Finkelstein reaction explicitly entails the conversion of an alkyl chloride or alkyl bromide to alkyl iodide by treatment with sodium iodide in acetone. $$\ce{R−X + NaI→[acetone] R−I + NaX↓ }\\ \ce{(X$=$Cl, Br; R$=$alkyl group)}$$
What I think I understand
Acetone is a polar aprotic solvent so $\ce{NaBr and NaCl}$ formed, being mostly ionic, are insoluble while $\ce{NaI}$ have covalent character (in accordance with Fazans's rules) & organic solvents can dissolve covalent compounds. so, $\ce{I-}$ will be in solution while $\ce{Br- or Cl-}$ will not be there due to being precipitated.
Le Chatelier's principle will make the reaction to be towards the product. Therefore, I also understand how a weaker Nucleophile can substitute a stronger Nucleophile.
Nucleophilicity order of Halides in Polar Aprotic solvent: $\ce{F- > Cl- > Br- > I-}$
What I ponder
- Why can't we do it on alkyl fluorides?
- Why only acetone? Will the reaction be same in other polar aprotic solvents too?
Some specific reactions which seem to deny all this
- $\ce{CH3-CH2-Cl →[NaF/DMF] CH3-CH2-F}$
- $\ce{CH3-CH2-Cl →[NaBr/DMSO] No rxn}$
- $\ce{CH3-CH2-Br\text{*} →[NaBr/DMSO] CH3-CH2-Br}$
where $\ce{Br\text{*}}$ is Isotopic bromine
Note-
- Here I am talking about major organic products.
- DMF & DMSO are polar aprotic solvents.
I would like to know the theoretical reasoning for the above reactions.