➠The reduction of carbonyl group of an aldehyde or a ketone to the methylene group with zinc-amalagamate (Zn-Hg) and concentrated hydrochloric acid (HCl) is known as Clemmenson Reduction.
➠The reaction undergoes reduction under acidic conditions which involves the protonation of a carbonyl group to which the electron transfer from the metal centre occurs (SET Mechanism).
➠E.g.
➠Mechanism:-
➠The reaction undergoes reduction under acidic conditions which involves the protonation of a carbonyl group to which the electron transfer from the metal centre occurs (SET Mechanism).
➠E.g.
➠Special Features:-
➥The acid should be concentrated to prevent intramolecular condensation of a carbonyl group.
➥The reaction fails with acid sensitive groups and high molecular weight carbonyl compounds.
➥The alpha-beta unsaturated ketones and other functional groups containing compounds can be used for the reduction.
➥This reaction is very particular for the reduction of a carbonyl centre.
➥A 1,3-dicarbonyl compound doesn't give the normal product, it depends upon the concentration of Zn metal.
➥This reduction can be used for the elimination of an alpha-substituted ketone.
➠Examples:-
➠Applications:-
➥It is generally used to form a long chain alkyl group in an aromatic ring through acyclation followed by Clemmenson Reduction.
➥Clemmenson Reduction can also be used to reduce the keto acids which upon further reaction can be converted into a naphthalene.
E.g.
➠Question:- Identify the reagents used?
➠Answer:- (c) Zn/HCl.
End
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