IB Chemistry Power Points
Topic 20
Organic Chemistry
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Elimination Reactions
Elimination of HBr from halogenoalkanes
Recall: nucleophilic substitution of a bromoalkane
Elimination of HBr from halogenoalkanes
Recall: nucleophilic substitution of a bromoalkane
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
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.
Elimination of HBr from halogenoalkanes
Elimination reaction mechanisms
E2 mechanism
- similar to SN2 - bimolecular process
- transition state, no intermediates
- typical of secondary or tertiary substituted alkyl halides
Elimination reaction mechanisms
E1 mechanism
-
similar to SN1
unimolecular process
carbocation intermediate
typical of tertiary substituted alkyl halides
Elimination reaction mechanisms
How the reactions happen – the role of OHThe 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-.