16 Preparation of Alkanes
16.0.1 Wurtz reaction
Wurtz reaction is coupling of haloalkanes using sodium metal in solvent like dry ether
2R-X + 2Na → R-R + 2Na+X−
Mechanism
The reaction consists of a halogen-metal exchange involving the free radical species R• (in
a similar fashion to the formation of a Grignard reagent and then the carbon-carbon bond
formation in a nucleophilic substitution reaction.)
One electron from the metal is transferred to the halogen to produce a metal halide and an
alkyl radical.
R-X + M → R• + M+X−The alkyl radical then accepts an electron from another metal atom to form an alkyl anion
and the metal becomes cationic. This intermediate has been isolated in a several cases.
R• + M → R−M+The nucleophilic carbon of the alkyl anion then displaces the halide in an SN2 reaction,
forming a new carbon-carbon covalent bond.
R−M++ R-X → R-R + M+X−16.0.2 Clemmensen reduction
Clemmensen reduction is a reduction of ketones (or aldehydes) to alkanes using zinc amal-
gam and hydrochloric acid
Figure 37 The Clemmensen reduction