Preparation of Alkanes
The Clemmensen reduction is particularly effective at reducing aryl-alkyl ketones. With
aliphatic or cyclic ketones, zinc metal reduction is much more effective The substrate must
be stable in the strongly acidic conditions of the Clemmensen reduction. Acid sensitive
substrates should be reacted in the Wolff-Kishner reduction, which utilizes strongly basic
conditions; a further, milder method is the Mozingo reduction. As a result of Clemmensen
Reduction, the carbon of the carbonyl group involved is converted from sp^2 hybridisation
to sp^3 hybridisation. The oxygen atom is lost in the form of one molecule of water.
16.0.3 Wolff-Kishner reduction
Figure 38 The Wolff-Kishner reduction
The Wolff–Kishner reduction is a chemical reaction that fully reduces a ketone (or aldehyde)
to an alkane. Condensation of the carbonyl compound with hydrazine forms the hydrazone,
and treatment with base induces the reduction of the carbon coupled with oxidation of the
hydrazine to gaseous nitrogen, to yield the corresponding alkane.
Mechanism
Figure 39 The mechanism of the Wolff-Kishner reduction
Themechanismfirstinvolvestheformationofthehydrazoneinamechanismthatisprobably
analogous to the formation of an imine. Successive deprotonations eventually result in the
evolution of nitrogen. The mechanism can be justified by the evolution of nitrogen as the
thermodynamic driving force. This reaction is also used to distinguish between aldehydes
and ketones.