Final answer:
Polar aprotic solvents are best for SN2 reactions because they dissolve the substrate and leave the nucleophile free for the reaction, whereas polar protic solvents hinder the nucleophile's approach and are therefore unsuitable for SN2 but suitable for SN1 mechanisms.
Step-by-step explanation:
For SN2 reactions, using a polar aprotic solvent is ideal because these solvents dissolve ionic compounds through ion-dipole interactions and leave the anion, which is the nucleophile, relatively free to approach the electrophilic carbon of the substrate. The lack of hydrogen bonding in polar aprotic solvents, like acetone or dimethylsulfoxide, minimizes solvation of the nucleophile, providing less hindrance for the nucleophilic attack necessary for an SN2 reaction. These characteristics make polar aprotic solvents the fastest and most effective for SN2 mechanisms.
On the other hand, polar protic solvents, such as water or methanol, which can form hydrogen bonds, solvate both the anions and cations extremely well, thereby hindering the nucleophile's approach towards the substrate due to increased solvation shell, which reduces the rate of SN2 reactions. This is why polar protic solvents are not suitable for SN2 reactions but can favor SN1 mechanisms, particularly with tertiary substrates where steric hindrance and the solvent's ability to stabilize the resulting carbocation are critical.