SN2 with ammonia: practice and mechanism
Ammonia replaces the halogen via SN2, giving a primary amine (–NH₂); the chain length stays the same.
Example
Ammonia replaces the halogen via SN2, giving a primary amine. The excess of NH₃ prevents the amine from being alkylated further to secondary and tertiary amines.
Mechanism step by step
- 1
The lone pair on the nitrogen of NH₃ attacks the carbon bearing the halogen (SN2).
- 2
An alkylammonium ion R–NH₃⁺ forms and the halide leaves.
- 3
A second NH₃ removes a proton: the free amine R–NH₂ forms.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- A nitrile forms with cyanide (NaCN), not with ammonia. NH₃ only brings a nitrogen atom and no carbon.
Often confused with: SN2 with hydroxide · SN2 with cyanide (chain extension)
Frequently asked questions
Why an excess of ammonia?
The amine formed is itself a good nucleophile and could be alkylated further (secondary, tertiary amine, ammonium salt). Lots of NH₃ makes it more likely that the haloalkane meets NH₃.
Is there a cleaner route to primary amines?
Yes, e.g. the Gabriel synthesis (phthalimide) or azide (SN2) followed by reduction.