step in the reaction between methane and chlorine?

  Cl2 ---> *Cl + *Cl

  • Two free radicals are formed
• What type of bond fission occurs during the initiation step of the free radical substitution reactions?

  Homolytic Fission
• E.G. How does the relative amount of chorine and methane present determine the identity and ratio of products produced in the reaction between CH4 AND Cl2

  • If methane is in excess, more CH3Cl will be produced
  • If chlorine is in excess, further substitution of H with Cl may take place to form CH2Cl2, CHCl3, CCl4
• Limitations of radical substitution reactions in organic synthesis? (S/E)

  Free radical substitution leads to a mixture of products being formed as a result of:

  • Further substitution at the same position in the carbon chain.
  • Substitution at different position in the carbon chain.
• E.G. Two equations for possible propagation steps in the reaction between methane and chlorine.

  *Cl + CH4 ----> HCL + *CH3

  *CH3 + Cl2 -----> CH3Cl + *Cl
• E.G. Three equations for possible termination steps in the reaction between methane and chlorine.

  1. *Cl + *Cl ----> Cl2
  2. *Cl + *CH3 ----> CH3Cl
  3. *CH3 + *CH3 ----> C2H6