nucleophilicity in an SN2 reaction

[pinkfld13]

I have found a discrepancy between the Ch. 4 organic chemistry section in the 2009 Kaplan DAT book and the material I was taught in my Rutgers University organic chemistry course.

In the Kaplan book, it says that order of nucleophilicity of halogens in an aprotic solvent is based on basicity: F->Cl->Br->I-

In my college class, I learned that the order of nucleophilicity in SN2 reaction conditions (which use an aprotic solvent) of halogens is the inverse: I->Br->Cl->F-, because good nucleophiles are less electronegative and are more polarizable.

I'm concerned about which material will be correct for the 2013 DAT. Which is it?

Thanks,
Misha

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[IsaacYankem]

I'm going to yield to the Godfather of Chemistry prep and point out that Chad has nucleophilicity increasing going up a column in aprotic solvent. So F->Cl->Br->I-

In protic solvent the order is reversed because H-bonding stabilizes the larger anions.

 

[Ari Rezaei]

Edit: It's F > Cl > Br > I for aprotic solvents.

F- is the strongest nucleophile normally, but for protic solvents it gets pronated so much because its so small so it loses its effectiveness. So for PROTIC solvents, yes I- is the strongest since its big and you need way more solvent to enclose it like F-.

But for aprotic solvents, there is nothing stopping the nucleophiles, and F- resumes being the strongest nucleophile.

Maybe your professor was talking about uncharged nucleophiles? In that case, H2Te > H2Se > H2S > H2O