Chapter 8, Problem 8.55P

Interpretation Introduction

(a)

Interpretation:

Whether the nucleophilic substitution reaction shown resulting in a mixture of constitutional isomers occurs via an ${\text{S}}_{\text{N}}\text{1}$ mechanism or an ${\text{S}}_{\text{N}}\text{2}$ mechanism is to be determined.

Concept introduction:

An ${\text{S}}_{\text{N}}\text{1}$ reaction is a two-step reaction. A carbocation is formed in the first reaction as a result of the leaving group breaking off, taking its bond pair with the carbon atom. The incoming nucleophile attacks the carbocation in the second step, forming the product.

The carbocation in the first step can rearrange to another one as a result of resonance if adjacent to a double or triple bond. A $\text{1, 2-hydride}$ shift or a $\text{1, 2-methyl}$ is also a possibility if it leads to the formation of a more stable carbocation. A rearrangement like this can lead to the formation of two constitutional isomers.

An ${\text{S}}_{\text{N}}\text{2}$ reaction is a single-step reaction. The incoming nucleophile attacks the carbon that is bonded to the leaving group, with the leaving group breaking off simultaneously with its bond pair. The incoming group must attack the carbon from a direction opposite to the leaving group. The result is generally a single product.

Interpretation Introduction

(b)

Interpretation:

A mechanism that accounts for the formation of each product is to be proposed.

Concept introduction:

An ${\text{S}}_{\text{N}}\text{1}$ reaction is a two-step reaction. A carbocation is formed in the first reaction as a result of the leaving group breaking off, taking its bond pair with the carbon atom. The incoming nucleophile attacks the carbocation in the second step, forming the product.

The carbocation in the first step can rearrange to another one as a result of resonance if adjacent to a double or triple bond. A $\text{1, 2-hydride}$ shift or a $\text{1, 2-methyl}$ is also a possibility if it leads to the formation of a more stable carbocation. A rearrangement like this can lead to two products.

An ${\text{S}}_{\text{N}}\text{2}$ reaction is a single-step reaction. The incoming nucleophile attacks the carbon that is bonded to the leaving group, with the leaving group breaking off simultaneously with its bond pair. The incoming group must attack the carbon from a direction opposite to the leaving group. The result is generally a single product.