Reaction Mechanism Steps

  1.   This is a reduction reaction

  1.   If R’ = H, then a methyl group (CH3) is formed

  1. If R’ = an alkyl group, then a methylene group (CH2) is formed

  1.   Zn(Hg) is a zinc-mercury “amalgam” (i.e. a mixture of metals) formed from mixing pure zinc metal with mercury metal.  They mix spontaneously

Reaction Characteristics

  1.   This reaction takes place under extremely acidic conditions.  Therefore the substrate must be able to withstand the reaction conditions

  1.   If the substrate is unable to withstand the acidic conditions required, the same product can be obtained via the Wolff-Kishner reduction, which takes place under basic conditions.  See HERE

1.  GENERAL REACTION characteristics
2.  reaction mechanism
3.  reaction examples

NOTE that these are the same products formed from the Wolff-Kishner reduction described HERE

the reaction begins by protonation of the carbonyl group by the HCl

the resulting protonated ketone or aldehyde has a resonance form that places positive charge on the carbon atom

once the carbon is protonated, the resulting alcohol undergoes protonation to form a water molecule which is able to depart as a leaving group to form a carbocation

the Zn(Hg) amalgam again transfers electrons to the positively charged carbon to form another transient anion that meets the same fate as the previous one, i.e. protonation by HCl to form the final reduced product

the Zn(Hg) amalgam transfers two electrons to the positively charged carbon atom to form a transient carbanion