Chapter 7, Problem 7.51P

Interpretation Introduction

(a)

Interpretation:

The electron-rich sites and electron-poor sites in the given elementary steps are to be identified.

Concept introduction:

An atom with partial or full negative charge is an electron-rich site whereas an atom with partial or full positive charge is an electron-poor site. In an elementary step, electrons tend to flow from an electron-rich site to an electron-poor site.

Interpretation Introduction

(b)

Interpretation:

In each of the given elementary steps, the appropriate curved arrows are to be drawn.

Concept introduction:

The curved arrow can be drawn from an electron-rich site to an electron-poor site to show the flow of electrons from the electron-rich site to the electron-poor site. The first curved arrow is drawn from the lone pair of the negatively charged atom of the electron-rich site to the less electronegative atom of the electron-poor site. The second curved arrow is drawn from the region between the less electronegative atom and more electronegative atom towards the more electronegative atom, indicating the breaking of the bond.

Interpretation Introduction

(c)

Interpretation:

The names of each elementary step are to be identified.

Concept introduction:

In the bimolecular substitution reaction ${\text{S}}_{\text{N}}\text{2}$, electron pair from the electron-rich site approaches the carbon atom of the substrate to which the electron-poor atom is attached. Simultaneously, the bond between the carbon atom and the electron-poor atom breaks. The making and breaking of bonds takes place simultaneously.

In nucleophilic addition step, the nucleophile adds to the polar $\text{π}$ bond. The nucleophile is relatively electron rich, and the polar $\text{π}$ bond is relatively electron poor. Thus, the curved arrow drawn from the nucleophile to the polar $\text{π}$ bond represents the flow of electrons from an electron-rich site to an electron-poor site.

In the nucleophilic elimination step, the more electronegative atom bears full negative charge or partial negative charge. This electronegative atom forms a $\text{π}$ bond with the adjacent carbon atom by removing the leaving group.