Chapter 7, Problem 78PP

(a)

Interpretation Introduction

Interpretation:

• Whether the given reaction proceeds via an ${\text{S}}_{\text{N}}\text{1}$ or ${\text{S}}_{\text{N}}\text{2}$ pathway has to be determined.
• The complete mechanism for the given reaction has to be drawn.

Concept Introduction:

$S_{N}2$Reaction: It is a nucleophilic substitution reaction in which the rate determining step depends on both (substrate and nucleophile) of the molecules involved. The bond making and the bond breaking process happens simultaneously in this reaction.

Structure of the substrate plays a major role in ${\text{S}}_{\text{N}}2$ reaction. If the substrate consists of more substitution the rate of the reaction 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 become slower.

Nucleophile: donates pair of electrons to positively charged substrate resulting in the formation of chemical bond.

Leaving group: it is the fragment that leaves the chemical compound with pair of electrons in a heterolytic bond cleavage.

Example

$S_{N}1$Reaction: it is a nucleophilic substitution reaction in which the rate determining step depends on one reactant. First step is the formation of more stable carbocation which is followed by the attack of nucleophile. Formation of more stable carbocation and the leaving group present in the substrate plays very important role in the reactivity of ${\text{S}}_{\text{N}}\text{1}$ reaction.

(b)

Interpretation Introduction

Interpretation:

The expected rate equation for the given process has to be found.

Concept Introduction:

Rate Equation: It describes the rate of the reaction with respect to the concentration of the reactants participated in the reaction.

The relationship between the rate of the reaction and concentrations of the given nucleophile and alkyl halide can be represented as follows.

  $\text{Rate}\text{α }\left[\text{alkyl halide}\right]\left[\text{nucleophile}\right]$

Thus,

  $\text{rate}\text{=}\text{k[alkyl}\text{halide][nucleophile]}$ ,

 Where, k is the rate constant.

(c)

Interpretation Introduction

Interpretation:

When the concentration of cyanide $CN$ was doubled then the process will occur at a faster rate or not has to be found.

Concept Introduction:

Rate of reaction: Rate of change for any species is inversely proportional to its co-efficient in a balanced equation.

Rate Equation: It describes the rate of the reaction with respect to the concentration of the reactants participated in the reaction.

The relationship between the rate of the reaction and concentrations of the given nucleophile and alkyl halide can be represented as follows.

  $\text{Rate}\text{α }\left[\text{alkyl halide}\right]\left[\text{nucleophile}\right]$

Thus,

  $\text{rate}\text{=}\text{k[alkyl}\text{halide][nucleophile]}$ ,

Where, k is the rate constant.

(d)

Interpretation Introduction

Interpretation:

Energy level diagram should be drawn for the given reaction process.

Concept Introduction:

Energy diagram of an E2 reaction has only one hump because only one step is involved in the E2 reaction.

Transition state is a state in between the reactant and product.

 In the transition state of E2 reaction: the abstraction of β-proton by the base, removal of leaving group (halo-group) and formation of double bond are taking place.