Chapter 15, Problem 15.45P

Interpretation Introduction

Interpretation:

The carbonyl stretch of $\text{acetyl chloride}$ (${\text{CH}}_{\text{3}}\text{COCl}$) is found at 1806 ${\text{cm}}^{-1}$. Whether the $\text{C=O}$ bond is stronger or weaker than an amide is to be determined. The thing that suggests the resonance contribution of a lone pair on Cl in an acid chloride compared to that of the lone pair on N in an amide is to be explained.

Concept introduction:

The lone pair on nitrogen atom can take part in resonance with $\text{C=O}$ bond in amide. 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. Weakening of $\text{C=O}$ bond lowers the stretching frequency of $\text{C=O}$ band. The $\text{C=O}$ bond in acid chloride is stronger than $\text{C=O}$ bond in amide.