The order of nucleophilicity for halide family in DMF is: $\ce{Cl- > Br- > I-}$. I understand the reason for this must be that DMF, being a polar aprotic solvent, is unable to solvate the halide ions.

I was told by my teacher that this is true only for DMF and DMSO, while in other aprotic polar solvents, the order is as usual: $\ce{F- < Cl- < Br- < I-}$.

I'd like to know what is special about DMF and DMSO (and which is not possessed by other aprotic solvents like acetone) that leads to this order.

I have verified the order in DMF from March's 7th edition (page 427). The book mentions the usual order ($\ce{Cl- < Br- < I-}$) for acetone (another aprotic polar solvent).


It depends on the ability of the solvent to solvate anions. If you draw structures of $\ce{DMF}$ with its resonance structure and check the side of the molecule that should interact with anions, you will see, that methyl groups prevent the interaction pretty much. Same for $\ce{DMSO}$. Thus these two solvents are commonly used to enhance the reactivity of nucleophiles. They solvate cations well but anions poorly. Maybe a bit strange, but it works well and it's pretty reliable.

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