1. IB Chemistry Power Points
Topic 20
Organic Chemistry
www.pedagogics.ca
Elimination Reactions

2. Elimination of HBr from
halogenoalkanes
Recall: nucleophilic substitution of a bromoalkane

3. Elimination of HBr from
halogenoalkanes
Recall: nucleophilic substitution of a bromoalkane

4. Elimination of HBr from
halogenoalkanes
Recall: nucleophilic substitution of a bromoalkane
Reaction occurs when
bromoethane is added to a
dilute NaOH solution.
If conditions change, the
nature of the reaction will
change

5. Elimination of HBr from
halogenoalkanes
An elimination reaction is one where a small molecule is
removed from the reactant molecule.
Substitution OH- + C2H5Br  C2H5OH + Br-
Elimination OH- + C2H5Br  C2H4 + H2O + Br-
Which reaction occurs depends on the reacting conditions.

6. Elimination or Substitution?

7. Elimination reaction mechanisms
E2 mechanism
- similar to SN2 - bimolecular process
- transition state, no intermediates
- typical of primary or secondary substituted alkyl halides

8. E2 mechanism (animation)

9. Elimination reaction mechanisms
E1 mechanism
- similar to SN1
- unimolecular process
- carbocation intermediate
- typical of tertiary substituted alkyl halides

10. Elimination reaction mechanisms
How the reactions happen – the role of OH-
The hydroxide ion hits one of the hydrogen atoms in the alkyl
halide and pulls it off. ACTS as a BASE! (proton acceptor)
This leads to a cascade of electron pair movements resulting
in the formation of a carbon-carbon double bond, and the loss
of the halogen as R-.