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.
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.
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.
An alcohol compound is given as the product from both aldehyde or ketone.
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 is an aldehyde which is reduced to ethanol by LiAlH4.
CH3CHO + LiAlH4 = CH3CH2OH
Propanal is reduced to propanol by LiAlH4.
CH3CH2CHO + LiAlH4 = CH3CH2CH2OH
A secondary alcohol is given as the product. As an example, Propanone (acetone) reacts with LiAlH4 to give 2-propanol.
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 can be reduced to 2-butanol from LiAlH4.
CH3COCH2CH3 + LiAlH4 = CH3CHOHCH2CH3
It's better to understand the mechanism of the reaction with an example.
Here, we discuss how ethanal reacts with LiAlH4 to give ethanol.
As LiAlH4, following reagents also can reduce aldehyde and ketone to alcohols.
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.
But, note that, LiAlH4 cannot reduce alkene and alkyne.
NaBH4 can be used in water or alcoholic solvents while LiAlH4 cannot be used.
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.