Chapter 15.12C, Problem 15.19P

Interpretation Introduction

Interpretation:

The explanation corresponding to the given statement that the $\left[4+2\right]$ Diels-Alder reaction is photochemically forbidden is to be proved.

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 strikes $1,3-\text{butadiene}$, 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.