Chapter 9, Problem 39P

(a)

Interpretation Introduction

Interpretation:

The effect on the rate of the reaction should be explained when the concentration of both methoxide and 1-bromobutane is increased by three times.

Concept Introduction:

S_N2 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 resulting in the formation of product with inversed configuration.

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.  Hence, primary substrate favors ${\text{S}}_{\text{N}}2$ mechanism.  The ${\text{S}}_{\text{N}}2$ reactivity increases in molecule with better leaving group.

Rate constant: Rate constant is an expression used to relate the rate of a reaction to the concentration of reactants participating in the reaction.

Rate of S_N2 reaction between an alkyl halide and nucleophile can be given as,’

$\begin{array}{l}\text{Rate}\text{=}\text{k}\left[\text{alkyl}\text{halide}\right]\left[\text{nucleophile}\right]\\ \\ where,\\ \\ \text{k}istherate\text{constant}\end{array}$

(b)

Interpretation Introduction

Interpretation:

The effect on the rate of the reaction should be explained when the solvent is change to ethanol.

Concept Introduction:

$S_{N}2$ 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 resulting in the formation of product with inversed configuration.

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.  Hence, primary substrate favors ${\text{S}}_{\text{N}}2$ mechanism. The ${\text{S}}_{\text{N}}2$ reactivity increases in molecule with better leaving group.

Protic solvent are polar solvent molecules which donates hydrogen bond and forms a strong ion-dipole interaction with the nucleophile.

Aprotic solvent are polar solvent molecules which do not have hydrogen having a partial positive charge and does not form any ion-dipole interactions with the nucleophile.

The stronger base is always a better nucleophile in an aprotic solvent.

(c)

Interpretation Introduction

Interpretation:

The effect on the rate of the reaction should be explained when the nucleophile is changed to ethanol.

Concept Introduction:

$S_{N}2$ 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 resulting in the formation of product with inversed configuration.

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.  Hence, primary substrate favors ${\text{S}}_{\text{N}}2$ mechanism.  The ${\text{S}}_{\text{N}}2$ reactivity increases in molecule with better leaving group.

Rate of S_N2 reaction between an alkyl halide and nucleophile can be given as,’

$\begin{array}{l}\text{Rate}\text{=}\text{k}\left[\text{alkyl}\text{halide}\right]\left[\text{nucleophile}\right]\\ \\ where,\\ \\ \text{k}istherate\text{constant}\end{array}$

The rate of the reaction increases when an alkyl halide reacts with a better nucleophile

(d)

Interpretation Introduction

Interpretation:

The effect on the rate of the reaction should be explained when the alkyl halide is changed to 1-chlorobutane.

Concept Introduction:

$S_{N}2$ 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 resulting in the formation of product with inversed configuration.

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.  Hence, primary substrate favors ${\text{S}}_{\text{N}}2$ mechanism.  The ${\text{S}}_{\text{N}}2$ reactivity increases in molecule with better leaving group.

Leaving group: it is a fragment that leaves substrate with a pair of electrons via heterolytic bond cleavage.

Down the group, atom size decreases which in turn reduces the tendency of atom for donating electrons and hence basicity decreases making them a better leaving group.

(e)

Interpretation Introduction

Interpretation:

The effect on the rate of the reaction should be explained when the alkyl halide is changed to 2-bromobutane.

Concept Introduction:

$S_{N}2$ 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 resulting in the formation of product with inversed configuration.

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.  Hence, primary substrate favors ${\text{S}}_{\text{N}}2$ mechanism.  The ${\text{S}}_{\text{N}}2$ reactivity increases in molecule with better leaving group.

Carbocation: It is carbon ion that bears a positive charge on it.  The increasing stability order of carbocation is as follows,

Primary carbocation < secondary carbocation < tertiary carbocation