[Winga]

If the solvent is aprotic!

[jdurg]

I'm not sure if this will help you out, but F- is a VERY weak reducing agent in comparison to the I- ion.

[movies]

In protic solvent the more basic halides (F, Cl) will be less nucleophilic because of their interaction with hydrogen atoms.  This is especially true with fluoride since HF has a pKa of 3 (cf., HI, pKa = -10).  So, there is much less F- in solution to undergo substitution reactions.

In aprotic solvents, however, there are no available protons to tie up the F- as HF.  You would expect F- to be a very good nucleophile because it has a dense negative charge.  I think that the rest of the observation is justified quite simply.

[dexangeles]

I agree with the above

With an aprotic solvent, F- cannot be solvated by H-bonds.  This is the reason why the more basic, F- is less nucleophilic in most cases than I-  It's charge to size ratio makes it more hindered in terms of nucleophilicity in protic solvents.

[dzxy]

This arguments i believe are misleading... it doesn not put into account the polarizability of the anions. I- is more polarizable tha Cl- or F- that it why it it works best for SN2 reactions (favored by APROTIC solvents). In addition an alkyl halide RCl can be stirred in NaBr to convert to RBr but the opposite does NOT work (chek Finkelstein eaction :animatedwink:)

[movies]

Polarizability doesn't explain the empirical reversal of rates in S_N2 reactions in protic vs. aprotic solvents.  (i.e. in protic solvents I- appears to be the best nucleophile, in aprotic solvents F- appears to be the best)

Furthermore, alkyl chlorides can indeed be prepared by the Finkelstein reaction.  NaBr is more soluble in aprotic solvents like acetone than is NaCl, so the reaction to form the bromide is driven by the precipitation of NaCl.  However, using conditions like TMS-Cl and imidazole in DMF can convert bromides to chlorides.  Finally, the Finkelstein reaction is one of the best ways to make alkyl fluorides from higher halides (especially iodides).

[Mitch]

I love TMS-Cl.

[movies]

I love TMS-Cl.

Ha!  Is that because of the faint burning sensation when you smell it, or the haze that it forms when you mix it with base?

[jdurg]

Heh.  A little from column A, a little from column B.  

[dexangeles]

I agree with movies.  The question relates to nucleophilicity, thus concerning halide ions already.  

The reason aprotic solvents is part of the question is because it doesn't cause solvolysis of the ions.  In polar protic solvents I- would be a better nucleophile eventhough it is a much weaker base than F-

Doesn't polarizability deal with the extent of distorting electrons in the presence of a magnetic field?  Thus it deals more with intermolecular forces between molecules.  Polarizability is one of the factors that explains differences in boiling point, etc....