How to know when we have to use clemmensen reduction and wolf kishner reduction??
Answers
Answer:
Explanation:
The Wolff Kishner reduction of ketones utilizes hydrazine (NH2NH2) as the reducing agent in the presence of strong base (KOH) in a high-boiling protic solvent (ethylene glycol, HO-CH2CH2-OH, boiling point 197 °C).
The driving force for the reaction is the conversion of hydrazine to nitrogen gas.
This is not exactly a gentle process; heating to almost 200 °C is required to make the reaction occur at a reasonable rate. [note]
The first step is formation of a hydrazone from the ketone (hydrazones are a cousin of imines, which we cover later in the course). Hydrazine (NH2NH2) adds to the carbonyl, and following a series of proton transfer steps, water is expelled. Click here to see an image of the mechanism for hydrazone formation
Once the hydrazone is formed, the real action in the Wolff-Kishner begins!
The NH2 of the hydrazone is reasonably acidic (pKa about 21) and can be deprotonated by strong base at a high enough temperature (the base is likely the conjugate base of ethylene glycol, not KOH). This deprotonation appears to be the rate-limiting step.
The next step is the trickiest: protonation on the carbon. With the caveat that resonance forms don’t really exist, it can be helpful to imagine forming the resonance form of this species that has a negative charge on the carbon, and then protonating it with solvent (ethylene glycol).
This gives a species with a nitrogen-nitrogen double bond , which , after deprotonation by base, decomposes irreversibly to give nitrogen gas and a carbanion (i.e. a negatively charged carbon).
The Clemmensen Reduction
A second way to go about reducing the carbonyl of an aromatic ketone is to use a reaction known as the Clemmensen Reduction. The reductant here is “zinc amalgam” (Zn-Hg) which is used under acidic conditions; one method calls for the presence of aqueous HCl, for example:
This process works best for aromatic ketones; non-aromatic ketones, not so much. The mechanism has not been thoroughly worked out; it’s thought to occur through a series of one-electron transfers from zinc amalgam.