Rosenmund reduction template

Pd inserts into the C–Cl bond of benzoyl chloride, hydrogen replaces chloride on Pd (HCl leaves) and the acyl and hydride join to give benzaldehyde.

Mechanism step by step

H₂ over poisoned Pd turns an acyl chloride into an aldehyde

1. Oxidative addition – Pd(0) inserts into the C–Cl bond
2. Hydrogenolysis – H₂ on the Pd surface replaces Cl by H; HCl is released
3. Reductive elimination – The acyl group and H join; Pd(0) is regenerated

Key points

• The reaction happens on the Pd surface; the steps shown are the usual molecular picture of it.
• BaSO₄ and quinoline–sulfur poison the Pd so the aldehyde is not reduced further to the alcohol.
• Formaldehyde cannot be made this way: formyl chloride (HCOCl) is unstable.
• Related: Stephen reduction (nitrile → aldehyde) and DIBAL-H reduction of esters.

Drawn with 78 atoms, 8 arrows, 7 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