SN2 with azide: practice and mechanism
Azide (N₃⁻) is a very good nucleophile and replaces the halogen via SN2. An alkyl azide R–N₃ forms, which can later be reduced cleanly to the primary amine.
Example
The azide ion is a strong, barely basic nucleophile and replaces the halogen via SN2. Alkyl azides can then be reduced to primary amines without any over-alkylation.
Mechanism step by step
- 1
The linear azide ion N₃⁻ attacks the carbon bearing the halogen from the back with a terminal N atom.
- 2
In one concerted step the C–N bond forms and the halide leaves (inversion at carbon).
- 3
The alkyl azide R–N₃ is usually used directly, e.g. reduced to the primary amine with H₂/Pd or LiAlH₄.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Background
Typical conditions
Sodium azide (NaN₃) in a polar aprotic solvent such as DMF or DMSO, often at 50–80 °C. Primary alkyl halides and tosylates work best; secondary ones react more slowly but still with inversion. Tertiary substrates do not react by SN2.
From azide to amine
The alkyl azide is an intermediate: with LiAlH₄, with H₂ and Pd/C, or mildly by the Staudinger reaction (PPh₃, then water) it becomes the primary amine and N₂ is released. This gives clean primary amines without the over-alkylation seen with ammonia.
Safety
Sodium azide is toxic, and with acid it forms toxic, explosive hydrazoic acid (HN₃). Small organic azides with few carbons per nitrogen can explode, so in the lab they are handled in small amounts and never heated to dryness.
Typical mistakes
Often confused with: SN2 with ammonia · SN2 with cyanide (chain extension)
Frequently asked questions
Why not use NH₃ directly?
The amine formed is nucleophilic itself and reacts further (overalkylation). The azide reacts only once; only the reduction then gives the amine, cleanly and without by-products.
Why DMF as the solvent?
DMF is polar aprotic: it dissolves NaN₃ but hardly solvates the anion. The azide stays reactive and SN2 is fast.
Is azide a good nucleophile?
Yes. N₃⁻ is small, linear and only weakly basic but strongly nucleophilic, so it reacts cleanly by SN2 with inversion and little elimination.
What are alkyl azides used for?
They can be reduced to primary amines (e.g. LiAlH₄ or H₂/Pd) without the over-alkylation seen with ammonia.