Chapter 15, Problem 15.8P

Interpretation Introduction

Interpretation:

Why $\text{trans-penta-1, 3-diene}$ absorbs at a much longer wavelength than $\text{penta-1, 4-diene}$ even though both have a total of four $\text{π}$ MOs is to be explained.

Concept introduction:

The ${\text{λ}}_{\text{max}}$ for the longest wavelength absorption in the UV-vis spectrum depends on the extent of conjugation. It increases with increasing conjugation though the difference decreases steadily with the extent of conjugation. Since this is directly related to the HOMO-LUMO energy difference, it implies that the energy difference also decreases as the extent of conjugation increases.