12.2 Chemical Reactions of the Alkanes

12.2 Chemical Reactions of the Alkanes

1. Reactivity of the Alkanes

   • Fairly Unreactive
     • C-C and C-H sigma bonds are strong
     • C-C bonds are non-polar
     • the electronegativity of C and H is so similar that the C-H bond can be considered non-polar

2. Complete Combustion of Alkanes

   • Plentiful supply of O2
   • Water
   • Carbon Dioxide
   • Exothermic
     • This reaction is exothermic as it gives out more heat than it takes in
   • Hexane (in petrol) = C6H14 (l) + 9.5O2 (g) --> 6CO2 (g) + 7H2O (l)
   • Propane = C3H8 (g) + 5O2 (g) --> 3CO2 (g) + 4H2O (l)
   • Ethane = C2H6 (g) + 3.5O2 (g) --> 2CO2 (g) + 3H2O (l)
   • Methane (as natural gas) = CH4 (g) + 2O2 (g) --> CO2 (g) + 2H2O (l)
   • Examples//

3. Incomplete Combustion of Alkanes

   • Limited supply of O2
   • Carbon Monoxide (CO)
     • Colourless
     • odourless
     • Highly toxic
     • Carbon could also be formed!
       • solid
   • Water
   • E.g// The incomplete combustion of Heptane
     • C7H16 (l) + 7.5O2 (g) --> 7CO (g) + 8H2O

4. Reactions of Alkanes with Halogens

   • UV radiation provides initial energy for reaction to take place
   • E.g// Methane + Bromine
     • CH4 (g) + Br2 (l) --> CH3Br (g) + HBr (g)
       • Forms bromomethane
   • Substitution reaction
     • Hydrogen atom int he alkane has been substituted by a halogen atom
   • Mechanism for bromination of Alkanes
     • Free Radical Substitution
       • Step 1: INITIATION
         • 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'
       • Step 2: PROPAGATION
         • These 2 propagation steps can keep reacting over and over in a chain reaction. Similar to that reaction with ozone.
         • 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
       • Step 3: TERMINATION
         • There can be a number of different termination reactions between two radicals
         • Two radicals collide, forming a molecule with all electrons paired
           • Br' + Br' --> Br2
           • CH3' + CH3' --> C2H6
           • CH3' + Br' --> CH3Br

5. Limitations of radical substitution in organic synthesis

   • 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.
     • Results in a mixture of compounds, E.g/ CH3Br --> CH2Br2 --> CHBr3 --> CBr4
   • Substitution at different points in the carbon chain
     • methane = only one monobromo compound possible as one carbon
     • Ethane = only on monosubstituted product C2H2Br possible
     • However pentane could have three monosubstituted isomers
       • 1-bromopentane
       • 2-bromopentane
       • 3-bromopentane

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