Wolf kishner Reduction reaction explain the Mechanism of Reaction and Application
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Wolff-Kishner reduction mechanism reaction steps application chemistry
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Wolff Kishner reduction mechanism application
Here is a thorough breakdown of the Wolff-Kishner Reduction - its mechanism, modifications, applications, and limitations.
Wolff-Kishner Reduction
Overview
The Wolff-Kishner reduction is an organic reaction that converts aldehydes and ketones (carbonyl groups, C=O) into methylene groups (CH₂), effectively deoxygenating the carbonyl compound to give an alkane. It was independently reported by Nikolai Kishner (1911) and Ludwig Wolff (1912).
Overall Reaction:
R₁-CO-R₂ → R₁-CH₂-R₂
Reagents required:
- Hydrazine (NH₂-NH₂)
- Strong base (KOH)
- High-boiling protic solvent (ethylene glycol, bp ~197°C)
- Heat (~200°C)
The driving force of the entire reaction is the thermodynamic stability of nitrogen gas (N₂) produced as a byproduct - N≡N has one of the strongest bonds in chemistry (945 kJ/mol).
Mechanism
The reaction proceeds in two major stages.
Stage 1 - Formation of the Hydrazone
This stage converts the carbonyl compound into a hydrazone intermediate (R₁R₂C=N-NH₂).
Step 1 - Nucleophilic addition of hydrazine to the carbonyl
Hydrazine (NH₂NH₂) acts as a nucleophile. One nitrogen attacks the electrophilic carbonyl carbon, forming a tetrahedral carbinolamine intermediate:
R₁-CO-R₂ + NH₂-NH₂ → R₁-C(OH)(NH-NH₂)-R₂
Step 2 - Proton transfer
The nitrogen bearing the OH group undergoes internal proton transfer to give a stable intermediate with both O and N present.
Step 3 - Elimination of water
The -OH leaves as water (acid-catalyzed or thermally driven), forming an iminium/imine-like bond with nitrogen:
R₁-C(OH)(NH-NH₂)-R₂ → R₁R₂C=N-NH₂ + H₂O
Step 4 - Deprotonation of nitrogen
The terminal -NH₂ group loses a proton under basic conditions to give the hydrazone anion. The product of Stage 1 is the hydrazone: R₁R₂C=N-NH₂
Stage 2 - Reduction of the Hydrazone to Alkane
This is where the actual "reduction" occurs. The base (KOH or the alkoxide of ethylene glycol) deprotonates the hydrazone, and N₂ is expelled with formation of a carbanion that is protonated.
Step 1 - Deprotonation of the terminal nitrogen (rate-determining step)
Strong base removes a proton from the -NH₂ of the hydrazone, generating a hydrazone anion (diimide anion):
R₁R₂C=N-NH₂ + OH⁻ → R₁R₂C=N-NH⁻ + H₂O
This is the rate-limiting step (confirmed by Szmant's kinetic studies - first order in both OH⁻ and hydrazone).
Step 2 - Resonance delocalization (negative charge shifts to carbon)
The anion delocalizes. The negative charge resonates from nitrogen to the carbon:
R₁R₂C=N-NH⁻ ⟷ R₁R₂C⁻-N=NH
This gives the carbanion character at carbon, which is key to protonation.
Step 3 - Protonation at carbon
The solvent (ethylene glycol or water) donates a proton to the carbanion carbon in a concerted step with loss of the second nitrogen proton:
R₁R₂C⁻-N=NH + H-solvent → R₁R₂CH-N=NH
Step 4 - Deprotonation of the remaining nitrogen
Base removes the proton from =NH to give another anion:
R₁R₂CH-N=NH + OH⁻ → R₁R₂CH-N=N⁻
Step 5 - Loss of nitrogen gas
The nitrogen leaves as N₂ gas, generating a carbanion at the alpha carbon:
R₁R₂CH-N=N⁻ → R₁R₂CH⁻ + N₂↑
Step 6 - Final protonation
The carbanion is protonated by solvent to give the alkane product:
R₁R₂CH⁻ + H-solvent → R₁R₂CH₂
Summary of Mechanistic Steps (at a glance)
| Stage | Step | What Happens |
|---|---|---|
| 1 | 1 | NH₂NH₂ attacks C=O |
| 1 | 2 | Proton transfer |
| 1 | 3 | -OH eliminated as water |
| 1 | 4 | Deprotonation → hydrazone |
| 2 | 1 | Base deprotonates -NH₂ (RDS) |
| 2 | 2 | Charge delocalizes to C |
| 2 | 3 | Carbon protonated by solvent |
| 2 | 4 | N-H deprotonated |
| 2 | 5 | N₂ expelled |
| 2 | 6 | Carbanion protonated → alkane |
Key Reaction Conditions
| Component | Role |
|---|---|
| Hydrazine (NH₂NH₂) | Reducing agent / forms hydrazone |
| KOH (strong base) | Deprotonates hydrazone (RDS) |
| Ethylene glycol | High-boiling solvent (~197°C) |
| Heat (~180-200°C) | Overcomes activation energy |
| N₂ gas (byproduct) | Thermodynamic driving force |
Important Modifications
1. Huang Minlon Modification (1946)
The most widely used modification. Instead of pre-forming the hydrazone in a separate step, the aldehyde/ketone is directly heated with hydrazine and KOH in ethylene glycol. Excess water and hydrazine are distilled off before the final heating step. This:
- Allows the reaction in one pot
- Significantly reduces reaction time
- Improves yields (especially for hindered ketones)
2. Bamford-Stevens Reaction
Tosylhydrazones (from p-toluenesulfonyl hydrazide) are used instead of hydrazine. Under basic conditions, this gives alkenes rather than alkanes (useful when a double bond is desired). Under acidic conditions, the mechanism shifts to give carbocations.
