Protic solvent for SN1 and aprotic/protic for SN2

[Minion]

I can memorize these, but I don't quite understand the concept behind it. I understand that for SN2, it would be faster for aprotic solvent because the strong nucleophile is one step reaction, so we wouldn't want our strong nucleophile to be protonated aka weaker.

I just can't understand the concept about SN1 and protic solvent. Why protic only? Thanks.

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[biomolecmed]

During Sn1 you are forming a carbonation. Protic solvent can stabilize a carbonation very well due to its dipole. The partial negative charge of the protic solvent will react and stabilize the plus charge of the carbonation. Aprotic does not have such a strong dipole, therefore cannot stabilize a carbonation as well.

Hope this helped. Good luck.

 

[Minion]

Oh wow ! Thank you, I never thought about the partial negative from dipole-dipole interactions. Because chad kept mentioning it does stabilize carbocation, but I was wondering how in the world would hydrogen atoms stabilize a carbocation ?

Little did I know, I have been looking at the wrong atom. Thanks !

 

[Chga]

The leaving group AND the carbocation are stabilized by the polar protic solvent. This is what enables the leaving group to leave the molecule.