Carbonyl deoxygenation — contrast
Clemmensen reduction
How the Clemmensen 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: Zn(Hg), conc. HCl. Conditions: Reflux, heterogeneous (metal surface). Bonds formed: CH₂ (two C–H); ZnCl₂, H₂O; bonds broken: C=O. Stereochemistry: Not stereocontrolled; mechanism debated. Rate and selectivity: Works best for aryl ketones; avoid acid-sensitive groups.
At a glance
Step 1: Electron transfer from the zinc surface
Zn(Hg) amalgam delivers electrons directly to the carbonyl carbon at the metal surface, forming a radical/organozinc species — an acidic, metal-surface mechanism instead of Wolff-Kishner's basic, solution-phase one.
Step 2: Further e⁻ / H⁺ transfers: zinc carbenoid
Sequential electron transfers and protonations (mechanism still debated in detail) convert the carbonyl through a zinc carbenoid-like intermediate.
Step 3: Protonation gives CH₂
The net result is the same as Wolff-Kishner — full deoxygenation to CH₂ — but run under strongly acidic conditions, making it the go-to choice for base-sensitive substrates.
Abbreviations and terms
- Zn(Hg)
- Zinc amalgam. Reductant for the Clemmensen reduction.
- HCl
- Hydrochloric acid / hydrogen chloride. Strong acid; chloride is the nucleophile in additions/substitutions.
How it compares with related reactions
| Clemmensen reduction | Mozingo reduction (thioketal desulfurization) | Wolff-Kishner reduction | |
|---|---|---|---|
| Reagents | Zn(Hg), conc. HCl | 1,2-Ethanedithiol, BF₃·OEt₂ (or TsOH); then Raney Ni (H-saturated) | N₂H₄; KOH (or t-BuOK) |
| Conditions | Reflux, heterogeneous (metal surface) | Step 1: CH₂Cl₂, 25 °C; step 2: EtOH, reflux | Diethylene glycol, 180–200 °C (Huang-Minlon) |
| Stereochemistry | Not stereocontrolled; mechanism debated | Any carbonyl stereocentre α is untouched; no new stereocentre | Carbanion is protonated; no stereocontrol |
| Rate / selectivity | Works best for aryl ketones; avoid acid-sensitive groups | Strong Ni–S bond formation drives desulfurization | N₂ loss is irreversible; needs base-stable substrate |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Clemmensen reduction?
Zn(Hg), conc. HCl. Conditions: Reflux, heterogeneous (metal surface).
Which bonds are formed and which are broken?
Formed: CH₂ (two C–H); ZnCl₂, H₂O. Broken: C=O.
What is the stereochemical outcome?
Not stereocontrolled; mechanism debated
What controls the rate or the selectivity?
Works best for aryl ketones; avoid acid-sensitive groups
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go