SN2 with cyanide (chain extension): practice and mechanism
Cyanide attacks with its carbon (SN2): a nitrile forms and the chain grows by one carbon.
Example
Cyanide is a strong nucleophile and replaces the halogen via SN2. A nitrile forms and the carbon chain grows by one carbon. Polar aprotic DMSO speeds up SN2.
Mechanism step by step
- 1
Cyanide is ambident (C or N could attack); the carbon is the softer, stronger nucleophile and attacks the haloalkane from the back.
- 2
In one step the new C–C bond forms and the halide leaves.
- 3
A nitrile with one more carbon forms; it can later become a carboxylic acid (hydrolysis) or an amine (reduction).
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- That is an isonitrile (attack through nitrogen). With NaCN in DMSO, cyanide attacks through carbon and the nitrile R–C≡N forms.
- That would be the carboxylic acid. It only forms if the nitrile is hydrolysed afterwards with acid and water; NaCN alone gives the nitrile.
Often confused with: SN2 with hydroxide · SN2 with ammonia
Frequently asked questions
Why DMSO as the solvent?
DMSO is polar aprotic: it dissolves NaCN but does not wrap the anion in hydrogen bonds. Cyanide stays "naked" and reactive, so SN2 is faster.
What is the chain extension good for?
It is one of the simplest ways to form a new C–C bond, e.g. 1-bromopropane → butanoic acid via the nitrile.