Addition of NaBH4 to aldehydes to give primary alcohols

by James

Description:  Addition of sodium borohydride (NaBH4) to aldehydes gives primary alcohols (after adding acid)

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:

NaBH4 is a source of hydride (H-) and the reaction begins with the addition of hydride to the carbonyl to 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 primary alcohol.

Notes: The choice of H2O / H2SO4 as acid isn’t crucial – this is just an example. Any source of proton (including water) will do.

{ 18 comments… read them below or add one }

Thomas

Thank you, very helpful and complete, answered every question!

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TJ

There is a problem in my book I am having a small issue with. It states “Draw the products formed when CH3COCH2CH2CH=CH2 is treated with NaBH4 (excess) in CH3OH”. I understand that NaBH4 selectively reduces C=O while leaving C=C inert, but what protonates the oxygen in the final step to give the alcohol? Does NaBH4 do that because it is in excess, or does methanol? I don’t 100% understand methanol’s role in the reaction, just that it is typically used with NaBH4.

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prabhu

in prescence of meoh, the MeOH acts as a proton source and also there will be formation of methoxy borohydride (mono,di,tri) which has higher susceptibility of donating the hydride. because of these two reasons MeOH or EtOH are often used as co-solvents in NaBH4 reduction

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James Ashenhurst

True, but actually methoxyborohydrides are not as nucleophilic as sodium borohydride itself.

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Jason Mathias

Hi James,
It seems like the BH4 in step 1 is nucleophilic, and the H-B bond is moving (Electrons and H) to the C=O, which is why the C-O bond ‘jumps’ up. Is this not a substitution reaction? Or perhaps my definition of an Addition reaction is a bit narrow?

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randy

Could you explain why in some reaction condition there were also iodine added?

I’m wonder whether its goes through IBH2 reactant or this author(http://totallymechanistic.wordpress.com/2006/12/11/sodium-borohydrideiodine-reduction/) states that the reductant is the borane (BH3) which is generated in situ. Yet some people think it’s the BH3-THF complex being reductant, and then I’m not sure what’s the Iodine for.

Thank you!!

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James

Iodine is used to form BH3 in situ from NaBH4 . It’s used, for instance, in the reduction of carboxylic acids to alcohols.

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Vanessa

Hey, what happens when there is an excess of NaBH4?

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James

Nothing bad should happen. Once the aldehyde is reduced to the alcohol, there’s nothing else that can happen (assuming you just have one aldehyde).

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aditya krishna

what if we use NaBD⁴ in H²0² with OH·-· to reduce propene ? The answer is ⛪ CH³CHDCH²CH²OH.. please give the mechanism

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Haydan White

Hey
Got a question regarding reduction of P-nitrobenzealdehyde to p-nitrobenzyl alcohol. We mixed sodium hydroxide and potassium borohydride and can’t figure out how the mechanism takes place. Does sodium hydroxide have anything to do with it, or is it only the aldehyd + potassium?

Thanks

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James

I’m not sure why you added NaOH with KBH4. By the way why did you use KBH4 – you usually have to make it, and NaBH4 is much cheaper?

The mechanism for reduction with a borohydride is simply that the B-H bond breaks, donating two electrons to form a new bond with the carbonyl carbon, and the C-O pi bond breaks. You end up with an alkoxide, which is then protonated to give the alcohol.

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prabu

can we use for ring reduction using sodium borohydride like pyridine and pyrrole are other heterocyclic answer me

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James

No, it is not a strong enough reductant.

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Mohamed

I just wanted to ask if there is any colour change that occurs during the reduction of the aldehydes to alcohols by NaBH4
Thanks in advance.

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James

Color changes are not a very diagnostic criteria for reactions. It’s very common to see a pale yellow colour, but that’s not unusual for a large number of other reactions.

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zia

how can the colour formation be prevented or can we decolourize it

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James

Color is not important for this process. Why is it important to you?

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