Chapter D, Problem D.10P

Interpretation Introduction

Interpretation:

It is to be determined whether the addition of a nucleophile to the O atom of a $\text{C=O}$ group is allowed or forbidden.

Concept introduction:

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.

The nucleophile is an electron-rich site, and the carbon of carbonyl carbon $\text{C=O}$ is an electron-poor site. Thus, the relevant HOMO will be contributed by the nucleophile, and the relevant LUMO will be contributed by the $\text{C=O}$ group. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) must have axial overlap or parallel overlap. The axial overlap between HOMO and LUMO has substantial net overlap, so this reaction is allowed. If the HOMO and LUMO do not have a substantial overlap, this overlap is forbidden.