Poisoned-catalyst partial hydrogenation
Rosenmund reduction
How the Rosenmund reduction works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Acid chloride, H₂, Pd/BaSO₄ + quinoline–S (poison). Conditions: Toluene or xylene, 25–110 °C, 1 atm H₂. Bonds formed: C–H; H–Cl; bonds broken: C–Cl; H–H. Stereochemistry: Not relevant (aldehyde carbon is not a stereocentre). Rate and selectivity: Poison slows aldehyde over-reduction; HCl scavenger (base) sometimes added.
At a glance
Step 1: Acid chloride adds to the Pd surface
An acid chloride's carbonyl carbon coordinates to a palladium surface poisoned with barium sulfate and a sulfur-containing additive (quinoline-S) — the poison deliberately lowers the catalyst's activity.
Step 2: H₂ delivers H, displacing Cl
H₂ dissociates on the metal surface and delivers hydrogen to the activated carbonyl carbon, displacing chloride — ordinary catalytic hydrogenation chemistry, just carefully tuned in rate.
Step 3: The weakened catalyst stops at the aldehyde
The reaction stops cleanly at the aldehyde stage, because the poisoned catalyst is deliberately too weak to further reduce the aldehyde all the way to an alcohol — a rare example where making a catalyst deliberately worse is exactly what gives useful selectivity.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- Pd/C
- Palladium on carbon. Heterogeneous hydrogenation catalyst.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- HCl
- Hydrochloric acid / hydrogen chloride. Strong acid; chloride is the nucleophile in additions/substitutions.
How it compares with related reactions
| Rosenmund reduction | Lindlar semihydrogenation of an alkyne | Acetic acid transfers a proton to water | |
|---|---|---|---|
| Reagents | Acid chloride, H₂, Pd/BaSO₄ + quinoline–S (poison) | H₂, Pd/CaCO₃, Pb(OAc)₂, quinoline | Acetic acid (pKa 4.76) + water |
| Conditions | Toluene or xylene, 25–110 °C, 1 atm H₂ | Hexane or EtOAc, 25 °C, 1 atm H₂ | Aqueous solution, 25 °C |
| Stereochemistry | Not relevant (aldehyde carbon is not a stereocentre) | syn addition gives the cis (Z) alkene | None (achiral species) |
| Rate / selectivity | Poison slows aldehyde over-reduction; HCl scavenger (base) sometimes added | Poisoned Pd: alkyne fast, alkene slow | Equilibrium: Ka = 1.8 × 10⁻⁵, lies mostly left |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Rosenmund reduction?
Acid chloride, H₂, Pd/BaSO₄ + quinoline–S (poison). Conditions: Toluene or xylene, 25–110 °C, 1 atm H₂.
Which bonds are formed and which are broken?
Formed: C–H; H–Cl. Broken: C–Cl; H–H.
What is the stereochemical outcome?
Not relevant (aldehyde carbon is not a stereocentre)
What controls the rate or the selectivity?
Poison slows aldehyde over-reduction; HCl scavenger (base) sometimes added
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go