AS Chemistry A OCR - Alkanes

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  • Created by: Shipreck
  • Created on: 11-02-16 10:22

Properties of Alkanes

  • Alkanes are saturated (only contain single bonds) hydrocarbons (only contain hydrogen and carbon)
  • The single bonds in Alkanes are σ bonds. 
    • σ bonds can rotate freely
    • σ bonds are the overlap of orbitals directly between the two nuclei.
  • Alkanes form a tetrahedral shape around the carbon atoms.
    • 4 bonding pairs, 0 lone pairs.
    • Electron pairs repel each other and thus have to be as far and equally spaced around the carbon as possible.
    • Bond angle of 109.5 degrees.
  • As the chain length of an alkane increases, so does the boiling point.
    •  The more electrons you have, and the more distance over which they can move, the bigger the possible temporary dipoles and therefore the bigger the london dispersion forces.
  • As branching of an alkane increases, the boiling point deceases.
    • Long thin molecules lie closer together and have more surface contact, making dispersion forces stronger.
  • Alkanes are relatively unreactive.
    • σ bonds have low polarity and high enthalpy
    • the C-C and C-H bonds require a lot of energy to break.
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Combustion of Alkanes

  • The more carbon atoms in the Alkane, the more energy released. But the more oxygen needed.
  • Complete Combustion
    • Products are CO2 and H2O
  • Incomplete Combustion
    • Products are CO/C and H2O.
    • Happens when there is a lack of oxygen.
  • CO is deadly as it binds with the blood in place of Oxygen.
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Radical Substitution

  • Conditions: Presence of UV Light to break the CL2.
  • Initiation - Cl2 -> 2*CL 
  • Propagation - *Cl + CH4 -> *CH3 + HCl
                        *CH3 + Cl2 -> CH3CL + *Cl
  • Termination - 2*Cl -> Cl2
                        2*CH3 -> C2H6
    • Termination is when 2 radicals react together.
  • Limitations
    • Mixture of Organic Products due to further substitution and termination.
    • Mixture of structural isomers.
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