REACTION MECHANISMS

REACTION MECHANISMS

1. Alkanes - Radical Substitution

   • Initiation
     • In the first stage, radicals are formed. Ultraviolet radiation provides the energy to break the covalent bond in a bromine molecule, Br-Br. The bond breaks by homiletic fission. Br2 --> 2Br-
   • Propagation
     • In the second stage, the products are made. Br- + CH4 --> CH3- + HBr CH3- + Br2 --> CH3Br + Br-
     • Bromine radicals catalyse the propagation stage. They are not used up in the overall reaction
     • A bromine radical is used up in the first step and regenerated in the second
   • Termination
     • Two radicals combine
       • CH3- + Br- --> CH3Br
       • Br- + Br- --> Br2
       • CH3- + CH3- --> C2H6

2. Alkenes - Electrophilic Addition

   • In this type of reaction the halogen adds across the double bond as shown:

3. Halogenoalkanes - Nucleophilic substitution

   • In this type of reaction the nucleophile (OH-) attacks the back of the carbon atom, donating an electron pair. A new bond is formed and the carbon-halogen bond breaks

4. NUCLEOPHILE is an electron pair donor

5. AN ELECTROPHILE is an electron pair acceptor

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