Chapter 2, Problem 47P

Interpretation Introduction

Interpretation:

For distinguishing the pairs (a), (c), (d), (e), (g), and (i) given in problem 2.46, the key IR absorption bands are to be predicted.

Concept introduction:

Infrared spectroscopy is a simple, instrumental technique, which helps to determine the presence of various functional groups.

It depends on the interactions of atoms or molecules with the electromagnetic radiation.

The molecules that have dipole moment are IR active and the molecules that do not have dipole moment are IR inactive.

The IR absorption frequency of an alcohol group lies in the range of $3200-3550{\text{ cm}}^{-1}$.

For $\text{C}-\text{O Streching}$, the IR absorption frequency of an ether group lies in the range of $1020-1275{\text{ cm}}^{-1}$.

For $\text{C}=\text{O}$ stretching, the IR absorption frequency of an aldehyde group lies in the range of $1690-1740{\text{ cm}}^{-1}$, for the ketone group, it lies in the range of $1680-1750{\text{ cm}}^{-1}$, and for esters, it is $1735-1750{\text{ cm}}^{-1}$.

In IR spectrum, for carboxylic acids, both carbonyl and hydroxyl stretching absorption is present.

The IR absorption frequency for the hydroxyl group of carboxylic acid extends from $3600{\text{ cm}}^{-1}$ to $2500{\text{ cm}}^{-1}$.

The IR absorption frequency of secondary amine lies in the range of $3300-3500{\text{ cm}}^{-1}$ whereas tertiary amines do not show such absorption due to absence of $\text{N-H}$ bond.

The higher the ring strain, the higher will be the stretching frequency.