Chapter 24, Problem 24.7P

Interpretation Introduction

Interpretation:

It is to be explained if diene A or diene B will react faster in a standard Diels-Alder reaction.

Concept introduction:

$\left[\text{4+2}\right]$ cycloaddition reactions are generally known as Diels-Alder reactions. A Diels-Alder reaction involves a diene and a dienophile. A diene shares four π electrons from the two conjugated π bonds while a dienophile shares two π electrons from a single π bond. The Diels-Alder reaction is concerted and proceeds through a six-membered aromatic transition state. For a Diels-Alder reaction to occur, the diene must be present in the s-cis conformation because both ends of the diene must be relatively close to the dienophile in the transition state. In a Diels-Alder reaction, the dienophile must be electron-poor, and the diene must be electron-rich. The electron-donating substituents bonded directly to the dienes carbons increase the rate of a Diels-Alder reaction. These are the same substituents as activating groups in electrophilic aromatic substitution. While electron-withdrawing substituents bonded directly to the dienophile carbons facilitate standard Diels-Alder reaction, these are the same substituents that act as deactivating groups in electrophilic aromatic substitution.