Why does SN2 favor aprotic?

1 Answer
Bio
Mar 26, 2016

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.

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