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Topic 20 3 elimination reactions


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Topic 20 3 elimination reactions

  1. 1. IB Chemistry Power Points Topic 20 Organic Elimination Reactions
  2. 2. Elimination of HBr fromhalogenoalkanesRecall: nucleophilic substitution of a bromoalkane
  3. 3. Elimination of HBr fromhalogenoalkanesRecall: nucleophilic substitution of a bromoalkane
  4. 4. Elimination of HBr fromhalogenoalkanes Recall: nucleophilic substitution of a bromoalkaneReaction occurs whenbromoethane is added to adilute NaOH solution.If conditions change, thenature of the reaction willchange
  5. 5. Elimination of HBr fromhalogenoalkanesAn elimination reaction is one where a small molecule isremoved from the reactant molecule.Substitution OH- + C2H5Br  C2H5OH + Br-Elimination OH- + C2H5Br  C2H4 + H2O + Br-Which reaction occurs depends on the reacting conditions.
  6. 6. Elimination or Substitution?
  7. 7. Elimination reaction mechanismsE2 mechanism- similar to SN2 - bimolecular process- transition state, no intermediates- typical of primary or secondary substituted alkyl halides
  8. 8. E2 mechanism (animation)
  9. 9. Elimination reaction mechanismsE1 mechanism- similar to SN1- unimolecular process- carbocation intermediate- typical of tertiary substituted alkyl halides
  10. 10. Elimination reaction mechanismsHow the reactions happen – the role of OH-The hydroxide ion hits one of the hydrogen atoms in the alkylhalide and pulls it off. ACTS as a BASE! (proton acceptor)This leads to a cascade of electron pair movements resultingin the formation of a carbon-carbon double bond, and the lossof the halogen as R-.