Inorganic — Pd(II)-catalyzed oxidation
Wacker oxidation: ethylene → acetaldehyde
How the Wacker oxidation 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: PdCl₂ (cat.), CuCl₂, O₂, H₂O. Conditions: H₂O/DMF or HCl(aq), 50–100 °C. Bonds formed: C–O, C=O; Pd–H (transient); bonds broken: C=C π; C–H (β). Stereochemistry: Anti-nucleopalladation; Markovnikov for terminal alkenes (methyl ketone). Rate and selectivity: Pd(0) reoxidized by 2 CuCl₂; Cu(I) reoxidized by O₂.
At a glance
Step 1: Water attacks the Pd-bound alkene (anti)
Pd(II) coordinates the alkene, activating it toward nucleophilic attack. Water's oxygen attacks the coordinated alkene carbon from the face opposite palladium — anti nucleopalladation.
Step 2: β-Hydride elimination
This forms a Pd–alkyl species bearing a pendant hydroxyl. β-hydride elimination transfers a hydrogen from carbon to palladium, forming a new Pd–H bond and a coordinated enol.
Step 3: Enol → acetaldehyde; Pd(0) reoxidized
The enol tautomerizes to acetaldehyde, and Pd(0) is reoxidized to Pd(II) by Cu(II)/O₂ so the catalytic cycle continues — the basis of industrial acetaldehyde production.
Abbreviations and terms
- DMF
- N,N-Dimethylformamide. Polar aprotic solvent – dissolves ionic nucleophiles but does not H-bond to them, speeding SN2 and SNAr.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- HCl
- Hydrochloric acid / hydrogen chloride. Strong acid; chloride is the nucleophile in additions/substitutions.
- Markovnikov
- Markovnikov’s rule. In HX addition, H goes to the carbon with more H’s so the more stable cation forms.
How it compares with related reactions
| Wacker oxidation | Associative ligand substitution at a square-planar Pt(II) complex | Asymmetric phase-transfer catalysis | |
|---|---|---|---|
| Reagents | PdCl₂ (cat.), CuCl₂, O₂, H₂O | K₂[PtCl₄] + NH₃ (aq) | Chiral cinchona-derived Q*⁺Br⁻ (1–10 mol%), 50% aq. KOH, glycine imine tBu ester, BnBr |
| Conditions | H₂O/DMF or HCl(aq), 50–100 °C | H₂O, 25 °C | Toluene/CH₂Cl₂, 0 °C, vigorous stirring |
| Stereochemistry | Anti-nucleopalladation; Markovnikov for terminal alkenes (methyl ketone) | Trigonal-bipyramidal intermediate; retention of geometry | High ee: chiral ion pair shields one enolate face |
| Rate / selectivity | Pd(0) reoxidized by 2 CuCl₂; Cu(I) reoxidized by O₂ | Rate = k₁[Pt] + k₂[Pt][NH₃] (associative; entering-ligand dependent) | Alkylation of the ion pair is rate-limiting |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Wacker oxidation?
PdCl₂ (cat.), CuCl₂, O₂, H₂O. Conditions: H₂O/DMF or HCl(aq), 50–100 °C.
Which bonds are formed and which are broken?
Formed: C–O, C=O; Pd–H (transient). Broken: C=C π; C–H (β).
What is the stereochemical outcome?
Anti-nucleopalladation; Markovnikov for terminal alkenes (methyl ketone)
What controls the rate or the selectivity?
Pd(0) reoxidized by 2 CuCl₂; Cu(I) reoxidized by O₂
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam