Alkanes

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    Strejan

    Cards (25)

    • 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
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