Clemmensen Reduction

Clemmensen Reduction transforms carbonyl compounds into hydrocarbons using Zn–Hg and HCl in organic synthesis.

• Type: Metal-acid reduction

Purpose of Clemmensen Reduction:

• Reduces aldehydes and ketones to alkanes.
• Used especially for carbonyl groups adjacent to aromatic rings (aryl ketones).

Reagents of Clemmensen Reduction:

• Zinc amalgam (Zn(Hg))
• Concentrated hydrochloric acid (HCl)
• Reaction is acidic

Mechanism (Simplified Concept):

1. Coordination to Zinc:

   • The carbonyl oxygen coordinates to the Zn surface, making the C=O bond more electrophilic.

2. Single Electron Transfer (SET):

   • Zn donates an electron to the carbonyl carbon → forms radical anion.

3. Protonation:

   • Proton from acid (HCl) stabilizes the intermediate.

4. Further Reduction and Protonation:

   • Repeated electron transfer and protonation convert the C=O group to -CH₂-.
   • Best used for aromatic ketones where acid-sensitive groups are absent.

Conditions:

• Harshly acidic, so acid-sensitive compounds may degrade.

Example:

• • Ar-CO-R + Zn(Hg)/HCl  →  Ar-CH₂-R
  • Benzophenone  →  Diphenylmethane

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