Question

13. Consider the nucleophiles, F, Cl, Br, and I. In a polar, aprotic solvent, which should be most reactive? WHY? (2) 14. Consider the nucleophiles, F', Cl, Br, and I. In a polar, protic solvent, which should be most reactive? WHY? (2)

Answer 1

F⁻ is a better nucleophile than all of the remaining nucleophile  in polar aprotic solvents. A protic solvent has an H atom bound to O or N. It can use its H atom to participate in H-bonding with a nucleophile.The nucleophile has to push this shell of solvent molecules out of the way to attack the carbon bearing the leaving group.

Aprotic solvents, like protic solvents, are polar but, because they lack a positively polarized hydrogen, they do not form hydrogen bonds with the anionic nucleophile. The result, with respect to solvation, is a relatively weak interaction between the aprotic solvent and the nucleophile.

The consequence of this weakened interaction is two-fold. First, by using an aprotic solvent we can raise the reactivity of the nucleophile. This can sometimes have dramatic effects on the rate at which a nucleophilic substitution reaction can occur. For example, if we consider the reaction between bromoethane and potassium iodide, the reaction occurs 500 times faster in acetone than in methanol.