My book says that to favour a reaction by SN1 mechanism we need to use a polar protic solvent and a polar aprotic solvent in a reaction by SN2. * My Prior Knowledge:*
There are two types of polar sovents : Polar Protic Solvents contain a hydrogen bonded to a highly electronegative atom. They can solvate both positively and negatively charged species. On the other hand Polar Aprotic Solvents can only solvate positively charged species to an appreciable extent as there is no hydrogen bonding.
A question which arises now is that why should we use a Polar Aprotic Solvent in an SN2 reaction. The leaving group (which usually is negatively charged) needs to be stabilized as well. Wouldn't it be perfectly fine if I used a Polar Protic Solvent? If this is fine then why does presence of Polar Protic Solvent favor SN1 over SN2? Also I asked this question on SE a while back. I am looking for a more satisfactory answer apart from the ones given. here