[se7en47]

So, going by several textbooks I found on google, they say that aprotic solvents are best for Sn2 and protic for Sn1 (I already knew this, just confirming); however, my professor today said that isn't necessarily the case, because you have to look at the anionic character. He lost me at that point. Before he said all that stuff, I thought i understood everything.

Could anyone explain?

Thank you

[Squirmy]

I'd guess he was talking about S_N2 reactions with neutral nucleophiles. Anionic nucleophiles are more reactive in polar, aprotic solvents because the anion is poorly solvated (less stabilized by solvent). e.g. Dimethylsulfoxide (CH₃SOCH₃; DMSO) solvates cations well because the negative oxygen is unhindered. However, it solvates anions poorly because the positively charged sulfur is crowded.

There are neutral nucleophiles that can participate in S_N2 reactions (SR₂, NR₃, PR₃), and I believe they react faster in polar, protic solvents. There are undoubtedly still differences in solvation of the nucleophile between protic and aprotic. However, I believe the bigger issue here is that the product is now a salt, which is stabilized by solvents that can solvate both the anion and cation (protic).

In a nutshell, for anions, the starting material is made more reactive/less stable by polar, aprotic solvents. For neutral compounds, the product is made more stable by polar, protic solvents.