Endeross

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in a protic solvent, the nucleophile strength of an ion generally follows the trend of going (from F) left and down, As such a carbocation (such as CN- ) is a strong nucleophile, so is I- . As you go up to the right it gets weaker till you reach F-. H2O is a weak nucleophile and can represent the protic solvent.

This changes in Aprotic situations where the nucleophilic strength trend changes to go (from F) left and up. so F would be a better nucleophile than I.

http://chemistry.umeche.maine.edu/CHY251/Ch7-Overhead4.html heres a site I found that may explain things better
 
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bigbutrealdreams

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in a protic solvent, the nucleophile strength of an ion generally follows the trend of going (from F) left and down, As such a carbocation (such as CN- ) is a strong nucleophile, so is I- . As you go up to the right it gets weaker till you reach F-. H2O is a weak nucleophile and can represent the protic solvent.

This changes in Aprotic situations where the nucleophilic strength trend changes to go (from F) left and up. so F would be a better nucleophile than I.

http://chemistry.umeche.maine.edu/CHY251/Ch7-Overhead4.html heres a site I found that may explain things better
cudos my friend!
 

orgoman22

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so kaplan says this:
CN->I->RO->HO->Br->Cl->F->H2O
Could someone please explain the reasoning to me?
The Kaplan nucleophile guide is just a " rough guide " of strength.

They needed to list the BIG DADDY of them all......Sulfur !!!! It turns out that SH-, CH3S- and even Phosphorus nucleophiles are great ways to do SN2 reactions in addition to CN-. These nucleophiles do not depend much on solvents for reasons the best organic chemists still argue over.....even sometimes want to fight over !!!! CH3O- and C2H5O- are also quite good.

The solvent often plays a huge role on nucleophile strength such as F-, Cl-, Br-, and I-. In a polar protic solvent such as water,,,,,,,I- is awesome....F- is a dud,,,,,,,BUT in an aprotic solvent F- is a superstar,,,,,I- not so great. This may seem a bit baffling......but remember,,,,,,nucleophiles such as water, alcohol,,,,,or similar species love to react with secondary or tertiary substrates to induce an SN1 process. Strong nucleophiles like CN-, SH- ( mercaptide ) prefer to do the SN2 reaction.

The David Klein text as well as books by Wade, Carey, Solomons, McMurray and Bruice all have great examples of these nucleophiles and the solvents they react in.

I hope this helps......Dont sweat it......I have been studying solvents for the past 30 years,,,,,and they are indeed baffling at times !

Dr. Jim Romano
 
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