Chapter 24, Problem 24.3YT

Interpretation Introduction

Interpretation:

The cycloaddition reaction involving two molecules of cyclopentene is as follows:

Whether the transition state is aromatic or antiaromatic is to be explained.

Concept introduction:

The Diels-Alder reaction is a single step concerted reaction, whereby all of the bonds are formed and broken simultaneously. Thus, the reaction proceeds through a cyclic transition state, which makes it a pericyclic reaction. All Diels-Alder reactions involve the cyclic flow of six π electrons. The number of electrons is important because these electrons are delocalized over a complete ring in the transition state. Six electrons is a Hückel number of electrons (i.e., an odd number of pairs), and the species is aromatic when the electrons are delocalized over an entire ring. Thus, Diels-Alder reaction proceeds through an aromatic transition state.