Final answer:
Polar aprotic solvents enhance SN2 reaction rates by providing strong ion-dipole interactions with cations while leaving the nucleophilic anions relatively free from solvation, making them more reactive and better able to approach the electrophilic carbon atom.
Step-by-step explanation:
Polar aprotic solvents are favourable for SN2 reactions because they do not solvate the nucleophile as effectively as polar protic solvents do. While polar protic solvents create a solvation shell around both cations and anions through ion-dipole interactions, thus hindering the nucleophile’s ability to approach the electrophilic carbon atom, polar aprotic solvents, due to steric hindrance, leave the anion (nucleophile) relatively free. This allows the nucleophile to be more reactive as it is less encumbered.
Furthermore, polar aprotic solvents such as dimethyl sulfoxide (DMSO) and N,N-dimethylformamide (DMF) enhance the rate of SN2 processes by providing strong ion-dipole interactions with cations while the nucleophile stays largely unsolvated and therefore more nucleophilic. However, in an SN1 or E1 reaction where the rate-determining step does not involve the nucleophile directly, polar aprotic solvents are not as effective because these reactions proceed well in polar protic solvents like water.
Examples of polar aprotic solvents include dimethylsulfoxide (DMSO) and acetone, which in the context of SN2 reactions, lead to faster reaction rates as opposed to polar protic solvents like water or methanol.