Leaving groups in protic/aprotic solvent

[iPodtosis]

On AAMC 10, ochem E2 reaction question. While alcohol is the solvent (polar protic) iodide should be the worst leaving group while fuoride is the best leaving group right? However, the practice solution says iodide is the best leaving group. Can someone please clear my confusion?
Thanks

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[wanderer]

wow, I just took AAMC 10 and had the same exact question. Anyways I think I have an answer. Leaving groups are not really stabilized much in protic solvents (for example HF is a weaker acid than HI, even in water). The reason that F- is a weaker nucleophile in protic solvents is that there is a sphere of hydrogens sequestering or shielding the flouride and preventing it from attacking the electrophile. If it's a leaving group, being more shielded by protic hydrogens doesn't make much of a difference, so the fact that it is smaller means that it can't stabilize its negative charge as well as iodide can.

 

[iPodtosis]

so the nature of the solvent is only significant when we look at the nucleophilicty?

 

[minutemen11]

On AAMC 10, ochem E2 reaction question. While alcohol is the solvent (polar protic) iodide should be the worst leaving group while fuoride is the best leaving group right? However, the practice solution says iodide is the best leaving group. Can someone please clear my confusion?
Thanks

know that for HF, it will hydrogen bond LIKE CRAZY because of electronegativity. In fact you need to put in a lot of energy to break an HF bond, so F will be a terrible leaving group (i doesnt want to part with its H ever). remember that since its a polar protic solvent, HF will be so shielded that its a bad nucleophile; nucleophillic ability doesnt correspond to basicity here- and larger molecules are better. I'm not sure if that last part has to do with the ionization energy or whatever.

BUT, in aprotic solvents the reactivity is flipped. F>Cl>Br>I because there is no solvation occurring. Aprotic means that the solvent does not have any H-bonds to F,O, or N. a good example of an aprotic solvent is DMSO..

i dunno if that helps you but it works for me.. good question

anyone else wanna chime in?

 

[iPodtosis]

thx minutemen, but the question has to do with better/worse leaving group, do you know how does protic or aprotic affects it?