[PoetryInMotion]

I am supposed to determine which of the following groups that will be the most nucleophilic – -OH, -NH₂ or -SH – in a reaction where
   a) acetone
   b) water
is used as a solvent.

a) Acetone is a polar aprotic solvent, right? And there will probably be much more solvent. So, the most basic group must be the correct answer to the question. -NH₂ is more basic than -OH, but what about -SH? It's much less electronegative than both N and O, so I as far as I can see, it must be the worst nucleophile of these three groups. The solution of this problem, however, says it's the best one, which makes me hesitate. Anyone who knows for sure?

b) Water is definitely protic, which means that that polarizability and size matters. The sulfur atom in -SH is larger than nitrogen and oxygen, and more polarizable, so I guess it's the best nucleophile, followed by -OH and -NH₂?

[TwistedConf]

but what about -SH? It's much less electronegative than both N and O, so I as far as I can see, it must be the worst nucleophile of these three groups.

Electronegativity is only part of the equation when it comes to nucleophilicity. You need to go back to your text and learn about other factors in something like O vs. S. Then, consider the effects of solvent hydrogen bonding on your nucleophiles in the protic solvent (Hint: it's not a good thing).

[opsomath]

Nucleophilicity increases with polarizability and decreases with electronegativity. (Think of polarizability - atom size, basically - as the atom's ability to "reach out and touch someone" with its electrons, its HOMO to be precise)

However, in a polar aprotic solvent, something weird happens. Less polarizable nucleophiles become much more reactive. This is because most of the solvation in these materials comes through polarizability, and so poorly polarizable things are higher in energy. So for instance I- is the best halogen nucleophile in alcohol, but F- is the best in DMSO. At least, that's the simple Organic I picture