Chapter 15, Problem 15.22P

Interpretation Introduction

Interpretation:

Which of the compounds $\text{A}$ or $\text{B}$ is consistent with the IR spectrum of unknown compound that has a strong absorption band at ${\text{1686 cm}}^{\text{-1}}$ with molecular formula ${\text{C}}_{\text{9}}{\text{H}}_{\text{10}}\text{O}$, it is to be explained.

Concept introduction:

IR spectroscopy enables to identify different functional groups in the given 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 $\text{C=O}$ absorptions for a ketone is a strong and relatively narrow that appears $\text{~1710}-{\text{1730 cm}}^{\text{-1}}$. The conjugated $\text{C=C}$ or $\text{C}\equiv \text{C}$ bonds lowers the absorption frequency $\text{C=O}$ by $\text{20}-40{\text{cm}}^{-1}$. For identifying correct structures from the given IR spectrum, the first step is to calculate the IHD from the given molecular formula of a compound. The IHD accounts for the number of rings or multiple bonds the structure could possess. A benzene ring has an IHD of 4, which accounts for the three double bonds plus a ring.