Remember that solvent can have a huge impact on nucleophilicity. Nucleophilicity is highest in polar aprotic solvents, and lower in polar protic solvents. That’s because polar protic solvents (such as water) can hydrogen-bond, and that means that the nucleophile will carry around a little “entourage” of solvent molecules everywhere it goes, making it more bulky.
Just in case you forgot, here’s a quick guide to how to figure out what kind of solvent you’re looking at.
After we ask ourselves about the substrate and the nucleophile, the next question to ask about a reaction to tell if it’s SN1/SN2/E1/E2 is to look at the solvent.
Now, sometimes, you’ll have a primary alkyl halide and no solvent listed. It’s generally safe to assume that when you have a primary alkyl halide you’re dealing with an SN2.
Where the identity of the solvent comes in most handy when you have a secondary alkyl halide, a strong nucleophile/base and have to figure out if it’s SN2 or E2.
The decision process is pretty straightforward. If the solvent is polar aprotic, it’s safe to assume the reaction is SN2. If the solvent is polar protic, the reaction will likely be E2.
Let’s look at the 4 examples again.
In the first example, the use of a polar aprotic solvent (DMSO) rules out the E2, so we now know that this reaction is SN2.
In the second example, the use of a polar protic solvent (EtOH) rules out the SN2, so we know the reaction is E2.
Solvent doesn’t help us with figuring out the third and fourth examples (deciding SN1 and E1). For that, we’ll have to ask the final question. More on that tomorrow.
Thanks for reading! James