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Stereoselective vs stereospecific

Discussion in 'MCAT Study Question Q&A' started by Hemichordate, Jul 30, 2009.

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  1. Hemichordate

    Hemichordate Pre-ophtho resident

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    I've read the definitions for both of these two terms but I still don't understand it. Can someone give a brief explanation and provide an example?
  2. loveoforganic

    loveoforganic -Account Deactivated-

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    I believe that the difference is stereospecific provides essentially one stereoisomer product while stereoselective provides some sort of mixture.

    If this is the case, my example for stereospecific would be an SN2 mechansim, while stereoselective would be an SN1 mechanism with a hindered face.
  3. sleepy425

    sleepy425

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    Your example is correct, but your definition isn't quite right. You don't have to have a mixture for it to be stereoselective. You can have 100% stereoselectivity without it being stereospecific.

    Stereospecificity is where the mechanism of the reaction only allows formation of one stereoisomer. So Sn2 is a great example of this because you can only have backside attack, which means that the mechanism only allows formation of the opposite stereochemical configuration.

    A stereoselective reaction is one where the mechanism allows formation of both products, but one product is preferred over the other. So a good example of this is the Sharpless epoxidation (go to the wikipedia page for this, but don't try to understand it fully unless you want to). Anyway, the Sharpless epoxidation allows you to form an epoxide selectively onto one face of the double bond versus the other. But the agent that forms the epoxide, a hydroperoxide (a peracid), can theoretically react from both sides, but prefers to react from one face due to the conditions of the reaction. These conditions don't affect the mechanism of the reaction, but they direct the hydroperoxide towards one face or the other. The enantioselectivity of the Sharpless epoxidation is very high.

    For stereospecific reactions, you cannot get a mixture because the mechanism precludes formation of a mixture. The stereospecificity is always 100%. For stereoselectivity, you can get a mixture, but 100% stereoselectivity is also possible, since the selectivity could be so good that none of the other isomer forms. However, in practice, you don't really see 100% stereoselective reactions, although many reactions are in the high 90s.
  4. SuperSaiyan3

    SuperSaiyan3

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    great explanation. sleep425 ftw
  5. loveoforganic

    loveoforganic -Account Deactivated-

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    Thanks for the clarification, very unambiguous
  6. boaz

    boaz shanah alef

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    LOL, I had this same question on my organic II midterm.

    Sleepy is stereospecifically right.
  7. Dieumerci

    Dieumerci

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    Thankssss alott
  8. Temperature101

    Temperature101

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    Thanks...I finally understand these concepts due to your outstanding explanation.
  9. straight2thapros

    straight2thapros

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    In EK Ochem: "Any reaction that is stereospecific is also stereoselective, but the converse is not true." Is the book wrong? It seems from sleepy425's definitions that a reaction can be either stereospecific or stereoselective, not both. Examples would be appreciated (hopefully outside EK). Thanks!!
  10. orangetea

    orangetea

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    So then what's regioselectivity :eek:

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