Chapter 24, Problem 24.3P

Interpretation Introduction

Interpretation:

Which molecule, A or B, will react faster with ethane in Diels-Alder reaction is to be explained.

Concept introduction:

The [4+2] cycloaddition reactions are generally known as a 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 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 to the s-cis form will react readily with a dienophile.