Clemmensen Reduction of Aldehydes and Ketones by Zn(Hg) and Concentrated HCl

Carbonyl group (C=O group) of aldehydes and ketones compounds is reduced to methylene (-CH2-) group by clemmensen reduction process. Zinc amalgam and concentrated hydrochloric acid ( Zn(Hg) / concentrated HCl ) is used as the clemmensen reagent.

clemmensen reduction


  1. Clemmensen reduction reagent
  2. What is clemmesen reduction
  3. Examples of Clemmensen reduction
    • Clemmensen reduction of Ethanal (Acetaldehyde)
    • Clemmensen reduction of Propanal
    • Clemmensen reduction of propanone
    • Zinc amalgam and concentrated DCl with ethanal
  4. Mechanism of Clemmensen reduction
  5. Clemmensen reduction and LiAlH4 reaction with aldehyde and ketone

Clemmensen reduction reagent

Zinc amalgam ( Zn(Hg) ) and concentrated hydrochloric acid (HCl) is used as the clemmensen reagent.

What is clemmensen reduction?

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. This clemmensen reduction reaction is a method of preparation of alkanes.

NOTE: alkenes and alkynes are not reduced by clemmensen reagent.

Examples of Clemmensen reduction

Here, we are going to observe som examples how Clemmensen reduction gives products when aldehydes and ketone compounds react with Zinc amalgam and concentrated HCl acid.

Clemmensen reduction of Ethanal (Acetaldehyde)

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.

clemmensen reduction of ethanal

Learn more ethanal reduction reactions Ethanol to ethane | See ethane preparing reactions

Clemmensen reduction of Propanal

Propanal is an aldehyde. When propanal and Zn(Hg) / concentrated HCl react, propane is given as the product. Propane also an alkane compound.

clemmensen reduction of propanal

Clemmensen reduction of propanone

Propanone is a ketone compound. Propanone gives propane as the product with Zn(Hg)/concentrated HCl. Propane is an alkane compound.

clemmensen reduction of propanone

Benzaldehyde and clemmensen reduction

Methylbenzene (toluene) is given when benzaldehyde reacts with clemmesen reagent.

Benzaldehyde and clemmensen reagent


Zinc amalgam and concentrated DCl with ethanal

Zinc amalgam and concentrated DCl also preform as the clemmensen reagent. Only difference is that, Hydrogen atom of HCl has been replaced by a deuterium atom. This Zn(Hg) and DCl reduces aldehydes and ketones to hydrocarbons. As the result, carbonyl group is reduced and two deuterium atoms are added to the Carbonyl carbon atom.

Zinc amalgam and concentrated DCl with ethanal

Clemmensen reduction mechanism

Several mechanisms have been proposed for the clemmensen reduction. One mechanism is explained below.

Clemmensen reduction mechanism

Clemmensen reduction and LiAlH4/ dry ether with Aldehydes and Ketones

Both aldehyde and ketones compounds can be reduced by clemmensen reagent or LiAlH4 / dry ether.

  • Aldehydes and ketones are reduced to alcohols by LiAlH4/ dry ether.
ketones and LiAlH4 ether reaction

  • Aldehydes and ketones are reduced to hydrocarbons by clemmensen reagent.


What is the difference in Wolff–Kishner reduction and clemmensen reduction

In the Wolff–Kishner reduction, the aldehyde or ketone is heated with hydrazine hydrate and a base (usually NaOH or KOH).

How to prepare zinc amalgam?

Zinc amalgam is prepared by mixing zinc metal (Zn) with mercury (Hg).

Is it possible to separate aldehydes and ketones from clemmensen reduction?

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.

Learn aldehydes and ketones separating methods

Is there a phase change in clemmensen reduction?

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.

  • Benzaldehyde to toluene : liquid to liquid - no phase change
  • Ethanal to ethane : Ethanal can be a liquid or gas. Ethane is a gas in room temperature.

Do esters react with Zn/Hg and concentrated HCl acid?

There seems no reaction between esters and Zn(Hg) and concentrated HCl in text books.

Reduction of acetaldehyde in presence of zinc amalgm ( Zn(Hg) ) with concentrated HCl

Zinc amalgam and concentrated HCl is used to reduce acetaldehyde to ethane. Acetaldehyde is CH3CHO and ethane is CH3CH3 which is an alkane compound.

What are the other methods of reducing aldehydes and ketones?

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 + Zn(Hg) + HCl which organic compound is given as the product?

Ketone is reduced and alkanes are given as the product.

C6H5COCH3 + Zn(Hg) / concentrated HCl

C6H5COCH3, acetaphenone is reduced to ethylbenzene C6H5CH2CH3 from Zn(hg) and HCl.

Ketone Zn/Hg H3O+

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

Toluene by clemmensen reduction reaction class 11

Benzaldehyde is treated with Zn[Hg] and concentrated HCl to produce toluene.

Does Zn, HCl, NaOH reduce alkyl groups?

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 reduction to aniline, Sn(Hg) and aqueous NaOH is used.

Reaction between propanone with zinc and chlorine water

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 pressure.

Zinc amalgam 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 amalgam.

Related tutorials to clemmensen reduction

Ethanal reduction Carboxylic acid to aldehyde Physical properties of aldehydes and ketones (carbonyl-compounds) Reaction of benzaldehyde and aniline Aldehyde and ketone reduction by LiAlH4