Clemmensen reduction reaction is applied to lower carbonyl compounds to simple hydrocarbons. The reaction gets via carbanion intermediate. Organic compounds are found all around us and build the elemental substances used in daily life. You will be amazed by the amount of organic chemistry going around you on an ordinary basis, even at this very moment! Among the list of these life-supporting molecules is one very essential species recognized as carbonyl compounds.

Carbonyls are majorly established by two classes of compounds—aldehydes and ketones. These organic molecules go through various reactions, including reduction reactions, such as Clemmensen’s reduction. This article analyzes the reaction in detail.

Aldehydes and ketones are organic compounds that have the carbonyl group integrated into them. A carbonyl group is one in which two of the four valences of a carbon atom are fulfilled by the double bond with an oxygen atom. It is the other two valences of the carbon which decide whether it is an aldehyde or a ketone. Aldehydes and ketones are detected in a wide collection in nature, and many of them are also incorporated artificially.

An aldehyde is a compound with a carbonyl group at the end. In aldehydes, one side of the carbonyl functional group is fulfilled by a hydrogen atom, while an alkyl group fills the other side. Alkyl groups are nothing but simple compounds of carbon. The aldehyde group is also known as a formyl or methanol group.

Vanillin (found in Vanilla), cinnamaldehyde (from cinnamon bark), and benzaldehyde (found in almonds) are some of the most frequently found natural aldehydes. Ketones are carbonyl compounds in which both the resting valences of the carbonyl functional group are fulfilled by alkyls. As a result, these compounds are more abundant and denser than aldehydes.

Camphor, R-carvone (from spearmint oil), and z-jasmone (found in jasmine) are some commonly occurring natural ketones.

What is Clemmensen Reduction?

Clemmensen reduction is an organic chemical reaction in which we transform ketone or aldehydes into an alkane. We require to use a catalyst for this reaction; it is amalgamated zinc (mercury alloyed with zinc ) with hydrochloric acid. Therefore, the mercury mixed with zinc does not engage in the reaction. It only arranges a clean, active surface for the reaction. The name of the processes evolved after the Danish scientist Erik Christian Clemmensen.

Clemmensen Reduction Mechanism
Clemmensen Reduction Mechanism

This process is eminently effective in the reduction of aryl-alkyl ketones. Moreover, zinc metal reduction is much more effective with aliphatic or cyclic ketones. More importantly, the substrate of this reaction has to be unreactive towards the greatly acidic conditions of the reaction.

Mechanism of Clemmensen Reduction Reaction:

The mechanism of this reaction is not fully understood, but there are two proposals;

Carbanionic mechanism:

The carbanionic mechanism of reaction shows that the zinc attacks straight to the protonated carbon.

Carbanionic Mechanism
Carbanionic Mechanism

Carbenoid mechanism:

While the carbenoid mechanism is a profound process and diminishes the happenings on zinc metal surface. The reduction takes place at the face of the zinc catalyst. In this reaction, alcohols are not postulated as intermediates, because subjection of the equivalent alcohols to these same reaction conditions does not manage to alkanes.

The elemental substance must not react to acidic cases. The acid-sensitive base substance reacts in the Wolff-Kishner reduction that has a strong base if it is milder than it is Mozingo reduction. The reaction is not for the substances that are sensitive to acids.

Carbenoid Mechanism
Carbenoid Mechanism

The heterogeneous nature forms the mechanism remains complicated, in spite of its antiquity, and the studies on the mechanism are challenging. There are only a few studies on the particular reaction introduced like possibly zinc carbenoids and organozinc intermediates.

Why is amalgamated zinc used as a catalyst?

Amalgamated zinc is Zinc dissolved in mercury, and is applied as a catalyst in this reaction. Powdered Zinc or pieces of metal cannot be used to precisely catalyze the reduction, as they are not activated enough. The addition of mercury to the Zinc smoothly distributes the metal throughout the amalgam, increasing the rate of reaction and also boosting the activation energy of Zinc to a level mandatory for the progress of the reaction.

Also, pure Zinc would quickly react with the hydrochloric acid exist, forming zinc chloride and discharging stable hydrogen gas that would quickly depart the reaction zone. However, when this reaction continues with zinc amalgam, the particular hydrogen molecules assembled by the reaction of HCL with zinc continue in a reactive state (sometimes announced “nascent hydrogen”) and act with the carbonyl compound (ketone or aldehyde) to start the reduction reaction.

Precautions to be taken during Clemmensen’s reduction

Clemmensen’s reduction is not good for substances sensitive to acids. If the reactant molecule blends an acid-sensitive group, such as the hydroxyl group (-OH), the hydrogen ions will attack them instead of attacking the carbonyl group, resulting in the misstep of the reaction.

Again, this operation cannot be utilized in reducing carboxylic acids, as the proton will not attack the reactive carbon of the acid group. Instead, soda-lime can be used for the same purpose.

Why Would You Prefer The Wolff-Kishner Over the Clemmensen, Or Vice Versa?

It’s somewhat rare to encounter conditions in an introductory class where a Wolff Kishner would be called for over a Clemmensen, or vice versa, but here are some things to think about.

The Wolff-Kishner is done under strongly basic conditions using high heat in a polar protic solvent.
The Clemmensen is performed in strongly acidic conditions. If you have a protecting group somewhere which can be removed with acids, such as an acetal or silyl ether, consider an alternative.

Two other methods deserve mention, although you might not see them covered until later in the course when ketone chemistry is addressed.

Frequently Asked Questions About Clemmensen Reaction

What Is Meant By Clemmensen Reduction?
Clemmensen Reduction Is A Chemical Reaction Described As A Reduction Of Ketones (Or Aldehydes) To Alkanes Using Zinc Amalgam And Concentrated Hydrochloric Acid. The Substrate Must Be Tolerant Of The Strongly Acidic Conditions Of The Clemmensen Reduction (37% HCl).
What Is Clemmensen Reduction Give An Example?
The Reaction Of Aldehydes And Ketones With Zinc Amalgam (Zn/Hg Alloy) In Concentrated Hydrochloric Acid, Which Reduces The Aldehyde Or Ketone To A Hydrocarbon, Is Called Clemmensen Reduction.
Does Clemmensen Reduction Reduce Alcohol?
The Clemmensen Reduction Occurs Over The Surface Of The Zinc Catalyst. As There Is No Formation Of Alcohol During The Reaction, This Method Is Not Useful To Reduce Alcohol To Alkanes.
Is Clemmensen Reduction Acid Sensitive?
However, Clemmensen Reduction Reaction Is Not Suitable For Substances Sensitive To Acids, I.E., The Substrate Must Be Unreactive To The Strongly Acidic Conditions Of The Clemmensen Reduction. This Mechanism Employs The Intermediacy Of Zinc Carbenoids To Rationalize The Mechanism Of The Clemmensen Reduction.