1. Elimination Reactions
IB Chemistry Power Points
Topic 20
Organic Chemistry
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 of HBr from halogenoalkanes

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

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

9. 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.
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-.