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.
Make certain that you can define, and use in context, the key term below.
- Wolff‑Kishner reduction
After studying this section, you can add yet another method of reducing organic compounds to your growing list of reduction reactions.
Reaction of Aldehydes or Ketones with Hydrazine Produces a Hydrazone
Aldehydes and ketones can be converted to a hydrazone derivative by reaction with hydrazine (H2NNH2). Hyrazone 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" represent 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 Produces 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 akanes called the Wolff-Kishner Reduction. Overall, the Wolff-Kihner reduction removes the carbonyl oxygen in the form of water by forming an intermediate hydrazone. They hydrazone then undgergoes loss of N2 gas along with protonation to give the alkane reaction product. Note that the Clemminson reduction accomplishes the same transformation of a carbonyl to an alkane.
Predicting the Products of a Wolff-Kishner Reduction
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.T he 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 the protonated to form a netural intermediate. A second weakly acidic N-H bond is deprotonated which causes the formation of N2 gas and a carbanion. In the final step the carbonion is protonated to form an alkane product.
3) Second deprotonation
4) The carbanion is protonated to form the alkane product.
1) Please draw the products of the following reactions.