[Andy87]

hi.I'm confused with predicting direction of nucleophilic substitution .Why I^- can replace Br^- eg CH₃CH₂Br +I^-->CH₃CH₂I +Br^- if iodide is easier leaving group and wearker base(because HI is strong acid) so weaker nucleophile?Also there's solvent factor..and becasue I is larger,charge is more diffuse and it is less solvatized so more reactive than Br or F?Those statements seem contradictory.If somebody can explain me how can I predict direction with F,Cl,Br,I and also along periode?Thanks

[nj_bartel]

You're confusing nucelophilicity and basicity.  Nucleophilicity increases down a group due to higher polarizability.  I have however, seen reactions where HBr is used in an S_N2, where Iodide is eliminated.  I don't understand the reasoning for this happening (essentially the question you're asking).

[Gregorian]

This substitution cannot happen in water because I- is a better leaving group than Br- and the general rule for substitution is the incoming nucleophile has to be a stronger base than the leaving group (the leaving group has to be a weak base and the basicity decreases as we go down the column.)   

But the order of nucleophilicity is reversed if the solvent used is acetone so the reaction you provided is valid in acetone but not in water. But I do not know why is that!! maybe because of the solvent's polarity effects!!