Polar Protic, Polar Aprotic and Non-Polar Solvents

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Organic Chemistry | Predicting the Mechanism as SN1 or SN2.

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Key Questions

  • Here's an example.

    Let's hypothetically react #Li^((+)) [(CH_2)_3CH_3]^((-))# (commonly #BuLi#) with acetone. Normally, #BuLi# is a fantastic nucleophile due to lithium's lewis acid characteristics.

    http://www.umich.edu/

    If you solvate #BuLi# in the optimal amount of ethanol (commonly #EtOH#), you have now in solution, before anything happens, #BuLi#, #EtOH#, and acetone.

    Acetone:
    https://upload.wikimedia.org/

    What would most likely happen is that since #BuLi# has such a high nucleophilicity, instead of reacting with acetone all the time, there is a good chance it would also steal a proton from #EtOH#.

    At that point, #BuLi# would become butane, which is clearly nonreactive as a poor nucleophile. Then, #EtO^(-)# forms and it becomes a potential nucleophile to attack acetone (but less often, as it's a worse nucleophile).

    At this point, you may realize that you now have a situation where:

    1. #BuLi# grabs a proton and loses its reactivity, allowing #EtO^(-)# to be an additional nucleophile (there's still some #BuLi# leftover)
    2. #BuLi# attacks acetone and the reaction proceeds to #EtOH# protonating the tetrahedral intermediate to form a tertiary alcohol.

    The result then is a mixture of the butane, #EtOH#, acetone, the tertiary alcohol, and the product of the mechanism where #EtO^(-)# attacks acetone. Ideally you don't want a mixture that you'd have to separate and purify later. If you got a pure product, that's what you should want.

    So naturally, it's a good idea, for example, to not use a protic solvent when using an anionic nucleophile, because it may actually deactivate the nucleophile.

  • Answer:

    Non-polar solvents are non-polar molecules that can be used as solvent.

    Explanation:

    Non-polar solvents are any non-polar molecules that can be used as a solvent.

    Example:
    Hexane, pentane, heptane, etc.
    Carbon tetrachloride #"C"Cl_4#.

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