Why polar protic solvents are unfavourable and polar aprotic solvents favourable for SN2 mechanism?

Protic solvents can H-bond to nucleophiles.They form a solvation shell around the nucleophile. This shell hinders the nucleophile from attacking the substrate.



Nucleophiles are less nucleophilic in protic solvents.

POLAR APROTIC SOLVENTS:
Examples of polar aprotic solvents are dimethylformimide (DMF), dimethylsulfuroxide (DMSO), ethylacetate and dichloromethane. (These molecules contain halides or oxygen to make it polar in general but no O-H or N-H bonds).

In Sn2 reactions , the nucleophile needs to react with the starting substance and this is the rate determining step. If the solvent is protic then it will stabilize the nucleophile so much that it basically won’t attack. Thus an aprotic solvent is needed, but remember that it still needs to be polar .Nucleophiles are more nucleophilic in aprotic solvents.

So,SN2reactions "prefer" aprotic solvents.