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15.18 It Is Important to Have More Than One Way to Carry Out a Reaction

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    16387
  • The reaction of aldehydes and ketones with zinc amalgam (Zn/Hg alloy) in concentrated hydrochloric acid, which reduces the aldehyde or ketone to a hydrocarbon, is called Clemmensen reduction.

    Introduction

    This alternative reduction involves heating a carbonyl compound with finely divided, amalgamated zinc in a hydroxylic solvent (often an aqueous mixture) containing a mineral acid such as HCl. The mercury alloyed with the zinc does not participate in the reaction, it serves only to provide a clean active metal surface.  

     

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    Example

     

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    Possible mechanism

    The mechanism of Clemmensen reduction is not fully understood; intermediacy of radicals are implicated. Clemmensen reduction is complementary to Wolff-Kishner reduction, which also converts aldehydes and ketones to hydrocarbons, in that the former is carried out in strongly acidic conditions and the latter in strongly basic conditions.

    http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Images2/clemsen2.gif

    References

    1. Clemmensen, E. Ber. Dtsch. Chem. Ges. 1913, 46, 1837–1843.
    2. Clemmensen, E. Ber. Dtsch. Chem. Ges. 1914, 47, 51–63.
    3. Clemmensen, E. Ber. Dtsch. Chem. Ges. 1914, 47, 681–687.

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