Chapter 17, Problem 42PP

Interpretation Introduction

Interpretation:

The absorption spectra of 1,3-butadiene showing absorption in the UV region ( ${\text{λ}}_{\text{max}}$= 217nm) and the absorption spectra of 1,2-butadiene is not showing any absorption in the UV region should be explained.

Concept introduction:

• The compounds having two $\text{C}\text{=}\text{C}$ π bonds or $\text{C}\text{=}\text{C}$ bond with $\text{C}\text{=}\text{O}$ bonds are called dienes.

Based on the proximity of $\text{C}\text{=}\text{C}$ bonds in dienes, they are classified into three.

• Cumulated dienes: the $\text{C}\text{=}\text{C}$ bonds are located adjacent carbons.
• Conjugated dienes: the $\text{C}\text{=}\text{C}$ bonds or $\text{C}\text{=}\text{C}$ and $\text{C}\text{=}\text{O}$ are separated by one σ-bond.
• Isolated dienes: the $\text{C}\text{=}\text{C}$ bonds are separated by two or more σ-bonds.
• The π-systems of conjugated dienes absorb UV or visible light (200nm – 800nm).
• The π-systems of cumulative diene absorbs below 200nm.

To explain: the absorption spectra of 1,3-butadiene showing absorption in the UV region ( ${\text{λ}}_{\text{max}}$= 217nm) and the absorption spectra of 1,2-butadiene is not showing any absorption in the UV region.