I ask this because SN2 requires a strong nucleophile, and I have read OHX− isn't a strong nucleophile in a polar protic solvent like water. So, how can OHX− replace, say IX− from CHX3I (methyl iodide)?
Answer
When you are considering a particular SN2 reaction, you have to consider the nucleophilicity as well as the nature of the leaving group of both the groups.
Here as you mentioned, OHX− and IX− (and other halide ions, except FX−, which is a very bad nucleophile) have comparable nucleophilicity in protic solvents. But, as for their leaving nature, IX− (as well as other halides) is far more better leaving group than OHX−, because we know weak bases are better leaving groups. That is why the reaction is feasible.
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