Clemmensen & Wolff-Kishner Reductions
Aldehydes and Ketones can be reduced directly to alkanes (converting $>C=O$ to $>CH_2$) using specific reducing agents. The two most important methods are the Clemmensen Reduction (Acidic conditions) and the Wolff-Kishner Reduction (Basic conditions).
1. Clemmensen Reduction
This method uses Zinc Amalgam ($Zn-Hg$) and Concentrated Hydrochloric Acid ($HCl$). It is ideal for substrates that are stable in hot acid.
Mechanism:
The reaction takes place on the surface of the Zinc metal. Zinc provides electrons, and the acid provides protons. The oxygen is protonated and removed as water, eventually replaced by two hydrogen atoms.
Substrate Limitations
Since the reaction uses strong acid ($HCl$), it cannot be used for compounds containing acid-sensitive groups like:
- Hydroxyl groups ($-OH$) - might undergo dehydration.
- Acetals or Ketals - might undergo hydrolysis.
- Alkene double bonds - might undergo addition of HCl.
2. Wolff-Kishner Reduction
This method uses Hydrazine ($NH_2NH_2$) followed by heating with a strong base like KOH or NaOH in a high-boiling solvent like Ethylene Glycol.
Step 1: Hydrazone Formation
$$ R-CO-R' + NH_2NH_2 \rightarrow R-C(=NNH_2)-R' + H_2O $$Step 2: Base Catalyzed Decomposition
$$ R-C(=NNH_2)-R' \xrightarrow{KOH, \ Glycol, \ \Delta} R-CH_2-R' + N_2 \uparrow $$Mechanism:
The ketone/aldehyde reacts with hydrazine to form a Hydrazone. Upon heating with a base, the hydrazone loses nitrogen gas ($N_2$) to yield the alkane.
Substrate Limitations
Since the reaction uses a strong base, it cannot be used for compounds containing base-sensitive groups like:
- Alkyl Halides ($-X$) - might undergo elimination or substitution.
- Esters - might undergo hydrolysis.
3. Comparison Table
| Feature | Clemmensen Reduction | Wolff-Kishner Reduction |
|---|---|---|
| Reagent | $Zn-Hg / Conc. HCl$ | $NH_2NH_2 / KOH / Glycol$ |
| Medium | Acidic | Basic |
| Intermediate | Carbenoid / Radical on Surface | Hydrazone |
| Gas Evolved | None (Water formed) | Nitrogen ($N_2$) |
| Best for | Base-sensitive compounds | Acid-sensitive compounds |
4. Examples
A. Acetone to Propane
$$ CH_3COCH_3 \xrightarrow{Zn-Hg/HCl} CH_3CH_2CH_3 $$ $$ CH_3COCH_3 \xrightarrow{NH_2NH_2/KOH} CH_3CH_2CH_3 $$B. Acetophenone to Ethylbenzene
$$ C_6H_5COCH_3 \rightarrow C_6H_5CH_2CH_3 $$C. Selective Reduction
To reduce 4-Hydroxycyclohexanone to Cyclohexanol:
- Use Wolff-Kishner (Basic medium protects -OH).
- Avoid Clemmensen (Acid might dehydrate -OH to alkene).
Reduction Quiz
Test your concepts on Carbonyl reduction. 10 MCQs with explanations.
No comments:
Post a Comment