Chapter 15.12C, Problem 15.20P

(a)

Interpretation Introduction

Interpretation:

The $\left[4+4\right]$ cycloaddition of two butadiene molecules to give $\text{cycloocta}-1,5-\text{diene}$ is thermally forbidden but photochemically allowed is to be shown.

Concept introduction:

When ultraviolet light is used rather than heat for $\left[4+2\right]$ Diels-Alder reaction, it is photochemically not allowed. A photon with correct energy strike $1,3-\text{butadiene}$, due to which one of the pi electrons gets excited to the next high molecular orbital. This high orbital is now occupied and is the new ${\text{HOMO}}^{\ast }$ of the excited molecule.

(b)

Interpretation Introduction

Interpretation:

The reaction of thermally allowed cycloaddition of two butadiene molecules is to be shown and an explanation corresponding to the fact that it is thermally allowed is to be stated.

Concept introduction:

Diels-alder reaction is a cycloaddition reaction in which two molecules combine to form a new ring. In this type of reaction, syn addition takes place. It is a reaction between diene with a dienophile to yield a cyclohexene. The dienophile adds to one side of the diene, and the diene adds to the one side of the dienophile. Thus, they have syn stereochemistry.