3. Caglioti Modification
Uses tosylhydrazone sodium salts under mild conditions (no need for elevated temperatures in some cases). Useful for sensitive substrates.
Applications
-
Deoxygenation of carbonyls: The primary use - converting ketones/aldehydes to CH₂ groups when the carbonyl is no longer needed after serving its synthetic purpose (e.g., as a directing group in previous steps).
-
Synthesis of alkyl-substituted benzenes (alkylbenzenes): Particularly useful when combined with Friedel-Crafts acylation. The sequence:
- Friedel-Crafts acylation (aromatic ring → aryl ketone)
- Wolff-Kishner reduction (aryl ketone → alkylbenzene)
- This is the preferred route over direct Friedel-Crafts alkylation (which suffers from carbocation rearrangements).
Example: Acetophenone → EthylbenzeneC₆H₅-CO-CH₃ → C₆H₅-CH₂-CH₃ -
Preparation of polycyclic aromatics: Used to build unbranched hydrocarbon side chains on aromatic rings.
-
Reduction of aliphatic and aliphatic-aromatic carbonyl compounds: Works on both simple and complex ketones/aldehydes.
-
Total synthesis of natural products: Applied in synthesis of:
- Scopadulcic acid B
- Aspidospermidine (via Huang Minlon modification as a key late step)
- Dysidiolide
- Steroid synthesis (reduction of steroidal ketones)
-
Large-scale industrial synthesis: Has been successfully scaled to kilogram quantities for synthesis of functionalized imidazole substrates when other reduction methods failed.
Comparison with Clemmensen Reduction
Both reactions accomplish the same overall transformation (C=O → CH₂), but differ in conditions:
| Feature | Wolff-Kishner | Clemmensen |
|---|---|---|
| Reagents | NH₂NH₂, KOH, heat | Zn(Hg), conc. HCl |
| Conditions | Strongly basic, high temp | Strongly acidic |
| Substrate preference | Acid-sensitive compounds | Base-sensitive compounds |
| Mechanism | Via hydrazone anion | Via organozinc intermediate |
| Solvent | Ethylene glycol | Aqueous HCl |
The choice between them is driven by the acid/base sensitivity of the substrate:
- Use Wolff-Kishner for acid-sensitive substrates (e.g., those containing acid-labile groups like acetals, certain esters).
- Use Clemmensen for base-sensitive substrates.
Limitations
- Requires very high temperature (~180-200°C) - not compatible with thermally sensitive substrates.
- Strongly basic conditions - incompatible with base-sensitive functional groups (esters, certain lactones, etc.).
- Double bond migration can occur as a side reaction when α,β-unsaturated carbonyl compounds are used, giving unexpected products instead of the desired alkane.
- Hindered ketones may react slowly or give incomplete conversion.
- The harsh conditions (high temp + strong base) limit functional group tolerance.
In summary, the Wolff-Kishner reduction is a reliable, well-understood method for deoxygenating aldehydes and ketones to alkanes under basic conditions, with N₂ loss as the thermodynamic driving force. Its most important synthetic use is in combination with Friedel-Crafts acylation to install clean, unbranched alkyl groups on aromatic rings - a strategy that avoids the carbocation rearrangements that plague direct Friedel-Crafts alkylation.
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Your PDF is ready. Here's what's included across 8 structured sections:
Sections in the PDF:
- Overall Reaction - equation, reagents table with roles
- Detailed Mechanism - full step-by-step tables for both stages (Stage 1: hydrazone formation in 4 steps; Stage 2: deoxygenation in 6 steps), reaction equations, and Szmant's kinetic insight highlighted
- Important Modifications - Huang Minlon, Bamford-Stevens, Caglioti, and Pre-formed Hydrazone methods (each in a colour-coded card)
- Synthetic Applications - 8 applications including Haworth synthesis, steroid chemistry, total synthesis (aspidospermidine, scopadulcic acid B), with the representative synthetic sequence shown
- Wolff-Kishner vs. Clemmensen Comparison - side-by-side feature table with a decision rule box
- Limitations - 6 key limitations in warning-style blocks
- Key Literature References - original papers (Wolff 1912, Kishner 1911, Szmant 1964, Huang Minlon 1946/1949)
- Summary at a Glance - quick-reference table of all key points
Wolff Kishner Reduction
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