Will a medium that dissolves the leaving group be beneficial for high reaction rates? E.g. would it be good to use alkaline water as a medium where Cl acts as a leaving group, or would acetone be a better option?
Well, behind every process there's a big Delta G (Gibbs free energy) and, associated with that, an equilibrium coefficient linked by DeltaG=-RTlnK. For the K/G of the whole process, you would have to include the solvatation equilibria into the consideration which will affect your yield.
Of course, that's just thermodynamics at this point which is about the final state, not the rate yet, but I've heard the sentence "the yield wins the fight" plenty of times during my studies.
For the actual rate, the decisive point is not the final state but the stabilization of transition states and intermediates. Maybe check out this here and the references therein:
Article Nucleophilic Substitution (SN2): Dependence on Nucleophile, ...
Thank you for your answer! Your thoughts are similar to what I have been thinking myself. Information about this topic is usually rather general (but what could I expect from textbooks). The article was actually very informative and helpful!
It depends on the reaction. For example, for SN1 - definitely - a polar, protic solvent stabilizes Cl- and weakens the C-Cl bond. SN1 wil be faster in water than in acetone.
For SN2, the effect on the leaving group is the same but it is more important that a protic solvent will neutralize your nucleophile. Water is no good.
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