[[V]]

Does a polar aprotic solvent actually encourage an SN2 reaction, or does it just not inhibit an SN2?

I understand how an polar protic solvent would form a shell around the nucleophile & prevent SN2 - But would do the same thing for a leaving group, therefore encourages an SN1.

So is it safe to say polar aprotic encourages both SN2 and SN1 equally? If not, why?

[Dan]

Polar protic solvents will solvate anions (such as the leaving group and nucleophile) by hydrogen bonding - only protic solvents can participate in hydrogen bonding.

Using an aprotic solvent makes the nucleophile more reactive because it cannot hydrogen bond to it. It is true that an aprotic solvent will also decrease the efficiency of the leaving group for the same reason - good leaving groups and strong nucleophiles are required for S_N2, and if that is the case the gain in nucleophilicity of the nucleophile in the absence of hydrogen bonding outwieghs the lack of solvation of the leaving group.

In an S_N1 reaction, any gain in nucleophilicity is irrelavent because the nucleophile does not appear in the rate expression (as it is not involved in the rate-determining step). Hence an aprotic solvent can only retard S_N1 by diminishing the efficiency of the leaving group.

It is worth pointing out that solvent effects are quite subtle in comparison to nucleophilic strength, leaving group ability, and the stability of any potential carbocations when determining whether a reaction will proceed by S_N1 or S_N2. For example, you can still do (and I have done) S_N2 reactions in polar protic solvents if the nucleophile is strong enough and the leaving group is good enough.