Chapter 7.5, Problem 7.9TIY

(a)

Interpretation Introduction

Interpretation: The expected product for the reactions is to be drawn and stereochemistry along with preferred substitution mechanistic pathway between ${\text{S}}_{\text{N}}1$ and ${\text{S}}_{\text{N}}2$ should be determined.

  

Concept introduction: Unimolecuar substitution or ${\text{S}}_{\text{N}}1$ proceeds via two-step mechanism. The first slow step that determines rate is the removal of leaving group from the substrate haloalkane and generates a carbocation. Since the rate is only governed by substrate alone and no other nucleophile or solvent it is termed as unimolecuar substitution. The final step is attack of nucleophile on carbocation generated and formation of racemic products.

The general reaction mechanism for the ${\text{S}}_{\text{N}}1$ reaction is given as follows:

Step 1: Formation of a carbocation.

  

Step 2: Attack of a nucleophile on electron-deficient carbon of a carbocation.

  

(b)

Interpretation Introduction

Interpretation: Whether treatment of $\text{R-2-chlorobutane}$ with aqueous ammonia is a good synthetic method for the preparation of $\text{R-2-butanamine}$ should be determined.

Concept introduction: It has been found experimentally that if the leaving group lies on tertiary carbon than reaction proceeds preferentially via ${\text{S}}_{\text{N}}1$ while if the leaving group lies on primary carbon reaction proceeds preferentially via ${\text{S}}_{\text{N}}2$ .

If the leaving group lies on secondary than both ${\text{S}}_{\text{N}}1$ and ${\text{S}}_{\text{N}}2$ mechanisms are probable. However in such cases the solvents govern the major pathway. For instance, with solvents such as DMF, acetone the ${\text{S}}_{\text{N}}2$ reaction occurs with secondary alkyl halide. These solvents are commonly termed as polar aprotic solvents. With solvents such as methanol, water the ${\text{S}}_{\text{N}}1$ reaction occurs with secondary alkyl halide. These are termed as polar protic solvents.

A good nucleophile can take part in both ${\text{S}}_{\text{N}}1$ and ${\text{S}}_{\text{N}}2$ . Strong nucleophiles are more favored to react via ${\text{S}}_{\text{N}}2$ pathway, poor nucleophile are less favored for ${\text{S}}_{\text{N}}2$ pathway.