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.


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


    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:

    Solvents that posses a polarity are called polar solvents.


    Polarity implies that the solvent has unequal charge distribution among the atoms of the solvent molecule. For eg. Water. Water molecule consists of H-O-H or H2O. There is a difference in the electronegativity of hydrogen and oxygen. Oxygen has greater capacity of pulling the electrons towards itself.

    Otherwise, in solvents like benzene, hexane, etc. molecules do not exhibit polarity. These are known as non-polar solvents. Also, solutes and solvents generally obey the rule of 'Like dissolves like' meaning, polar solvents dissolve polar substances and form polar bonds or hydrogen bonds.

  • Answer:

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


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

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