Reduction of Aldehydes and Ketones to Alcohols by LiAlH₄

Lithium aluminium hydride (LiAlH₄) is a strong reduction reagent used in organic chemistry. LiAlH₄ can reduce aldehyde and ketone compounds to alcohols. When an aldehyde is reduced by LiAlH₄, a primary alcohol is given as the product. But, reduction of ketone with the same reagent will give a secondary alcohol as the result. In this tutorial, we will learn in detail how these reactions are occurred and products are given.

Content

1. Lithium Aluminum Hydride - LiAlH₄
   • Advantages of using LiAlH₄ in reduction of aldehyde and ketone
2. Steps of adding the reagent
3. Products of Aldehyde and Ketone reduction by LiAlH₄
   • Aldehyde reduction by LiAlH₄
   • Ketone reduction by LiAlH₄
4. Mechanism of Reduction of aldehydes and ketones by LiAlH₄
   • Aldehyde reduction mechanism - Ethanal + LiAlH₄
5. LiAlH₄ as a reducing agent in organic chemistry
   • Reagents which can reduce aldehyde and ketone to alcohols
   • LiAlH₄ does not reduce alkenes and alkynes to alkanes
   • Comparison and difference of LiAlH₄ and NaBH₄ in reduction reactions

Lithium Aluminum Hydride - LiAlH₄

Lithium aluminum hydroxide is a strong reducing agent in organic chemistry. It can reduce aldehyde, ketone, carboxylic acids, amide and more organic compound types. In this tutorial we mostly consider about, reduction of aldehyde and ketone by LiAlH₄. To keep LiAlH₄ stable, it should be stored with ether or THF (Tetrahydrofuran).

Lithium aluminum hydride can easily reduces aliphatic, aromatic, alicyclic, and heterocyclic aldehydes, containing double or triple bonds and/or nonreducible groups.

Advantages of using LiAlH₄ in reduction of aldehyde and ketone

1. LiAlH₄ do not reduce carbon - carbon double or triple bonds (with the exception of propargylic alcohols)
2. All four carbon atoms have the ability to participate the reaction

Steps of adding the reagent

As the reducing reagent, first LiAlH₄ / ether should be added to the aldehyde or ketone compound. Then water is added to complete the reaction, for obtaining alcohol. Remember that LiAlH₄ is a strong reducing agent we have in organic chemistry.

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Products of Aldehyde and Ketone reduction by LiAlH₄

An alcohol compound is given as the product from both aldehyde or ketone.

Aldehyde reduction by LiAlH₄

A primary alcohol is given as the product when aldehyde is reduced by LiAlH₄. As examples, reduction of ethanal, propanal will give ethanol and propanol respectively with LiAlH₄/ ether.

Ethanal reduction from LiAlH₄

Ethanal is an aldehyde which is reduced to ethanol by LiAlH₄.

CH₃CHO + LiAlH₄ = CH₃CH₂OH

Propanal reduction from LiAlH₄

Propanal is reduced to propanol by LiAlH₄.

CH₃CH₂CHO + LiAlH₄ = CH₃CH₂CH₂OH

Ketone reduction by LiAlH₄

A secondary alcohol is given as the product. As an example, Propanone (acetone) reacts with LiAlH₄ to give 2-propanol.

Reduction of propanone

Propanone is reduced to propanol by LiAlH₄ Propanone is a ketone with three carbon atoms and propanol also has three carbon atoms.

CH₃COCH₃ + LiAlH₄ = CH₃CHOHCH₃

Butanone reduction

Butanone can be reduced to 2-butanol from LiAlH₄.

CH₃COCH₂CH₃ + LiAlH₄ = CH₃CHOHCH₂CH₃

Mechanism of Reduction of aldehydes and ketones by LiAlH₄

It's better to understand the mechanism of the reaction with an example.

Aldehyde reduction mechanism - Ethanal + LiAlH₄

Here, we discuss how ethanal reacts with LiAlH₄ to give ethanol.

• First, Al-H bond attacks the positively charged carbonyl carbon. That, Al-H bond is broken and carbonyl carbon takes the hydrogen atom. With that, one bond of carbonyl carbon and oxygen will be broken and electron pair is being transferred onto the oxygen atom.
• Electronegativity of oxygen is high. Therefore O^- attacks the positively charged hydrogen atom of water molecule. Then water molecule gets hydrolyzed and the final product, ethanol (a primary alcohol) is given.

LiAlH₄ as a reducing agent in organic chemistry

• Carboxylic acids are reduced to alcohols.
• Amide is converted to amine by LiAlH₄.
• Esters give a alcohol compounds mixture with LiAlH₄.

Reagents which can reduce aldehyde and ketone to alcohols

As LiAlH₄, following reagents also can reduce aldehyde and ketone to alcohols.

• NaBH₄
• H₂ / Ni
• Na / ethanol

If H₂ / Ni is used to reduce aldehyde or ketone group in a organic compound which has a alkene or alkyne group also, that alkene or alkyne group is also reduced to an alkane group.

LiAlH₄ does not reduce alkenes and alkynes to alkanes

But, note that, LiAlH₄ cannot reduce alkene and alkyne.

Comparison and difference of LiAlH₄ and NaBH₄ in reduction reactions

NaBH₄ can be used in water or alcoholic solvents while LiAlH₄ cannot be used.

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References

1. MARCH'S ADVANCED ORGANIC CHEMISTRY, Chapter 19, OXIDATIONS AND REDUCTIONS,19-36 Reduction of Aldehydes and Ketones to Alcohols911

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Questions

Ask your chemistry questions and find the answers

Can I identify aldehyde and ketone from reaction with lithium aluminium hydride?

Aldehyde and ketone reacts with LiAlH₄ and give primary alcohol and secondary alcohol respectively. So we cannot identify aldehyde and ketone directtly from this reaction. But after products are given, they can be tested to identify whether they are primary or secondary alcohol.

• From Lucas' reagent test
• Oxidation by a strong oxidizing agent

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