Clemmensen reduction

The Clemmensen reduction is a reaction in the field of organic chemistry. It was named after its discoverer, Erik Christian Clemmensen, who in 1913 first described in a publication. Using this reaction can be reduced in hydrochloric acid to the underlying alkanes, ketones and aldehydes by reaction with zinc amalgam.

In general, the procedure is such that in a first step, the zinc is amalgamated with mercury salts and washed with hydrochloric acid, to remove an oxide passivation. Then it is mixed with fresh concentrated hydrochloric acid and added the carbonyl compound at the boiling point. Typical reaction times are about 20 hours.

Serve as substrates aldehydes, aliphatic ketones and araliphatic ketones ( with an aliphatic and an aromatic residue), with diaryl ketones generally react badly The Clemmensen reduction is more suitable for low molecular weight compounds low steric hindrance. A variant using activated zinc in a cooled to 0 ° C ethereal solution in which hydrogen chloride gas is passed. Both variants are naturally limited to acid-stable compounds. Acyloins and α - β -unsaturated carbonyl compounds are poor substrates. Complementary Wolff -Kishner reaction, which in turn is suitable for base-stable compounds.

The mechanism is not limited to secured. We discuss two mechanisms. One thing is certain:

• The reaction proceeds on the metal surface.
• The reaction does not proceed on the free alcohol, as such an alcohol, is added as substrate purpose can not be reduced.
• It is a repeated one-electron transfer ( engl.: Single Electron Transfer = SET), in the course of a zinc -carbene complex is formed.

Mechanism 1: the acidity of the oxygen of the carbonyl group is protonated. Then electron transfer a zinc -carbon bond is formed. After re- protonation and electron transfer water is split off and formed the zinc -carbene complex. By a double protonation and two-time electron transfer of the hydrocarbon is released.

Mechanism 2: It is transferred to the carbonyl carbon of an electron. Thus an oxygen -zinc bond is formed, and also gives rise to a carbon radical. By two-time electron transfer of oxygen to the zinc is transferred and formed a carbon -zinc carbene. Then, the hydrocarbon is released as described above by protonation and electron transfer.

In both cases ( not shown ) is released, the appropriate amount of zinc in the form of zinc chloride.