Stephen reduction template

Benzonitrile takes HCl to form the imidoyl chloride, SnCl₂ reduces it to the aldimine (as its salt), and hydrolysis gives benzaldehyde and ammonia.

Mechanism step by step

SnCl₂ and HCl reduce a nitrile to an imine; water then gives the aldehyde

1. Protonation – HCl protonates the nitrile nitrogen
2. Chloride adds – Cl⁻ attacks the nitrilium carbon: an imidoyl chloride
3. Reduction – SnCl₂ gives two electrons (Sn²⁺ → Sn⁴⁺) and HCl the H: the aldimine
4. Iminium salt – In HCl the imine is protonated (it separates as a tin salt)
5. Water adds – Water attacks the iminium carbon
6. Proton transfer – H⁺ moves from O to N (through the solvent)
7. Loss of NH₃ – The O lone pair pushes out ammonia
8. Deprotonation – Benzaldehyde

Key points

• The electron transfer from SnCl₂ is drawn as one step without curly arrows: its detailed path is not established.
• The aldimine separates as the stannichloride salt (RCH=NH₂⁺)₂SnCl₆²⁻ and is hydrolysed with warm water.
• Works best for aromatic nitriles; DIBAL-H is the modern way to stop a nitrile at the aldehyde.
• Compare Rosenmund reduction (acyl chloride → aldehyde).

Drawn with 173 atoms, 23 arrows, 12 shapes – every one editable in MolDraw.

Sources

• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020 Textbook