Chapter 9, Problem 9.58P

Interpretation Introduction

Interpretation:

Which solvent, ethanol or dimethyl sulfoxide, would be better for carrying out the given reaction is to be determined.

Concept introduction:

Polar aprotic solvents favor ${\text{S}}_{\text{N}}\text{2/E2}$ reactions. In ${\text{S}}_{\text{N}}\text{2}$ reaction, the nucleophile starts forming a bond with the electrophilic carbon before the leaving group has departed. In E2 reaction, the attacking species acts as a base, removing an acidic proton. In general, strong negatively charged nucleophiles (or bases) work best in these reactions. Negatively charged species are strongly solvated by protic solvents, reducing their strength as a nucleophile or a base. Aprotic solvents do not effectively solvate the nucleophile/base, and therefore, work better.

Polar protic solvents favor ${\text{S}}_{\text{N}}\text{1/E1}$ reactions. Both reactions start the same way. The leaving group departs or the base extracts an acidic proton to form a carbocation in the first step. The nucleophile/base has not participated in this step. A protic solvent will therefore promote an ${\text{S}}_{\text{N}}\text{1/E1}$ reaction in two ways; by stabilizing the carbocation as well as the nucleophile/base.