Chapter 15, Problem 15.46P

Interpretation Introduction

Interpretation:

In Section 15.5c, we learned that the frequency of a $\text{C=O}$ stretch decreases when the $\text{C=O}$ bond is conjugated to a $\text{C=C}$ bond. The pertinent resonance contributor of a conjugated carbonyl ($\text{C=C-C=O}$) is to be drawn. Based on the resulting resonance hybrid, the reason for decrease in the frequency is to be explained.

Concept introduction:

When a $\text{C=O}$ group is conjugated to a $\text{C=C}$ bond, the frequency of the $\text{C=O}$ absorption is typically lowered by 20–40 ${\text{cm}}^{-1}$. The conjugation decreases the stiffness of the $\text{C=O}$ bond. If the $\text{C=O}$ bond is conjugated to a $\text{C=C}$ bond, the resonance structure is possible. The $\text{C-O}$ single bond is weaker than $\text{C=O}$ bond. The $\text{C-O}$ single bond character decreases the stiffness of $\text{C=O}$ bond. The stretching frequency of $\text{C=O}$ band lowers by the Weakening of $\text{C=O}$ bond.