IB Chemistry Power Points Topic 20 Organic
Chemistrywww.pedagogics.ca Elimination Reactions
Elimination of HBr fromhalogenoalkanesRecall: nucleophilic
substitution of a bromoalkane
Elimination of HBr fromhalogenoalkanesRecall: nucleophilic
substitution of a bromoalkane
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
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.
Elimination or Substitution?
Elimination reaction mechanismsE2 mechanism- similar to SN2 -
bimolecular process- transition state, no intermediates- typical of
primary or secondary substituted alkyl halides
E2 mechanism (animation)
Elimination reaction mechanismsE1 mechanism- similar to SN1-
unimolecular process- carbocation intermediate- typical of tertiary
substituted alkyl halides
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-.
