F322: Alkanes

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Introduction

  • Alkanes are saturated hydrocarbons, meaning they have all single C-C bonds.
  • General Formula = CnH2n+2
  • General Formula of a cycloalkane = CnH2n

    -These are unreactive because only have C-H and C-C bonds, which are very strong.
    -Also are non-polar, meaning they don't attract attacking species. 

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Boiling/Melting Points

  • As carbon chain length (Also Mr) increases, so does boiling/melting point:
    -Number of electrons increases.
    -Strength of van der Waals' forces increases.
  • As branching increases the melting point/boiling point decreases:
    -Molecules cannot pack as tight, so less surface contact.
    -So weaker and fewer van der Waals' forces.
     
  • Therefore Octane has a hight mpt/bpt than butane. 
  • And 2,3-diethylpentane has a lower mpt/bpt than hexane.
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Fractional Distillation

  • Crude oil is seperated by fractional distillation, which is: the seperation of components in a liquid mixture into fractions which differ by boiling points by means of distillation.
  • Different fractions have different boiling points due to chain length and branching.

    -Uses: Fuels = Methane for domestic use, Butane for camping gas, and Octane for petrol.
              Petrochemicals = Converted into alkenes for plastics etc.
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Fractional Distillation (Cont.)

  • Cracking = The breaking down of long-chained alkanes into shorter-chained ones and alkenes.
    E.g. C10H22 -> C2H4 (solvents) + C8H18 (petrol) {ZEOLITE CATALYST, 450 DEGREES}
  • Reforming = Conversion of unbranched into cyclic or aromatic, by removing H.
    E.g. Hexane (C6H14) -> Cyclohexane (C6H12) + H2
           Cyclohexane -> Benzene (C6H6, an arene) +3H2
  • Isomerism = Conversion of unbranched into branched alkanes.
    E.g. Octane (C8H18) -> 2,3,4-triethylpentane (C8H18)
    Alkanes formed by reforming and isomerism are added to fuel to increase efficiency
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Alkanes As Fuels

  • ALKANE + O2 -> CO2 + H2O
    E.g. C4H10 + O2 -> CO2 + H2O
    -Incomplete sometimes occurs, which produces CO rather than CO2; this happens frequently in car engines.
    -CO is a colourless and odourless gas, toxic because it inhibits the haemoglobin from binding with O2, often fatal.
    -Plentiful O2:           C3H8 +    5O2  ->  3CO2 +     4H2O
    -Limited O2:            C3H8 +    7/2 O2  ->  3CO +     4H2O 
  • Hydrocarbons make good fuels because they:
    -Release large amounts of energy.
    -Readily available.
  • Problems:
    -Non-renewable (Becoming more expensive).
    -Produces pollutants (NO, CO, SO2 which affect acid rain and destruction of forests).
    -They can add to climate change.
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Alternative Fuels

  • A biofuel which is a fuel made from recently living material such as plants/animal waste.
    E.g. -Sugar cane grown for energy production.
           -Ethanol made by fermentation, used to burn or added to fuels.
           -Biodisel derived from plants. 
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Radical Substitution Of Alkanes

  • A type of substitution where a radical replaces a different atom or group of atoms.
  • Overall reaction for methane: CH4 + Cl2 -> CH3Cl + HCl
  • Mechanism:
    INITIATION - Formation of radicals by homolytic fission by UV light.
    Cl2  ->  2Cl.
    PROPOGATION
    - Radicals used up and regenerated.

    CH4 +    Cl.   ->  CH3. +    HCl
    CH3. +    Cl2  ->   CH3Cl    +    Cl. 

    TERMINATION - Radicals used up to form product.
    CH3. +    CH3. ->   C2H6
    CH3. +    Cl.   ->   CH3Cl 

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