Clemmensen reduction
The Clemmensen reduction is a reaction from the field of organic chemistry . It was named after its discoverer Erik Christian Clemmensen , who described it for the first time in a publication in 1913. With the help of this reaction, ketones and aldehydes can be reduced to the underlying alkanes by reaction with amalgamated zinc in hydrochloric acid .
mechanism
The general procedure is that, in a first step, the zinc is amalgamated with mercury salts and washed with hydrochloric acid in order to remove oxidic passivation . Fresh concentrated hydrochloric acid is then added and the carbonyl component is added at the boiling point . Typical reaction times are over 20 hours.
Aldehydes, aliphatic ketones and araliphatic ketones (with one aliphatic and one aromatic radical) are used as substrates, with diaryl ketones generally reacting poorly. The Clemmensen reduction is more suitable for low molecular weight compounds with little steric hindrance . A variant uses activated zinc in an ethereal solution cooled to 0 ° C , into which hydrogen chloride gas is passed. Both variants are naturally limited to acid-stable compounds. Acyloins and α, β-unsaturated carbonyl compounds are unsuitable substrates. The Wolff-Kishner reaction is complementary and is in turn suitable for base-stable compounds.
The mechanism is not fully secured. Two mechanisms are discussed. That's for sure:
- The reaction takes place on the metal surface.
- The reaction does not proceed via the free alcohol , since such an alcohol, which is intentionally added as a substrate, cannot be reduced.
- It is a repeated single electron transfer (SET), in the course of which a zinc- carbene complex is formed.
Mechanism 1: The acid protonates the oxygen of the carbonyl group. A zinc-carbon bond is then formed with electron transfer. After renewed protonation and electron transfer, water is split off and the zinc-carbene complex is formed. The hydrocarbon is released through two protonation and two electron transfers.
Mechanism 2: An electron is transferred to the carbonyl carbon. Characterized an oxygen-zinc bond is formed and simultaneously a carbon arises radical - anion . The oxygen is transferred to the zinc by two electron transfers and a carbon-zinc-carbene is formed. Then, as above, the hydrocarbon is released through protonation and electron transfer.
In both cases (not shown here) the corresponding amount of zinc is released in the form of zinc chloride .
See also
Individual evidence
- ↑ Erik Clemmensen: Reduction of ketones and aldehydes to the corresponding hydrocarbons using amalgamated zinc and hydrochloric acid. In: Reports of the German Chemical Society. 46, No. 2, 1913, pp. 1837-1843, doi : 10.1002 / cber.19130460292
- ↑ Heinz GO Becker, Rainer Beckert, Günter Domschke, Egon Fanghänel , Wolf D. Habicher, Peter Metz, Dietrich Pavel, Klaus Schwetlick : Organikum , 21st edition, Wiley-VCH, Weinheim, 2001, p. 590.