Why does an aprotic solvent increase the rate of an S_N2 reaction?

• A. An aprotic solvent solvates both the cation and the anion of the nucleophile. This spreads out the electron density around the nucleophile and makes the nucleophile less nucleophilic.
• B. An aprotic solvent solvates the cation, but not the anion, of the nucleophile. This increases the electron density around the nucleophile and makes the nucleophile more nucleophilic.
• C. It doesn't. An aprotic solvent increases the rate of an S_N1 reaction.
• D. An aprotic solvent solvates the anion, but not the cation, of the nucleophile. This shifts the electron density to the cation, leading to a faster reaction.

Answer: (B)

Verified step by step guidance

1. Understand the mechanism of an SN2 reaction: SN2 reactions involve a single concerted step where the nucleophile attacks the electrophile, leading to the displacement of a leaving group. The rate of this reaction is dependent on the concentration of both the nucleophile and the electrophile.
2. Recognize the role of solvents in SN2 reactions: Solvents can influence the nucleophilicity of the nucleophile. In SN2 reactions, the nucleophile must be strong to effectively attack the electrophile.
3. Differentiate between protic and aprotic solvents: Protic solvents have hydrogen atoms bonded to electronegative atoms, which can form hydrogen bonds with nucleophiles, potentially hindering their reactivity. Aprotic solvents lack these hydrogen bonds, allowing nucleophiles to remain more reactive.
4. Analyze the effect of aprotic solvents on nucleophiles: Aprotic solvents solvate the cation associated with the nucleophile but do not solvate the anion. This lack of solvation around the anion increases its electron density, enhancing its nucleophilicity and thus increasing the rate of SN2 reactions.
5. Conclude why aprotic solvents favor SN2 reactions: By increasing the nucleophilicity of the nucleophile through selective solvation, aprotic solvents facilitate the nucleophilic attack on the electrophile, thereby accelerating the SN2 reaction.