19.10: Nucleophilic Addition of Hydrazine - The Wolff-Kishner Reaction
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Objectives
After completing this section, you should be able to
- write an equation to illustrate the Wolff‑Kishner reduction of an aldehyde or ketone.
- identify the product formed from the Wolff‑Kishner reduction of a given aldehyde or ketone.
Key Terms
Make certain that you can define, and use in context, the key term below.
- Wolff‑Kishner reduction
Study Notes
After studying this section, you can add yet another method of reducing organic compounds to your growing list of reduction reactions.
Aldehydes and ketones can be converted to a hydrazone derivative by reaction with hydrazine (H 2 NNH 2 ). Hydrazone formation is a variation of the imine forming reaction discussed in the previous section.
Reaction with a Base and Heat Converts a Hydrazone to an Alkane
Hydrazones can be further converted to the corresponding alkane by reaction with a base, usually KOH, and heat. Typically a high boiling point solvent, such as ethylene glycol, is used to provide the high temperatures needed for this reaction to occur. In the examples below the symbol " Δ" represents the addition of heat to a reaction. During this reaction nitrogen gas, which contains a very stable N-N triple bond, is produced.
Both Reactions Together Produce the Wolff-Kishner Reduction
These two steps previously discussed can be combined to provide a general reaction for the conversion of aldehydes and ketones to alkanes called the Wolff-Kishner Reduction. Overall, the Wolff-Kishner reduction removes the carbonyl oxygen in the form of water by forming an intermediate hydrazone. The hydrazone then undergoes loss of N 2 gas along with protonation to give the alkane reaction product. Note that the Clemmensen reduction accomplishes the same transformation of a carbonyl to an alkane under acidic conditions.
Predicting the Products of a Wolff-Kishner Reduction
Examples
Conversion of Cyclopentanone to cyclopentane
Conversation of Acetophenone to Ethylbenzne
Mechanism of the Wolff-Kishner Reduction
Hydrazine reacts with a carbonyl to form a hydrazone using a mechanism similar to that of an imine formation discussed in the previous section. The weakly acidic N-H bond is deprotonated to form the hydrazone anion. The hydrazone anion has a resonance structure that places a double bond between the nitrogens and a negative charge on carbon. The hydrazone anion is then protonated to form a neutral intermediate. A second weakly acidic N-H bond is deprotonated which causes the formation of N 2 gas and a carbanion. In the final step the carbanion is protonated to form an alkane product.
Step 1: Deprotonation
Step 2: Protonation
Step 3: Second deprotonation
Step 4: The carbanion is protonated to form the alkane product.
Exercise \(\PageIndex{1}\)
Please draw the products of the following reactions.
- Answer
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