SN2 Polar Aprotic Solvent

[MedPR]

Just wanted to make sure that my understanding of SN2 preferred solvent is correct.

Aprotic so the nucleophile doesn't get protonated, and thus weakened.

Polar so the leaving group anion is stabilized by the solvent.

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[Pisiform]

Yes, aprotic is used because protic solvent would make the reaction less effective by hydrogen bonding with the nucleophile. And SN2 involves a backside attack, anything that interferes with nucleophile attacking will definitely slow down the reaction rate. And polar solvent is needed just to dissolve nucleophile.

 

[SaintJude]

Ah, yes--I just had this question in Kaplan & got it wrong b/c I didn't recognize another related reason (or the word choice)

An aprotic solvent is also preferred because it accelerates the Sn2 reaction by "solvating the cation of the nucleophile compound", thus improving its nucleophicity. It's in effect saying very similar things, but just noting that solvation and ion separation both work to accelerate the SN2 process.

 

[MedPR]

Ah, yes--I just had this question in Kaplan & got it wrong b/c I didn't recognize another related reason (or the word choice)

An aprotic solvent is also preferred because it accelerates the Sn2 reaction by "solvating the cation of the nucleophile compound", thus improving its nucleophicity. It's in effect saying very similar things, but just noting that solvation and ion separation both work to accelerate the SN2 process.

I'm not sure if that makes sense to me. Protic solvents basically disable SN2/E2 because the strong nucleophile (often an anion) gets protonated and allows SN1/E1 to occur more readily.

Is that directly from Kaplan? I don't think any strong nucleophiles, or any nucleophiles exist as cations.

 

[Ssina]

I think s/he is referring to the fact that if the Nu is OH it will be as e.g. NaOH so the aprotic solvent can stabilize the Na+ once the reaction is under way with Nu

 

[SaintJude]

I think s/he is referring to the fact that if the Nu is OH it will be as e.g. NaOH so the aprotic solvent can stabilize the Na+ once the reaction is under way with Nu

Yup and that was directly from Kaplan.

 

[MedPR]

I think s/he is referring to the fact that if the Nu is OH it will be as e.g. NaOH so the aprotic solvent can stabilize the Na+ once the reaction is under way with Nu

Oh, I see. I didn't realize we were talking about the counterion and not the nucleophile itself.

Good point.

 

[SKation]

Just wanted to make sure that my understanding of SN2 preferred solvent is correct.

Aprotic so the nucleophile doesn't get protonated, and thus weakened.

Polar so the leaving group anion is stabilized by the solvent.

Hwey,

I has a question regarding solvents I realize that polar, protic favors sn1 but is it still able to occur in polar, aprotic??
Thanks!