Master Organic Chemistry Reaction Guide

Addition of NaBH4 to aldehydes to give primary alcohols

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.

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(Advanced) References and Further Reading:

  1. First example
    Brown, H.C.; Wheeler, O.H.; Ichikawa, K. Tetrahedron 1:214 (1957)
    DOI: 10.1016/0040-4020(57)88041-7
    Early paper by Nobel Laureate H. C. Brown describing the reactivities of simple aldehydes and ketones to reduction by NaBH4, in which it is shown that aldehydes are more reactive than ketones to nucleophilic reactions.
  2. Mechanistic studies
    Brown, H. C.; Ichikawa, K. Tetrahedron 1:221 (1957)
    DOI: 10.1016/0040-4020(57)88042-9
    This paper and the above are both mechanistic studies on the reduction of carbonyls – this paper investigates the effect of ring size on the reduction of cyclic ketones (e.g. reduction of cyclobutanone vs. cyclopentanone, cyclohexanone, etc.).
  3. Reference To An Experimental Procedure
    Antonio Bermejo Gómez, Nanna Ahlsten, Ana E. Platero-Prats and Belén Martín-Matute
    Org. Synth. 2014, 91, 185
    DOI: 10.15227/orgsyn.091.0185
    The first step in this procedure uses NaBH4 to reduce a cinnamyl ketone to the alcohol.

Comments

Comment section

50 thoughts on “Addition of NaBH4 to aldehydes to give primary alcohols

  1. 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.

    1. 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

    2. Yes,ofcourse Methanol would get protonate the oxygen in final step because it is present in excess and it release it’s H-atom easily under given condition.

  2. 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?

    1. 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).

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

  4. 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

    1. 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.

  5. 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.

    1. 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.

        1. dear Zia,
          if you have color impurity(if you are very sure) then you can treat your compound with charcoal. And this mostly works with liquid compounds. You can add a pinch of charcoal in your compound and let it stirring for some 10 minutes.

          1. thanks
            i used activated carbon ,the orange color remained unchanged. would you tell me if there is any substance that could decolorise primary alcohol made by reacting sodium boro hydride on aldehyde

    1. Hi – I would expect that KBH4 is slightly worse, as K+ is not as good a Lewis acid as Na+.

      LiBH4 , for example, is a stronger reducing agent than NaBH4. LiBH4 can reduce esters whereas NaBH4 cannot.

      Hope that helps!

      James

  6. Hi James
    What happens if a group like C=NH is reduced using NaBH4.
    Does NaBH4 reduces every polar π bonded group?

    1. Imines can be reduced with NaBH4 but usually as the iminium ion (i.e. the conjugate acid of imines). This is the key process in “reductive amination”. NaBH4 certainly does NOT reduce very polar pi bonded group – it does not always reduce C=O bonds, for example. Esters and amides are generally not reduced by NaBH4 for example.

  7. Hi, I am using spray mirror silver, ammonia, sodium hydroxide and in my second solution of boric acid and sodium borohydride. Can I introduce formalin or other material to achieve better reaction and increase the quality of mirror quality? do .
    Matt used hydrazine hydrate and in the old days I also used glucose and formalin, but now sodium hydrochloride is rapidly increasing.

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