Nucleophilic Substitution
Nucleophilic Substitution of Halogenoalkanes with Hydroxide Ions
• Functional Group: Halogenoalkane ! Alcohol
• Reagent: AQUEOUS Potassium or Sodium Hydroxide
• Conditions: Heat under reflux
• Nucleophile: :OH-
Extra:
• The reagent must be aqueous (not ethanolic) otherwise elimination will occur.
• Tertiary halogenoalkanes will under nucleophilic substitution in a different way (an understanding of
this mechanism is not required):
o The tertiary carbocation is stabilised by the electron releasing alkyl groups around it.
o The alkyl groups are also bulky and prevent the OH- ion from attacking the halogenoalkane
in the same way as the above mechanism.
Nucleophilic Substitution of Halogenoalkanes with Cyanide Ions
• Functional Group: Halogenoalkane ! Nitrile
• Reagent: Ethanolic Sodium or Potassium Cyanide
• Conditions: Heat under reflux
• Nucleophile: :CN-
Extra:
• This reaction can be used to increase the length of the carbon chain.
• The nitrile group can be hydrolysed to produce a carboxylic acid by heating under reflux with a strong acid,
such as HCl.
• E.g. CH3CH2CN + 2H2O + H+ ! CH3CH2COOH + NH4+
, Nucleophilic Substitution
Nucleophilic Substitution of Halogenoalkanes with Ammonia
• Functional Group: Halogenoalkane ! Primary Amine
• Reagent: Ethanolic Ammonia
• Conditions: Heat under pressure (in a sealed tube)
• Nucleophile: :NH3
Extra:
• The reagent must be ethanolic (not aqueous) because…
o Halogenoalkanes will react with water.
o Halogenoalkanes are less soluble in water than ammonia since ammonia can hydrogen bond
with water).
• The product of the above reaction is a primary amine, which itself can act as a nucleophile, as the
nitrogen atom still has a lone pair of electrons.
• Successive reactions between the amine and the halogenoalkane can form secondary and tertiary
amines as well as quaternary ammonium salts.
• This means that the desired product would have to be separated.
o To favour production of the primary amine, use excess ethanolic ammonia.
o To favour production of the quaternary ammonium salt, use excess halogenoalkane.
Formation of Secondary Amine:
• Step 1: The primary amine acts as a
nucleophile.
• Step 2: Either the primary amine or
ammonia could act as the base to form
the secondary amine.
Formation of Tertiary Amine: Formation of Quaternary Ammonium Salt:
(Only step 1 occurs)
Nucleophilic Substitution of Halogenoalkanes with Hydroxide Ions
• Functional Group: Halogenoalkane ! Alcohol
• Reagent: AQUEOUS Potassium or Sodium Hydroxide
• Conditions: Heat under reflux
• Nucleophile: :OH-
Extra:
• The reagent must be aqueous (not ethanolic) otherwise elimination will occur.
• Tertiary halogenoalkanes will under nucleophilic substitution in a different way (an understanding of
this mechanism is not required):
o The tertiary carbocation is stabilised by the electron releasing alkyl groups around it.
o The alkyl groups are also bulky and prevent the OH- ion from attacking the halogenoalkane
in the same way as the above mechanism.
Nucleophilic Substitution of Halogenoalkanes with Cyanide Ions
• Functional Group: Halogenoalkane ! Nitrile
• Reagent: Ethanolic Sodium or Potassium Cyanide
• Conditions: Heat under reflux
• Nucleophile: :CN-
Extra:
• This reaction can be used to increase the length of the carbon chain.
• The nitrile group can be hydrolysed to produce a carboxylic acid by heating under reflux with a strong acid,
such as HCl.
• E.g. CH3CH2CN + 2H2O + H+ ! CH3CH2COOH + NH4+
, Nucleophilic Substitution
Nucleophilic Substitution of Halogenoalkanes with Ammonia
• Functional Group: Halogenoalkane ! Primary Amine
• Reagent: Ethanolic Ammonia
• Conditions: Heat under pressure (in a sealed tube)
• Nucleophile: :NH3
Extra:
• The reagent must be ethanolic (not aqueous) because…
o Halogenoalkanes will react with water.
o Halogenoalkanes are less soluble in water than ammonia since ammonia can hydrogen bond
with water).
• The product of the above reaction is a primary amine, which itself can act as a nucleophile, as the
nitrogen atom still has a lone pair of electrons.
• Successive reactions between the amine and the halogenoalkane can form secondary and tertiary
amines as well as quaternary ammonium salts.
• This means that the desired product would have to be separated.
o To favour production of the primary amine, use excess ethanolic ammonia.
o To favour production of the quaternary ammonium salt, use excess halogenoalkane.
Formation of Secondary Amine:
• Step 1: The primary amine acts as a
nucleophile.
• Step 2: Either the primary amine or
ammonia could act as the base to form
the secondary amine.
Formation of Tertiary Amine: Formation of Quaternary Ammonium Salt:
(Only step 1 occurs)
