Wolff-Kishner Reduction
Aldehydes and ketones can be
converted to a hydrazine derivative by reaction with hydrazine. These
"hydrazones" can be further converted to the corresponding alkane by
reaction with base and heat. These two steps can be combined into one reaction
called the Wolff-Kishner Reduction which represents a general method for
converting aldehydes and ketones into alkanes. Typically a high boiling point
solvent, such as ethylene glycol, is used to provide the high temperatures
needed for this reaction to occur. Note! Nitrogen gas is produced as part of
this reaction.
Wolff-Kishner reduction
mechanism begins with the formation of a hydrazone anion which then releases
the nitrogen molecule to form a carbanion. This carbanion then reacts with the
water in the system to give a hydrocarbon. Typically, diethylene glycol is used
as a solvent for this method.
This reduction is an organic
reaction where aldehydes and ketones are reduced to alkanes. Some
carbonyl compounds are stable in strongly basic conditions, hence they can be
easily reduced to alkanes (The carbon-oxygen double bond becomes two
carbon-hydrogen single bonds).
Although the mechanism
usually begins with the condensation of hydrazine to give a hydrazone, the
usage of a pre-formed hydrazone can have advantages such as reduced reaction
time, reactions that proceed at room temperature or very mild reaction
conditions. The pre-formed hydrazone substrates that can be used in this
reduction also require different solvents and reaction temperatures.
Wolff Kishner Reduction Mechanism
Step 1: Formation of hydrazone
The aldehyde or ketone is
subjected to hydrazine. This yields the hydrazone required for the process. The
reaction is illustrated below.
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| Wolff-Kishner-Reduction-Mechanism |
Step 2: Deprotonation of Nitrogen
The terminal nitrogen atom
is deprotonated, and it proceeds to form a double bond with the neighbouring
nitrogen atom. The released proton attaches itself to the hydroxide ion from
the basic environment to form water.
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| Wolff-Kishner-Reduction-Mechanism |
Step 3: Protonation of the Carbon
Since oxygen is more
electron-withdrawing than carbon, the carbon is protonated by the water
molecule as shown below.
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| Wolff-Kishner-Reduction-Mechanism |
Step 4: Deprotonation of Nitrogen
The terminal nitrogen is
deprotonated again, this time forming a triple bond with its neighbouring
nitrogen atom. This results in the formation of a carbanion where the two
triple-bonded nitrogens are released as nitrogen gas. Similar to step 2, the
ejected proton forms water along with the basic environment.
Step 5: Protonation of Carbon
Similar to step 3 of the
Wolff-Kishner reduction mechanism, the carbon is protonated by water, resulting
in the formation of the desired hydrocarbon product as shown. Thus, the
aldehyde or ketone is converted to an alkane.
The rate-determining step of
this reaction is the bond formation of the terminal carbon with hydrogen (in
the hydrazone anion). The carbon-hydrogen bond formation is helped by
mildly electron-withdrawing substituents. Highly electron-withdrawing
substituents decrease the negative charge of the terminal nitrogen, making it
difficult to break the N-H bond.
The Wolff-Kishner reduction has been modified into several techniques, each with its own advantages and disadvantages. For example – the Huang Minlon modification (using the carbonyl compound, 85% hydrazine and potassium hydroxide as the reagent) offers reduced reaction time and the achievement of higher temperatures but requires distillation.
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