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Formation of glycoside: rx of monosaccharides

Discussion in 'MCAT Study Question Q&A' started by SaintJude, 03.18.12.

  1. SaintJude

    SaintJude

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    Last monosaccharide rx type to cover...

    3. Glycosidic reactions: Basically a hemiacetal converts to an acetal. Why does this have to occur under acidic condition?

    View attachment Glycoside .jpg


    And Kaplan says (like it's obvious) that "this rx is not new to us..it's really a dressed up SN2 reaction with the sugar acting as the nucleophile" It is?!

    P.s. what's so darn special about these glycosides anyway? I know that glycosides do not! reduce a tollen's reagent, b/c they are closed acetal rings...
    Last edited: 03.18.12
  2. MedPR

    MedPR

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    You need the acid to protonate the OH into water so it leaves the molecule and the alkoxy group can add in its place. If it is catalyzed by a base, you will only get as far as hemiketal/hemiacetal because the OH will never leave. They're probably special because they are just like glucose and appear in biochem/biology all the time.

    I don't know about it being a dressed up SN2 though.. I don't think the H2O leaves at the same time the ROH adds, so it appears more like an SN1.
  3. SaintJude

    SaintJude

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    Ah, yes! Thank you MedPr. And please note I made a mistake earlier that wasn't corrected in your quote! Glycosides do NOT reduce Tollin's reagant
  4. chiddler

    chiddler

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    Plus SN2 will only have inversion product. This has both.

    It COULD be SN2 if you think about the ROH being the nucleophile.
  5. MedPR

    MedPR

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    Hey do you know where in TBR the tollen's test is? I keep seeing it on AAMCs and other places, but those questions only ask you simple stuff that has nothing to do with the tollen's mechanism or how it works. I really need to look at amines and carbonyls and sugars.
  6. chiddler

    chiddler

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    yes i remember reading it in the ochem sugar chapter.
  7. SaintJude

    SaintJude

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    This is an SN2 reaction.When it comes to conceptual explanations in Kaplan's review books, I have not learned of a single errata. This was in their "Key concept" section--so this is a SN2 and the sugar is acting as the nucleophile.

    Besides, the products are just a depiction of mutarotation--which happens for all alpha & beta anomers


    And, SN2 reactions do not always produce an inversion of the absolute (R/S) configuration!! I remember a thread about this right when I joined SDN. The inversion of an absolute configuration only occurs when the leaving group and nucleophile have the same priority.
  8. chiddler

    chiddler

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    1. mutorotation occurs spontaneously, right?

    2. yeah but it's very difficult to find a situation where retention occurs during SN2. unless you have like...a non leaving group heavy atom and a light leaving group atom. possible, but not common.

    can you think of any examples?
  9. SaintJude

    SaintJude

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    1. Yeah--and Kaplan says mutarotation occurs more rapidly in acid or base catalyzed rx.

    2. You're def right. An example from my notes:
    R-CH3-BrFI + Cl-, with Cl as the nucleophile. Iodide will leave.

    I think my problem to the original question in this thread, is that I've kind of forgotten how to recognize an SN2--what do you look for first in a question like this?
  10. chiddler

    chiddler

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    Go look at the mechanism again. The features i look for are just an apt nucleophile and a possible leaving group.
  11. MedPR

    MedPR

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    Good nucleophile, non-tertiary substrate, aprotic solvent for SN2.
  12. SaintJude

    SaintJude

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    "Good nucleophile, non-tertiary substrate, aprotic solvent for SN2." Yeah, so then the methanol must indeed then be the substrate in this case.


    Edit. If the sugar is acting as the nucleophile then it has to be one doing the attacking...I wish they included a mechanism for this!
    Last edited: 03.18.12
  13. SaintJude

    SaintJude

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    Actually, sorry Chiddler

    Glycoside do NOT undergo mutarotation b/c the ring structure does not open to become the open-chain structure, so these 2 products won't spontaneously convert...
  14. chiddler

    chiddler

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    good call...i think it only happens when it's an alcohol group --> carbonyl, yes?

    then why do two products show in OP?

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