Elimination vs Substitution

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Elimination vs Substitution
Curriculum Outcome:
8. Understand the competition
between substitution and elimination
Two Outcomes
If we react a haloalkane with hydroxide ions
there are two possible outcomes.
1. Elimination  Removal of halogen and
formation of double bond.
2. Substitution  Removal of halogen which is
replaced by the –OH group
Nucleophillic Substitution
In this example, 2-bromopropane is changed to
2-propanol.
Elimination
In this example the 2-bromopropane reacts to
give propene.
Notice that the hydrogen is removed from one
of the end carbons.
Markovnikov’s rule in reverse ?
What decides whether you get
substitution or elimination?
• The same reagents are used for both the
substitution and elimination reactions.
• In all cases you will get a mixture of both
reactions happening!
• The reaction conditions determine the major
product.
Reaction Conditions
Type of haloalkane
Solvent used for the reaction
The temperature
Concentration of the OH- solution
Type of Haloalkane
This factor has the greatest influence on the
product formed
Type of Haloalkane
Primary
Secondary
Substitution or Elimination
Mainly substitution
Both substitution and elimination
Tertiary
Mainly elimination
Draw structures to represent the primary,
secondary and tertiary structures.
Solvent
NaOH or KOH are soluble in water.
The haloalkanes are soluble in ethanol.
The proportion of water to
ethanol in the solvent changes
the reaction outcome.
• More water encourages
substitution
• More ethanol encourages
elimination
Temperature
Higher temperatures encourage elimination.
Lower temperatures encourage substitution.
Concentration of OHHigher concentrations of OH- favours elimination
Make a table to summarise
Reaction Condition
Type of Haloalkane
Solvent
Temperature
Concentration of OH-
Substitution
Elimination
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