Chapter 6, Problem 52P

(a)

Interpretation Introduction

Interpretation:Whether $\text{1-butyl-3-methylimidazolium}$ hexafluorophosphate acts as polar or non-polar and protic or aprotic solvent should be characterized.

Concept introduction:There are three classes of major solvents namely non-polar, polar-protic and polar aprotic. The non-polar solvents include cyclohexane, ${\text{CCl}}_4$ , benzene and decane that are essentially all the covalent organic solvents. They can dissolve only pure organic compounds and are immiscible with any ionic or inorganic substance.

Polar-protic solvents possess partial positive and negative charges that confer polarity to such molecules. Through their $\text{δ+}$ and $\text{δ}–$ charges they can form hydrogen bonds with negative and positive ions respectively. For instance,water has oxygen with $\text{δ}–$ and hydrogen with $\text{δ+}$ charges that allow water to associate with other water molecules. This makes water a polar solvent. Similarly,ethanol, methanol, solution of $\text{NaBr}$ in water are polar-protic solvents.

Polar-aprotic solvents possess partial positive and negative charges also. However, they do not liberate any proton to the substrate unlike water, methanol solvents. Acetone, DMSO, THF or DMF are categorized as polar aprotic solvent.

(b)

Interpretation Introduction

Interpretation:Effect on rate of nucleophilic substitution reaction between sodium cyanide and $\text{1-chloropentane}$ reaction when solvent is changed from ethanol to BMIM should be described.

Concept introduction:Bimolecular substitution or ${\text{S}}_{\text{N}}2$ proceeds via single-step mechanism. Thus it is well known as concerted mechanism. Nucleophile approaches carbon while the leaving group still departs from the rear side (opposite to leaving group). The transition state only illustrates the geometric orientation of the substrates and reagents as they pass through the maxima in the single-step mechanism.

A general ${\text{S}}_{\text{N}}2$ reaction mechanistic pathway is illustrated below:

  

${\text{S}}_{\text{N}}2$ pathway as it is a stereospecific reaction. This essentially means the R stereoisomer can only lead to inverted S stereoisomer and vice versa. Thus the outcome is the rear side displacement of leaving group.

Polar-aprotic solvents accelerate the rate of ${\text{S}}_{\text{N}}2$ reactions. These solvents possess partial positive and negative charges also. However, they do not liberate any proton to the substrate unlike water, methanol solvents. Acetone, DMSO, THF or DMF are categorized as polar aprotic solvent.