Here's an example.
Let's hypothetically react Li^((+)) [(CH_2)_3CH_3]^((-))Li(+)[(CH2)3CH3](−) (commonly BuLiBuLi) with acetone. Normally, BuLiBuLi is a fantastic nucleophile due to lithium's lewis acid characteristics.
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If you solvate BuLiBuLi in the optimal amount of ethanol (commonly EtOHEtOH), you have now in solution, before anything happens, BuLiBuLi, EtOHEtOH, and acetone.
Acetone:
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What would most likely happen is that since BuLiBuLi 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 EtOHEtOH.
At that point, BuLiBuLi would become butane, which is clearly nonreactive as a poor nucleophile. Then, EtO^(-)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:
- BuLiBuLi grabs a proton and loses its reactivity, allowing EtO^(-)EtO− to be an additional nucleophile (there's still some BuLiBuLi leftover)
- BuLiBuLi attacks acetone and the reaction proceeds to EtOHEtOH protonating the tetrahedral intermediate to form a tertiary alcohol.
The result then is a mixture of the butane, EtOHEtOH, acetone, the tertiary alcohol, and the product of the mechanism where EtO^(-)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.
