[Lo.Lee.Ta.]

It would be nice if I could logically reason which solvent to use instead of just memorizing what solvent goes with each type of reaction...
Is there a way to know?

For instance, in reactions that convert alcohols to alkyl halides, we use pyridine with SOCl3, ether with PBr3, ether with HX, and pyridine with HF.

Then in other reactions, we have to use ethanol as the solvent...

Thanks!

[orgopete]

You should start with inert, virtually inert, reagent, and catalytic roles for your solvents. If the solvent is actually a diluting reagent, that should explain itself. Use ethanol to make an ethyl ester. Even thionyl chloride is sometimes used as solvent and reagent.

If you treat an ester with a base in an alcohol (Claisen condensation), then the alcohol should match the ester. The esters would actually be exchanged, but because they match, then it will return the ester of the starting material. The reaction occurs as if the solvent were inert. If you used a different alcohol, then you would create an exchange reaction in addition to the condensation.

Hydrocarbons, halocarbons, aromatics, and ethers are generally inert and do not react. They are often chosen based upon solubility, inertness, boiling point, and product isolation. Water, alcohols, ketones, and esters may also be inert to some reactions.

Pyridine is a base, a diluent, and/or a catalyst. It probably could be used in place of other solvents in some reactions, but it would be like using a gold can opener instead of steel. Pyridine is usually used because of its basic properties.

In summary, I never thought that much about the solvents. The chemistry and chemical compatibility, solubility, boiling points generally dictated the choices. One doesn't use ethyl acetate in a saponification reaction because it too reacts. This choice is determined by the reaction mechanism. If you are trying to learn solvents by use of flashcards (no mechanisms), then this problem is going to be more difficult. Reaction mechanisms are a strategy to simplify questions of this type. I completely endorse learning them.