Vilsmeier–Haack reaction template

DMF and POCl₃ form the chloroiminium ion (Vilsmeier reagent). N,N-dimethylaniline attacks it at the para position, and water hydrolyses the iminium to 4-(dimethylamino)benzaldehyde.

Mechanism step by step

DMF and POCl₃ formylate an electron-rich arene

1. O attacks P – The DMF oxygen attacks POCl₃; chloride leaves
2. Chloride adds – Cl⁻ adds to the carbon
3. Vilsmeier reagent – The N lone pair pushes out dichlorophosphate: the chloroiminium ion
4. Electrophilic attack – The ring attacks at the para position (activated by NMe₂)
5. Rearomatisation – Loss of H⁺ restores the aromatic ring
6. Hydrolysis – Water turns the α-chloroamine (iminium) into the aldehyde

Key points

• The Vilsmeier reagent is a weak electrophile: only electron-rich rings react (anilines, phenol ethers, pyrrole, indole, furan).
• Pyrrole and furan react at C-2; indole at C-3.
• Other amides give ketones (N,N-dimethylacetamide gives an acetyl group).
• Gattermann–Koch and Gattermann are other formylations; they suit benzene and phenols respectively.

Drawn with 169 atoms, 17 arrows, 10 shapes – every one editable in MolDraw.

Sources

• Kürti & Czakó, Strategic Applications of Named Reactions in Organic Synthesis, Elsevier 2005 Textbook
• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020 Textbook