12.2 Chemical Reactions of the Alkanes

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AS - Level Chemistry (12 - Alkanes) Mind Map on 12.2 Chemical Reactions of the Alkanes, created by Bee Brittain on 22/03/2016.
Bee Brittain
Mind Map by Bee Brittain, updated more than 1 year ago
Bee Brittain
Created by Bee Brittain over 8 years ago
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Resource summary

12.2 Chemical Reactions of the Alkanes
  1. Reactivity of the Alkanes
    1. Fairly Unreactive
      1. C-C and C-H sigma bonds are strong
        1. C-C bonds are non-polar
          1. the electronegativity of C and H is so similar that the C-H bond can be considered non-polar
        2. Complete Combustion of Alkanes
          1. Plentiful supply of O2
            1. Water
              1. Carbon Dioxide
                1. Exothermic

                  Annotations:

                  • This reaction is exothermic as it gives out more heat than it takes in
                  1. Hexane (in petrol) = C6H14 (l) + 9.5O2 (g) --> 6CO2 (g) + 7H2O (l)
                    1. Propane = C3H8 (g) + 5O2 (g) --> 3CO2 (g) + 4H2O (l)
                      1. Ethane = C2H6 (g) + 3.5O2 (g) --> 2CO2 (g) + 3H2O (l)
                        1. Methane (as natural gas) = CH4 (g) + 2O2 (g) --> CO2 (g) + 2H2O (l)
                          1. Examples//
                          2. Incomplete Combustion of Alkanes
                            1. Limited supply of O2
                              1. Carbon Monoxide (CO)
                                1. Colourless
                                  1. odourless
                                    1. Highly toxic
                                      1. Carbon could also be formed!
                                        1. solid
                                      2. Water
                                        1. E.g// The incomplete combustion of Heptane
                                          1. C7H16 (l) + 7.5O2 (g) --> 7CO (g) + 8H2O
                                        2. Reactions of Alkanes with Halogens
                                          1. UV radiation provides initial energy for reaction to take place
                                            1. E.g// Methane + Bromine
                                              1. CH4 (g) + Br2 (l) --> CH3Br (g) + HBr (g)
                                                1. Forms bromomethane
                                              2. Substitution reaction
                                                1. Hydrogen atom int he alkane has been substituted by a halogen atom
                                                2. 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
                                                        1. Each bromine atom takes one electron from the pair in the covalent bond
                                                          1. This forms two highly reactive bromine radicals
                                                            1. Br-Br --> Br' + Br'
                                                      2. Step 2: PROPAGATION

                                                        Annotations:

                                                        • 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
                                                          1. Propagation step 2: CH3' + Br2 --> CH3Br + Br'
                                                            1. Each methyl radical reacts with another bromine molecule, forming bromomethane (CH3Br) and a new bromine radical (Br')
                                                            2. Bromine radical reacts with a C-H bond in the methane to form a methyl radical (CH3') and a molecule fo hydrogen brominde
                                                          2. Step 3: TERMINATION

                                                            Annotations:

                                                            • There can be a number of different termination reactions between two radicals
                                                            1. Two radicals collide, forming a molecule with all electrons paired
                                                              1. Br' + Br' --> Br2
                                                                1. CH3' + CH3' --> C2H6
                                                                  1. CH3' + Br' --> CH3Br
                                                          3. 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
                                                                  1. Ethane = only on monosubstituted product C2H2Br possible
                                                                    1. However pentane could have three monosubstituted isomers
                                                                      1. 1-bromopentane
                                                                        1. 2-bromopentane
                                                                          1. 3-bromopentane
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