Chapter 24, Problem 24.26P

Interpretation Introduction

Interpretation:

Figure 24-13 for $\text{buta-1, 3-diene}$ in its excited state and ethene in its ground state is to be redrawn. The same is to be done for $\text{buta-1, 3-diene}$ in its ground state and ethene in its excited state. If these results are consistent with a Diels–Alder reaction being photochemically forbidden is to be determined.

Concept introduction:

A UV photon with sufficient energy ($\text{hν}$) is capable of promoting an electron from ethene’s HOMO to its LUMO to generate the excited state. The excited state’s HOMO is the ground state’s LUMO. When a molecule of $\text{buta-1, 3-diene}$ absorbs a photon with sufficient energy, an excited state of $\text{buta-1, 3-diene}$ is generated, in which an electron from what was originally the HOMO now occupies what was originally the LUMO. When this excited state species encounters a molecule of ethene in the ground state, the HOMO and LUMO no longer have the appropriate symmetries to interact, making the reaction forbidden. Consequently, the Diels–Alder reaction is thermally allowed, but photochemically forbidden.