Chapter 16, Problem 16.37P

Interpretation Introduction

Interpretation:

The IR spectrum of unknown 3 contains a strong absorption at ${\text{1670 cm}}^{\text{-1}}$ and no absorption above ${\text{3000 cm}}^{\text{-1}}$. From this data, three possible structures of unknown $\text{ 3}$ are to be drawn.

Concept introduction:

The IR spectroscopy is limited to know which functional group is present in the compound. The peaks in the IR spectroscopy are also called IR bands. Some characteristic bands of important functional groups are given below:

The alcoholic and acidic $\text{OH}$ groups give a broad band above ${\text{3000 cm}}^{\text{-1}}$ due to H bonding. Carbonyl group gives strong absorption in a range from ${\text{1850 – 1630 cm}}^{\text{-1}}$. Especially carbonyl group of an ester gives strong absorption in a range of ${\text{1735 – 1750 cm}}^{\text{-1}}$, and that of an amide group is ${\text{1630 - 1690 cm}}^{\text{-1}}$. The carbonyl group of an aldehyde gives absorption band at ${\text{1720 -1740 cm}}^{\text{-1}}$. 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 ${\text{3500 - 3000 cm}}^{\text{-1}}$ (medium); the range for the same for an amide group is ${\text{3500 - 3050 cm}}^{\text{-1}}$ (medium). The stretching band due to C-X (X: Halogen atom) is generally observed in the fingerprint region. 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.