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Thread: What is the mechanism of this reaction?

  1. #1 What is the mechanism of this reaction? 
    Forum Junior ArezList's Avatar
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    Hi all.
    I wonder what' the exact mechanism of this reaction?

    NaBH4 + a ketone -----> an alcohol

    and also what's the intermediate if any?

    what's the nuclerphile in the reaction?


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  3. #2  
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    Sample answer (free):

    The borohydride anion functions as the nucleophilic source, by donating a hydride to the electrophilic carbonyl center, giving the following intermediate:

    [Visit http://www.chemistryhighschool.com for all your organic chemistry needs.]


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  4. #3  
    Forum Masters Degree organic god's Avatar
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    K this could be wrong as i just went through it quickly in my head.

    we have the borohydride anion right which can give off H- ions, this is attracted to the positive carbon atom right. which is like ch3m tutor said. So the Carbon momentarily has 5 bonds which it doesn't want. Therefore, the oxygen takes all of the electron density to itself and you get an intermediate like this.

    H
    I
    R-----C----H so this is kind of like the intermediate
    I
    O(-)

    So this is kind of unstable right and then i guess the borohydride anion is also kind of unstable having given off H-, so it gives off an H+ to become more stable.
    and the H+ bonds to the O(-) giving you the alcohol,

    Probably makes no sense but you never know, i think it is like the reaction with Carbonyl compounds and the Cyanide anion

    EDIT - i cant make the Hydrogen and oxygen bond the carbon, so it looks like they are bonded to the R which they aren't lol
    everything is mathematical.
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  5. #4  
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    As mentioned we will be using an absorbance spectrometer to determine the amount of iron present in a solution made from an iron tablet. This will be done by first making a set of standards containing
    "NaBH4 + a ketone -----> an alcohol"

    As stated, the hydride attacks the partial positive charge on the ketone's carbon, resulting in an intermediate much like described, with a negative charge on the oxygen.

    Because hydrogen and the two R (alkyl) groups on the ketone will almost never ever leave the structure, the negative charge sits on the oxygen leaving the stable intermediate.

    The oxygen will subsequently be protonated giving the resulting alcohol.

    Organic God's answer was mostly right, except the structure was not completely correct, ketones contain RCOR type structure, what was shown was an aldehyde.

    To my knowledge, I donít think the proton comes from the boron, but I may be wrong in that. It would be just as valid to suggest another source for protons
    Stumble on through life.
    Feel free to correct any false information, which unknown to me, may be included in my posts. (also - let this be a disclaimer)
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  6. #5  
    Forum Masters Degree organic god's Avatar
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    oops yeh didn't realise i had drawn an aldehyde sorry about that,

    and yeh the thing that i wasn't sure about was where the H+ comes from to protonate the intermediate.

    I mean when the Borahydride ion donates (H-) it will have 0 charge i think, so i can't see it donating another atom...
    I know you do this reaction in water so perhaps this is involved somehow in the protonation step
    everything is mathematical.
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