Question

Why does SN2 favor aprotic?

Answer 1

It should be the other way round: aprotic solvent favors `"S"_N2`.

Explanation:

For `"S"_N1`, the rate determining step depends on the leaving group. The nucleophile does not play an important role.

For `"S"_N2`, that is not the case. The more nucleophilic the nucleophile, the faster the reaction.

In protic solvents, the nucleophile is "trapped" in a cage of solvent molecules, and becomes less nucleophilic. Consequently, the `"S"_N2` reaction occurs as a slower rate.

Hence, `"S"_N2` is favored in an aprotic environment, where there is less hindrance to the nucleophiles.