19.2.1—Preparation of nitriles
- Syllabus
- 9701–2028–2029
- Objective
- 19.2.1
- Level
- AS
KCN substitution makes a nitrile and increases the carbon chain by one
CN⁻ attacks a halogenoalkane in ethanol on heating, replacing X and forming a nitrile, R–C≡N. The cyanide carbon becomes part of the product skeleton.
This route is useful for chain extension: count the starting carbon atoms and add one for the nitrile carbon. The reaction is nucleophilic substitution, not elimination.
CH₃CH₂Br + KCN → CH₃CH₂CN + KBr. Bromoethane has two carbons; propanenitrile has three.
Do not write HCN as the carbon source or forget the extra carbon in the product.
ConceptA-Level CAIE Chemistry AS