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CLEMMENSEN REDUCTION : EXPLANATION

* The Clemmensen reduction is used to conveniently reduce the carbonyl compounds, which are stable to strongly acidic conditions, to alkanes. 

clemmensen reduction 1-1

* In Clemmensen reduction, the amalgamated zinc in HCl is used as reducing agent. 

* The C=O group is converted to CH2 group.

* The Clemmensen reduction is complementary to Wolff-Kishner reduction, which may be used to reduce acid sensitive compounds.

 

MECHANISM OF CLEMMENSEN REDUCTION

 

* The Clemmensen reduction occurs over the surface of zinc catalyst. The probable mechanism is shown below. 

clemmensen reduction 1-2 

* There is a net flow of electrons from zinc to the carbonyl compound.

* As there is no formation of alcohol during the reaction, this method is not useful to reduce alcohols to alkanes.

 

ILLUSTRATIONS

1) 

  Zn-Hg  
CH3CHO + 4(H)   -----------------> CH3CH3 + H2O
   HCl    

2) clemmensen reduction 1-3

3) In the following reaction, along with the reduction of carbonyl group, the -OH group is substituted by the -Cl group (side reaction). However this side reaction can be avoided by employing Wolff-Kishner method.

clemmensen reduction 1-4

4) But the phenol group is not affected in Clemmensen reduction. (Why? Ans: Nucleophilic substitution is not easy on sp2 carbon of benzene ring!)

clemmensen reduction 1-5

 

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Author: Aditya vardhan Vutturi Google+ profile