Question

Question #c736e

Answer 1

The best solvent is `"CH"_3"CH"_2"SOCH"_3`

Explanation:

Ethyl methyl sulfoxide is a polar aprotic solvent.

It preferentially solvates the cations in an `"S"_text(N)2` reaction.

Thus, the anions (the nucleophile) are relatively "bare" of solvating molecules and they become even better nucleophiles.

But don't we require nonpolar solvents for `"S"_text(N)2` reactions?

That's only partially correct.

One factor to consider when choosing a solvent is solubility. We must get the reactants into the same phase so they can collide and react.

Butane is a gas at -1 °C. Pentane (boiling point 68 °C) would be a better nonpolar solvent.

However, a nonpolar solvent won't dissolve ionic nucleophiles. We need polar solvents to get ionic reactants into solution.

Polar solvents are important for `"S"_text(N)1` reactions, because they can stabilize the intermediate carbocation.

Polar protic solvents like water hinder `"S"_text(N)2` reactions because:

• they solvate the starting materials more than the transition state, so they increase the activation energy
• they surround the anionic nucleophiles with a hydrogen-bonded solvation shell, making them less reactive

However, polar aprotic solvents like ethyl methyl sulfoxide preferentially solvate the cations, leaving the anions relatively "bare". Thus, the anions have enhanced nucleophilicity in these solvents.