C=O group of aldehydes and ketones is reduced to -CH2- group by clemmensen reduction. Zinc amalgam and concentrated hydrochloric acid ( Zn(Hg) / concentrated HCl ) is used as the reagent for clemmensen reduction.
Zn(Hg) and concentrated HCl
In this tutorial, we study about
Clemmesen reduction reduces aldehydes and ketones to hydrocarbons. Hydrocarbons are compounds which contains only hydrogen and carbon such as alkanes, alkenes, alkynes. In most occasions, alkanes are given by clemmensen reduction reaction.
NOTE: alkenes and alkynes don't react with clemmensen reagent.
Clemmensen reduction and its products of some aldehydes and ketones are explained below.
Ethanal is an aldehyde. Ethanal is reduced to ethane by clemmensen reduction. Zinc amalgam and concentrated HCl is used as the clemmensen reducing reagent. Ethane is an alkane compound.Learn more ethanal reduction reactions Ethanol to ethane | See ethane preparing reactions
Propanal is an aldehyde. When propanal and Zn(Hg) / concentrated HCl react, propane is given as the product. Propane also an alkane compound.
Propanone is a ketone compound. Propanone gives propane as the product with Zn(Hg)/concentrated HCl. Propane is an alkane compound.
Methylbenzene (toluene) is given when benzaldehyde reactes with clemmesen reagent ( Zn(Hg) / concentrated HCl ).
Zinc amalgam and concentrated DCl also preform as clemmensen reagent. This Zn(Hg) and DCl reduces aldehydes and ketones to hydrocarbons. Hydrogen atom in the HCl is replaced by deuterium atom. hydrogen and deuterium are Isotopes of hydrogen.
Several mechanisms have been proposed for the clemmensen reduction. One mechanism is explained below.
Zinc amalgam is prepared by mixing zinc metal (Zn) with mercury(Hg).
You can't separate aldehyde and ketone from this test because both aldehyde and ketone react with Zn[Hg] / concentrated HCl and give hydrocarbons as the products. Therefore you are unable to see any difference during the reaction or after the reaction.
As an example, you cannot separate propanal and propanone from clemmesen reduction reaction.
In the Wolff–Kishner reduction, the aldehyde or ketone is heated with hydrazine hydrate and a base (usually NaOH or KOH).
Clemmensen reduction gives alkane from aldehydes and ketones. Most of aldehydes and ketones exist in liquid or solid phase. But first alkanes are in gaseous phase. Sometimes there may be a phase change in reactants and products. Some examples are listed below.
both aldehydes and ketones can be reduced by clemmensen reagent or LiAlH4/ dry ether.
Zinc amalgam and concentrated HCl is used to reduce acetaldehyde to ethane. Acetaldehyde is CH3CHO and ethane is CH3CH3 which is an alkane compound.
Aldehydes and ketones can be reduced to alcohols by following reagents. Ketones are reduced to secondary alcohols and aldehydes are reduced to primary alcohols.
Ketone is reduced and alkanes are given as the product.
C6H5COCH3, acetaphenone is reduced to ethylbenzene C6H5CH2CH3 from Zn(hg) and HCl.
In the "MARCH’S ADVANCED ORGANIC CHEMISTRY" book, for clemmesen reduction reaction, it is mentioned that zinc amalgam and aqueous HCl is used. If aqueous HCl is used, H3O+ is given.
MARCH’S ADVANCED ORGANIC CHEMISTRY, 19-61, Reduction of Carbonyl to Methylene in Aldehydes and Ketones Dihydro-de-oxo-bisubstitution, pg 1835
Benzaldehyde is treated with Zn[Hg] and concentrated HCl to produce toluene.
Alkyl groups are not reduced from Zn or HCl or NaOH. But groups such as carbonyl groups are reduced from Zn[Hg] and concentrated HCl.
In nitrobenzene reuction to aniline, Sn(Hg) and aqueous NaOH is used.
If you are thinking to do clemmesen reduction in this way, it may not give desired product you are wishing to form.
You may think, when chlorine will react with water and produce hydrochloric acid (HCl). Not that only. With HCl, hypochlorous acid also is produced. So, we don't know what will react and what will happen in this reaction until we do an experiment or research to find what are the products.
Concentrated HCl is required. To form concentrated HCl by dissolving chlorine gas in the water is very hard and have to feed a large chlorine amount with high presssure.
Zinc amalgm is required to do the clemmensen reduction.
According to the question, we cannot say propanone will give propane as the only product and there is huge problem because there is only zinc instead of zinc amalgm.
There seems no reaction between esters and Zn(Hg) and concentrated HCl in text books.