Reduction: Aldehydes, Ketones and Carboxylic Acids, Videos & Examples

Reduction of Aldehydes and Ketones lead to the formation of alcohols. Do you know through reduction process we can obtain many important disinfectants which is a very necessary item in our daily life? This process is used in industries for the production of much important and wide range of solvents necessary during cosmetic production, esterification, disinfectant and many more. In this topic, we will study about the reduction of aldehyde and ketones for the production of alcohols and alkanes.

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There are primarily subdivisions of reduction of aldehydes and ketones on the basis of the type of end products. Reduction of aldehydes and ketones to:

Alcohols
Alkanes

Reduction to Alcohols

Aldehydes and ketones can undergo reduction process for the formation of either primary alcohol or secondary alcohol with the help of reagents, sodium borohydride (NaBH₄) or lithium aluminium hydride (LiAlH₄). Aldehydes and ketones can also form alcohol by the process of catalytic hydrogenation.

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Reducing Agents

Formation of alcohol from aldehydes or ketones requires either of the particular reducing agents. The reducing reagents are sodium borohydride (NaBH₄ ) or lithium aluminium hydride (LiAlH₄). Even though the reagents sound very complicated but the structure of the compounds are quite simple. The structure consists of four hydrogen atoms around aluminium for LiAlH₄ and around boron for the compound NaBH4.

Structure of Lithium tetrahydridoaluminate/ lithium aluminium hydride:

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Structure of Sodium Tetrahydridoborate/ sodium borohydride:

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Each negative ion has one coordinate covalent bond formed due to the lone pair present in the hydride ion. This helps in the formation of a bond with the empty orbital present on the aluminium or boron.

Overall Reactions Using Either of the Two Reducing Agents

Both the reagents (use lithium tetrahydridoaluminate or sodium tetrahydridoborate) result in the formation of the same product. There is no difference in the end product on use of either lithium tetrahydridoaluminate or sodium tetrahydridoborate. For instance, ethanal will produce ethanol irrespective of the reagent (sodium borohydride or lithium aluminium hydride).

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Therefore, Reduction of aldehydes produces primary alcohol. Similarly, ketones from the process lead to the formation of secondary alcohol. Moreover, there is no change in the end product irrespective of the reagent (sodium borohydride or lithium aluminium hydride). For example, propanone reacts to produce propan-2-ol.

Details of the Reactions

When the Reagent is Lithium Aluminum Hydride

The reaction with respect to lithium aluminium hydride excludes common solvents such as water and alcohol. This is because the reagent lithium aluminium hydride is more reactive in comparison to sodium borohydride. Thus, it can react violently with alcohol or water. Hence, solvents like these are avoided when this process takes place with the reagent lithium aluminium hydride.

The reaction takes place at room temperature in solution in the presence of dried ether (diethyl ether). The reaction will occur in two steps:

First Step: In the first step, the reaction produces an intermediate complex ion (salt along with aluminium ion). Refer to the below skeletal representation of any general aldehyde or ketone reaction to understand the two-step process.

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Second Step: In this step, treatment of the intermediate complex with the help of dilute acids such as dilute hydrochloric acid or dilute sulphuric acid takes place in order to release alcohol from the intermediate complex ion.

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When the Reagent is Sodium Borohydride

Sodium borohydride is less reactive than lithium tetrahydridoaluminate. Therefore, this reagent can work with solutions in water and solutions in alcohol. However, the solution must be alkaline in nature. This reaction is also a two-step process.

First step: In the first step, the solution of aldehyde or ketone in an alcohol (methanol, ethanol) will react on the addition of sodium borohydride to form an intermediate complex. The reaction condition varies which means the reaction is sometimes kept at room temperature and sometimes it is heated under reflex depending on the basis of nature of aldehydes and ketones.
Second Step: In the second stage, the addition of water to the mixture takes place and boiled together until alcohol is released from the complex. The recovery process is fractional distillation.

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Reduction of Aldehydes/Ketones to Hydrocarbons

There are two different reactions under this category.

Clemmensen Reduction: In this reaction reduction of the carbonyl group of aldehydes and ketones takes place into the CH₂ group on treatment of zinc amalgam along with conc. HCl.
Wolff-Kishner Reduction: In this reaction carbonyl group of aldehydes and ketones takes place into a CH₂ group on treatment with hydrazine. Thereafter the reaction involves heating with potassium or sodium hydroxide in the high boiling solvent like ethylene glycol.

1) Clemmensen Reduction

When aldehyde or ketone reduction occurs under zinc amalgam (an alloy of Zn and Hg) and conc. HCl, in order to produce hydrocarbon, refers to Clemmensen reduction.

In this reaction process, heating of carbonyl compound occurs with fine amalgamated zinc particles in an aqueous mixture of hydroxylic solvent containing a mineral acid like HCl. The zinc amalgam just serves as a clean active metal surface but do not participate in the reaction.

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2) Wolff-Kishner Reduction

Aldehydes and Ketones undergo reaction with hydrazine to form hydrazine derivative. The hydrazine derivative can undergo further reaction with base and heat to form the corresponding alkane. Combination of these two reactions is called Wolff-Kishner Reduction.

This method together represents a general method for converting aldehydes and ketones into alkanes. Usually, a solvent having high boiling point such as ethylene glycol for providing high temperature to the reaction. Nitrogen gas is released as part of the reaction.