Chapter 16, Problem 16.87P

Interpretation Introduction

Interpretation:

The product of the given reaction is to be determined, and the complete, detailed mechanism of the reaction is to be drawn based on the given ${}^1\text{H}$ NMR spectrum.

Concept introduction:

In Hofmann elimination reaction, the quaternary ammonium iodide salt is formed when quaternary ammonium is reacted with excess of methyl iodide. This product is then treated with silver oxide, water and heat. The substitution of the iodine by a hydroxyl anion is done by an elimination reaction. This gives the least substituted alkene.

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 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 $\text{H-O}$ in carboxylic group appears to be $\text{~ 10-12 ppm}$. The alpha hydrogen attached to the nitrogen has a frequency range $\text{~ 2-3 ppm}$.

The integration of each signal indicates 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, one can build molecular fragments with multiple carbon atoms.