Chapter 12.9, Problem 18ATS

(a)

Interpretation Introduction

Interpretation:

The structure of the expected product (2) has to be drawn.

Concept introduction:

${\text{PBr}}_{\text{3}}$ is a good brominating agents.  In bromination process it is used to convert alcoholic group in to alkyl bromide.

${\text{S}}_{\text{N}}2$Reaction: It is a nucleophilic substitution reaction in which the rate determining step depends on both of the molecules involved.  The bond making and the bond breaking process happens simultaneously in this reaction.  A general ${\text{S}}_{\text{N}}2$ reaction mechanism is given as,

Structure of the substrate plays major role in the reactivity of ${\text{S}}_{\text{N}}2$ reaction.  If the substrate is more substituted then the rate of the reaction will becomes slower.  This is due to reason that the mechanism of ${\text{S}}_{\text{N}}2$ reaction proceeds through backside attack on the substrate, it depends on steric factor that if more groups are attached near the leaving group then reactivity becomes slower.  The ${\text{S}}_{\text{N}}2$ reactivity increases in molecule with better leaving group.

(b)

Interpretation Introduction

Interpretation:

The possible mechanism for formation of by-product 3 and its stereochemical outcome have to be discussed.

Concept introduction:

${\text{PBr}}_{\text{3}}$ is a good brominating agents.  In bromination process it is used to convert alcoholic group in to alkyl bromide.

${\text{S}}_{\text{N}}2$Reaction: It is a nucleophilic substitution reaction in which the rate determining step depends on both of the molecules involved.  The bond making and the bond breaking process happens simultaneously in this reaction.  A general ${\text{S}}_{\text{N}}2$ reaction mechanism is given as,

Structure of the substrate plays major role in the reactivity of ${\text{S}}_{\text{N}}2$ reaction.  If the substrate is more substituted then the rate of the reaction will becomes slower.  Since the mechanism of ${\text{S}}_{\text{N}}2$ reaction proceeds through backside attack on the substrate, it depends on steric factor that if more groups attached near the leaving group the reactivity becomes slower.  The ${\text{S}}_{\text{N}}2$ reactivity increases in molecule with better leaving group.