Chapter 20, Problem 20.70P

Interpretation Introduction

Interpretation:

When $\text{methyl 5-oxopentanoate}$ is treated with vinyl Grignard, a compound is produced whose formula is ${\text{C}}_{\text{7}}{\text{H}}_{\text{10}}{\text{O}}_{\text{2}}$. In the IR spectrum of ${\text{C}}_{\text{7}}{\text{H}}_{\text{10}}{\text{O}}_{\text{2}}$, an intense absorption appears at ${\text{1740 cm}}^{\text{-1}}$ and a weaker absorption appears at ${\text{1650 cm}}^{\text{-1}}$. Seven signals appear in the ${}^{\text{13}}\text{C}$ NMR spectrum of ${\text{C}}_{\text{7}}{\text{H}}_{\text{10}}{\text{O}}_{\text{2}}$. The DEPT spectrum shows that there is one carbon that is bonded to no hydrogens, two $\text{CH}$ carbons, and four ${\text{CH}}_{\text{2}}$ carbons. From this given information, the structure of ${\text{C}}_{\text{7}}{\text{H}}_{\text{10}}{\text{O}}_{\text{2}}$ is to be drawn.

Concept introduction:

To deduce the structure of an unknown compound, the first step is to find the index of hydrogen deficiency (IHD) if the molecular formula is given, which gives a basic idea about the carbon skeleton of the unknown compound. ${}^{13}\text{C}$ NMR provides valuable information about the carbon skeleton. Each signal in the ${}^{13}\text{C}$ NMR equals the number of distinct carbon atoms in the given compound. In most of the ${}^{13}\text{C}$ NMR spectra, almost every time, all the signals would appear as singlets when the carbon atoms of the compound are not directly attached to other spin active nuclei (such as F, ${}^{\text{31}}\text{P}$ etc.). The IR spectra of an unknown compound give an idea as to which functional group may be present in the compound. For carbonyl compounds, the strong absorption band ranging from ${\text{1640 - 1850 cm}}^{\text{-1}}$ due to the stretching of the carbonyl group must be observed. The characteristic absorption band for an ester group is ${\text{1735 - 1750 cm}}^{\text{-1}}$ (strong), and that for $\text{C=C}$ is ${\text{1630 - 1680 cm}}^{\text{-1}}$ (weak), and for isolated $\text{C=C}$, it is near ${\text{1640 -1650 cm}}^{\text{-1}}$ in IR spectrum.