Addition of LiAlH4 to aldehydes to give primary alcohols

by James

Description: Addition of lithium aluminum hydride to aldehydes leads to formation of primary alcohols (after addition of acid) [private_ReactionGuide]

Notes: The choice of specific acid is unimportant, so long as it gives the neutral alcohol in the end.

Examples: 

Notes: Lots of different acids can be used in the last step, it’s not important which specific acid is used, just that something is present that can form the alcohol.

Mechanism: LiAlH4 is a source of hydride (H–) and the reaction begins with 1,2-addition of hydride to the carbonyl of the aldehyde (Step 1, arrows A and B). Upon addition of acid the oxygen is protonated (Step 2, arrows C and D) to give the neutral alcohol.

Notes:  The choice of H2O/H2SO4 as acid here isn’t crucial.

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{ 5 comments… read them below or add one }

Nes

Hi,i have a question about the reduction of an aldehyd to a primary alcohol,in the class I learned that it can be done either with adding NaBH4+etoh(weak acid) or with LiAlH4 and than we have to add et2o and not an acid????

Reply

james

Reduction of aldehydes can be performed with NaBH4 or LiAlH4. If ether is mentioned, it acts as a solvent only. The second step is addition of some kind of proton source to give the neutral alcohol. This is sometimes written as H3O+ or NH4Cl.

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Tasdique Quadery

Hi,
Will it work with just H2O workup without any acid?

Reply

Alexis Espinoza

What is the difference between the reduction of aldehydes between NaBH4 and LiAlH4?

Reply

James

They work the same way. NaBH4 is milder than LiAlH4 but the reaction works equally well.

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