Why is nucleophilicity is solvent dependent?

Answer 1

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.

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.

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Answer 2

Nucleophilicity is solvent-dependent because the nature of the solvent can influence the reactivity of the nucleophile. Solvents can stabilize or destabilize the charge on the nucleophile, affecting its ability to donate electrons. Polar solvents, such as water or alcohols, can solvate the nucleophile and stabilize its charge, making it less reactive. Conversely, nonpolar solvents, like hydrocarbons, can shield the nucleophile from solvation and promote its reactivity. Additionally, the solvent can also affect the stability of the leaving group, which indirectly influences nucleophilicity. Overall, solvent polarity plays a significant role in determining nucleophilicity.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

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