12.2 Chemical Reactions of the Alkanes

Reactivity of the Alkanes
1. Fairly Unreactive
  1. C-C and C-H sigma bonds are strong
  2. C-C bonds are non-polar
  3. the electronegativity of C and H is so similar that the C-H bond can be considered non-polar
Complete Combustion of Alkanes
1. Plentiful supply of O2
2. Water
3. Carbon Dioxide
4. Exothermic
  Anmerkungen:
  - This reaction is exothermic as it gives out more heat than it takes in
5. Hexane (in petrol) = C6H14 (l) + 9.5O2 (g) --> 6CO2 (g) + 7H2O (l)
6. Propane = C3H8 (g) + 5O2 (g) --> 3CO2 (g) + 4H2O (l)
7. Ethane = C2H6 (g) + 3.5O2 (g) --> 2CO2 (g) + 3H2O (l)
8. Methane (as natural gas) = CH4 (g) + 2O2 (g) --> CO2 (g) + 2H2O (l)
9. Examples//
Incomplete Combustion of Alkanes
1. Limited supply of O2
2. Carbon Monoxide (CO)
  1. Colourless
  2. odourless
  3. Highly toxic
  4. Carbon could also be formed!
    1. solid
3. Water
4. E.g// The incomplete combustion of Heptane
  1. C7H16 (l) + 7.5O2 (g) --> 7CO (g) + 8H2O
Reactions of Alkanes with Halogens
1. UV radiation provides initial energy for reaction to take place
2. E.g// Methane + Bromine
  1. CH4 (g) + Br2 (l) --> CH3Br (g) + HBr (g)
    1. Forms bromomethane
3. Substitution reaction
  1. Hydrogen atom int he alkane has been substituted by a halogen atom
4. Mechanism for bromination of Alkanes
  1. Free Radical Substitution
    1. Step 1: INITIATION
      1. Covalent bond in a bromine molecule is broken by HOMOLYCTIC FISSION
        Each bromine atom takes one electron from the pair in the covalent bond
        This forms two highly reactive bromine radicals
        Br-Br --> Br' + Br'
    2. Step 2: PROPAGATION
      Anmerkungen:
      - These 2 propagation steps can keep reacting over and over in a chain reaction. Similar to that reaction with ozone.
      1. Propagation step 1: CH4 + Br' --> CH3' + HBr
        Propagation step 2: CH3' + Br2 --> CH3Br + Br'
        Each methyl radical reacts with another bromine molecule, forming bromomethane (CH3Br) and a new bromine radical (Br')
        Bromine radical reacts with a C-H bond in the methane to form a methyl radical (CH3') and a molecule fo hydrogen brominde
    3. Step 3: TERMINATION
      Anmerkungen:
      - There can be a number of different termination reactions between two radicals
      1. Two radicals collide, forming a molecule with all electrons paired
        Br' + Br' --> Br2
        CH3' + CH3' --> C2H6
        CH3' + Br' --> CH3Br
Limitations of radical substitution in organic synthesis
1. Bromomethane CH3Br was formed from radical substitution. Another bromine radical can collide with a bromomethane molecule, substituting it further. This can carry on in a kind of chain reaction.
  1. Results in a mixture of compounds, E.g/ CH3Br --> CH2Br2 --> CHBr3 --> CBr4
2. Substitution at different points in the carbon chain
  1. methane = only one monobromo compound possible as one carbon
  2. Ethane = only on monosubstituted product C2H2Br possible
  3. However pentane could have three monosubstituted isomers
    1. 1-bromopentane
    2. 2-bromopentane
    3. 3-bromopentane

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12.2 Chemical Reactions of the Alkanes

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