In DMSO the nucleophilicity of the halide ions toward CH3I follows the order

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In DMSO the nucleophilicity of the halide ions toward $CH X_{3} I$ follows the order

DMSO is aprotic and does not hydrogen bond to anions, so solvation differences vanish and nucleophilicity follows basicity (charge density), i.e.

F X^{-} > Cl X^{-} > Br X^{-} > I X^{-}

. This is the reverse of the order in protic solvents.