Chapter 15, Problem 15.50P

Interpretation Introduction

Interpretation:

The difference between $\text{C=O}$ stretching frequencies for butanal centered at $1746{\text{ cm}}^{-1}$ and for $\text{butan-2-one}$ centered at $1735{\text{ cm}}^{-1}$ is to be explained.

Concept introduction:

IR spectroscopy enables us to identify different functional groups in the molecule.

One of the greatest advantages of IR spectroscopy is that the frequency of a particular type of vibration is typically found within a characteristic range of frequencies regardless of the functional group with which the vibration is associated. The fact that absorptions by certain vibrations appear with characteristic frequency ranges enables us to use IR spectroscopy to obtain structural information about a molecule. The resonance effect alters the bond absorption frequency of $\text{C=O}$ in IR spectrum. Greater the stabilization of carbocation greater is the contribution from the resonance structure. This weakens the bond, and hence lowers the vibrational frequency.