Oh ok. Why can't the book just say that?
So can I choose ANY aprotic solvent for ANY SN2 reaction?
Not all aprotic solvents are the same. This is probably a bit beyond the scope of what you'll need, but different solvent have different dielectric constants. That could potentially affect how well certain species are dissolved in the solution. Sometimes toluene works. Other times THF is a better choice.http://en.wikipedia.org/wiki/Solvent
While I'm rambling, one of my favorite 'gotcha' questions for students involves reactions with alkoxide bases as reagents. If you're doing a reaction with sodium ethoxide (NaOCH2
), you probably don't want to use methanol as the solvent (H3
COH). The sodium ethoxide will react with the methanol to produce ethanol and sodium methoxide. If you're using the alkoxide base as a nucleophile, you have just produced two competent nucleophiles in your reaction (sodium ethoxide and sodium methoxide). Both nucleophiles will react indiscriminately.
Additionally, you want to make sure the solvent doesn't react under the reaction conditions. You don't want to use methanol as a solvent in a Grignard reaction.
Although, sometimes it's helpful if the solvent can BE the reactant. I did an esterification the other day where I used methanol as both the solvent AND nucleophile.
As for why the book can't just say that. You've got enough to worry about right now. You were just introduced to this whole new language (nucleophile, transition state, carbocation, Markovnikov's rule...) and you're being thrown all these arrows which mean different things depending on the number of barbs they have... Usually the solvent is an afterthought - learn the reactions, learn the mechanisms, that's the theory that's important in an undergrad organic lecture class.
Solvent selection is something you learn as you gain perspective on chemistry. It usually requires some time in the lab, reading literature, accidentally using ethyl acetate when you quench the reaction with sodium hydroxide... then you can't figure out why you have this huge peak at 2 ppm in your NMR... (not that I ever did that or anything...)
For now, and for these introductory reactions, just worry about pushing electrons and have an idea which class of solvent works for which reactions. You'll pick up the rest as you go.
ps - the Finkelstein is usually run in acetonehttp://en.wikipedia.org/wiki/Finkelstein_reaction