Chapter 24, Problem 24.5P

Interpretation Introduction

Interpretation:

The product of a hypothetical reaction between benzene and ethene is to be drawn, and why it does not occur is to be explained.

Concept introduction:

The [4+2] cycloaddition reactions are generally known as Diels-Alder reaction. 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 p 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. The s indicates that the cis designation describes the orientation of groups about a single bond, rather than a double bond or the plane of the ring. The s-trans conformation of the diene is more stable than s- cis conformation due to steric repulsion. Thus, the conformer which easily undergoes s-cis form will react readily with a dienophile. Diels-Alder reaction will not easily occur in case of aromatic compounds when they are considered as a diene source because it will disturb the aromaticity.