Chapter 7.8, Problem 26CC

(a)

Interpretation Introduction

Interpretation:

The given substrates should be determined that whether they favor ${\text{S}}_{\text{N}}2$ or ${\text{S}}_{\text{N}}\text{1}$ reaction or either both the reaction mechanism.

Concept introduction:

${\text{S}}_{\text{N}}\text{1}$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. The tertiary carbo cation substrate favors this mechanism strongly since it is the most stable species among the carbo cation.

Carbocation: it is carbon ion that bears a positive charge on it.

Carbocation stability order:

Carbo-cation Rearrangement:

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

 (b)

Interpretation Introduction

Concept introduction:

${\text{S}}_{\text{N}}2$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.

Structure of the substrate plays an 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.

Leaving group: it is a fragment that leaves the chemical compound with pair of electrons in a heterolytic bond cleavage. The ${\text{S}}_{\text{N}}2$ reactivity increases in molecule with better leaving group.

Nucleophile: Nucleophiles are electron rich compounds which donates electrons to electrophilic compounds which results in bond formation.

Nucleophilic nature depends on the negative charge present in the molecule, the solvent in which it present and the electronegativity of the atom.

 (c)

Interpretation Introduction

Concept introduction:

Factors that favors both ${\text{S}}_{\text{N}}2$ and ${\text{S}}_{\text{N}}\text{1}$ reaction,

Secondary substrates, benzylic and allylic substrates undergoes via both the reaction mechanism.

Secondary substrates being in the middle between tertiary and primary they will undergo via both the mechanisms.

Allylic halides results in secondary substrate as halide group leaves and it favors ${\text{S}}_{\text{N}}2$ and also with ${\text{S}}_{\text{N}}\text{1}$ since the cation formed by this is resonance stabilized.

Benzylic halides also react via both mechanisms since it forms sterically unhindered secondary substrate and more stable resonance stabilized cation.

 (d)

Interpretation Introduction

Concept introduction:

Factors that favors both ${\text{S}}_{\text{N}}2$ and ${\text{S}}_{\text{N}}\text{1}$ reaction,

Secondary substrates, benzylic and allylic substrates undergoes via both the reaction mechanism.

Secondary substrates being in the middle between tertiary and primary they will undergo via both the mechanisms.

Allylic halides results in secondary substrate as halide group leaves and it favors ${\text{S}}_{\text{N}}2$ and also with ${\text{S}}_{\text{N}}\text{1}$ since the cation formed by this is resonance stabilized.

Benzylic halides also react via both mechanisms since it forms sterically unhindered secondary substrate and more stable resonance stabilized cation.

(e)

Interpretation Introduction

Concept introduction:

Substrates that do not undergo in either of the two given mechanism are vinyl and aryl substrates. They do not follow ${\text{S}}_{\text{N}}2$ since backside attack is restricted and also that the cations are not resonance stabilized means that ${\text{S}}_{\text{N}}\text{1}$ cannot be followed.

Vinyl group: It is functional group achieved by removing one hydrogen atom from the ethylene molecule ( ${\text{H}}_{\text{2}}{\text{C=CH}}_{\text{2}}$).

${\text{-HC=CH}}_{\text{2}}\to \text{vinyl}\text{group}$

Aryl group: It is functional group derived from aromatic substances.