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 cm − 1 . For C − O Streching , the IR absorption frequency of an ether group lies in the range of 1020 − 1275 cm − 1 . For C = O stretching, the IR absorption frequency of an aldehyde group lies in the range of 1690 − 1740 cm − 1 , for the ketone group, it lies in the range of 1680 − 1750 cm − 1 , and for esters, it is 1735 − 1750 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 cm − 1 to 2500 cm − 1 . The IR absorption frequency of secondary amine lies in the range of 3300 − 3500 cm − 1 whereas tertiary amines do not show such absorption due to absence of N-H bond. The higher the ring strain, the higher will be the stretching frequency.

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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 cm−1.

For C−O Streching, the IR absorption frequency of an ether group lies in the range of 1020−1275 cm−1.

For C=O stretching, the IR absorption frequency of an aldehyde group lies in the range of 1690−1740 cm−1, for the ketone group, it lies in the range of 1680−1750 cm−1, and for esters, it is 1735−1750 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 cm−1 to 2500 cm−1.

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

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

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