Chapter 20, Problem 20.69P

Interpretation Introduction

Interpretation:

A pain reliever has the formula ${\text{C}}_{\text{8}}{\text{H}}_{\text{9}}{\text{NO}}_{\text{2}}$. When it is heated in water under basic conditions, followed by an acid workup, two compounds are formed: acetic acid and a compound whose formula is ${\text{C}}_{\text{6}}{\text{H}}_{\text{7}}\text{NO}$. From the given information, the structures of that compound and the pain reliever are to be drawn.

Concept introduction:

In ^{${}^1\text{H}$} NMR spectroscopy, protons in different environments within a molecule have different chemical shifts, that is, they experience different degrees of shielding.

In addition to chemical shift, a ${}^1\text{H}$ NMR spectrum provides structural information based on Number of signals, which tells how many different kinds of protons are there; Integrated areas, which tells the ratios of the various kinds of protons; and Splitting pattern, which gives information about the number of protons that are within two or three bonds of the one giving the signal. Spin-spin splitting of NMR signals results from the coupling of the nuclear spins that are separated by two bonds (geminal coupling) or three bonds (vicinal coupling). In these cases, the number of peaks into which a signal is split is equal to $\text{(n+1)}$, where n is the number of protons to which the proton in question is coupled. Protons that have the same chemical shift do not split each other’s signals.

Complicated splitting patterns can result when a proton is unequally coupled to two or more protons that are different from one another. The chair form of cyclohexane is stable in which all the bulky groups occupy the equatorial position.

The ideal range for alkane protons is $\text{δ}0.9\text{- δ}1.4$; for alcohol it is $\text{δ}2\text{- δ}5.0$, and for aromatic protons, it is $\text{δ}6.5\text{- δ}8.5$. The chemical shift of a signal prompts about the aromatic rings, double bonds, or nearby electronegative atoms. The integration of each signal suggests the number of protons responsible for that signal. The splitting pattern of a signal indicates the number of neighboring protons that are distinct from the protons responsible for that signal. To deduce the structure of an unknown compound, the first step is to find the index of hydrogen deficiency if the molecular formula is given. Based on the data given in the ${}^1\text{H}$ NMR, build molecular fragments with multiple carbon atoms.

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. The characteristic absorption band for an ester group is ${\text{1735 - 1750 cm}}^{\text{-1}}$ (strong) in the IR spectrum, and that for amide group is ${\text{1630 - 1690 cm}}^{\text{-1}}$. A broad band due to O-H stretching ranges from ${\text{3600 - 3000 cm}}^{\text{-1}}$. A primary amine gives two signals in the range of ${\text{3500 - 3200 cm}}^{\text{-1}}$ (medium).